Darwin, Charles - On the Origin of Species

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The Origin of Species Vol. 1

Additions and Corrections to

the Sixth Edition. NUMEROUS small corrections have been made in the last and present editions on various subjects,

according as the evidence has become somewhat stronger or weaker. The more important corrections

and some additions in the present volume are tabulated on the following page, for the convenience of

those interested in the subject, and who possess the fifth edition. The second edition was little more than

a reprint of the first. The third edition was largely corrected and added to, and the fourth and fifth still

more largely. As copies of the present work will be sent abroad, it may be of use if I specify the state of

the foreign editions. The third French and second German editions were from the third English, with some

few of the additions given in the fourth edition. A new fourth French edition has been translated by

Colonel Moulinié; of which the first half is from the fifth English, and the latter half from the present

edition. A third German edition, under the superintendence of Professor Victor Carus, was from the

fourth English edition; a fifth is now preparing by the same author from the present volume. The second

American edition was from the English second, with a few of the additions given in the third; and a third

American edition has been printed from the fifth English edition. The Italian is from the third, the Dutch

and three Russian editions from the secondEnglish edition, and the Swedish from the fifthEnglish edition.

Fifth Edition Sixth Editon Chief Additions and Corrections.

Page Page vol. i.

100 106 Influence of fortuitous destruction on natural selection.

158 156 On the convergence of specific forms.220 221 Account of the Ground-Woodpecker of La Plata modified.

225 227 On the modification of the eye.

230 233Transitions through the acceleration or retardation of the period of

reproduction.

231 234 The account of the electric organ of fishes added to.

233 237 Analogical resemblance between the eyes of Cephalopods and Vertebrates.

234 239 Claparède on the analogical resemblance of the hair-claspers of the Acaridæ.

248 254 The probable use of the rattle to the Rattle-snake.

248 254 Helmholtz on the imperfection of the human eye.

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255 262

The first part of this new Chapter consists of portions, in a much modified

state, taken from chap. iv. of the former editions. The latter and larger part is

new and relates chiefly to the supposed incompetency of natural selection to

account for the incipient stages of useful structures. There is also a discussion

on the causes which prevent in many cases the acquisition through natural

selection of useful structures. Lastly, reasons aregiven for disbelieving in great

and sudden modifications. Gradations of character, often accompanied bychanges of function, are likewise here incidentally considered.

268 333The statement with respect to young cuckoos ejecting their foster-brothers

confirmed.

270 334 On the cuckoo-like habits of the Molothrus.

vol.ii

307 9 On fertile hybrid moths.

319 22The discussion on the fertilityof hybrids not having been acquired through

natural selection condensed andmodified.

326 28 On the causes of sterility of hybrids, added to and corrected.377 81 Pyrgoma found in the chalk.

402 107 Extinct forms serving to connect existing groups.

440 148 On earth adhering to the feet of migratory birds.

463 172 On the wide geographical range of a species of Galaxias, a fresh-water fish.

505 218 Discussion on analogical resemblances, enlarged and modified.

516 232 Homological structure of the feet of certain marsupial animals.

518 236 On serial homologies, corrected.

520 237 Mr. E. Ray Lankester on morphology.

521 240 On the asexual reproduction of Chironomus.541 262 On the origin of rudimentary parts, corrected.

547 262 Recapitulation on the sterility of hybrids, corrected.

552 275Recapitulationon the absence of fossils beneath theCambrian system,

corrected.

568 293 Natural selection not exclusive agency in the modification of species, as

alwaysmaintained in thiswork.

572 297The belief in the separate creation of species generally held by naturalists, until

a recent period.

"But with regard to the material world, we can at least go so far as this—we can perceive that events are brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the

establishment of general laws."

Whewell: Bridgewater Treatise .

"The only distinct meaning of the word 'natural' is stated, fixed , or settled; since what is natural as much

requires and presupposes an intelligent agent to render it so,i. e ., to effect it continually or at stated

times, as what is supernatural or miraculous does to effect it for once."

Butler: Analogy of Revealed Religion .

"To conclude, therefore, let no man out of a weak conceit of sobriety, or an ill-applied moderation, think





or maintain, that a man can search too far or be too well studied in the book of God's word, or in the

book of God's works; divinity or philosophy; but rather let men endeavour an endless progress or

proficience in both."

Bacon: Advancement of Learning .

Down, Beckenham, Kent ,

First Edition, November 24th , 1859.

Sixth Edition, Jan. 1872.

|Go to Contents |

Chapter I

Variation Under Domestication.

Causes of Variability—Effects of Habit and the use or disuse of Parts—Correlated

Variation—Inheritance—Character of Domestic Varieties—Difficulty of distinguishing between Varieties

andSpecies—Origin of DomesticVarieties from one or more Species—Domestic Pigeons, their

Differences andOrigin—Principles of Selection, anciently followed, their Effects—Methodical and

Unconscious Selection—UnknownOrigin of our Domestic Productions—Circumstances favourable to

Man's power of Selection.

Causes of Variability.

WHEN we compare the individuals of the same variety or sub-variety of our older cultivated plants and

animals, one of the first points which strikes us is, that they generally differ more from each other than dothe individuals of any one species or variety in a state of nature. And if we reflect on the vast diversity of

the plants and animals which have been cultivated, and which have varied during all ages under the most

different climates and treatment, we are driven to conclude that this great variability is due to our

domestic productions having been raised under conditions of life not so uniform as, and somewhat

different from, those to which the parent species had been exposed under nature. There is, also, some

probability in the view propounded by Andrew Knight, that this variability may be partly connected with

excess of food. It seems clear that organic beings must be exposed during several generations to new

conditions to cause any great amount of variation; and that, when the organisation has once begun to

vary, it generally continues varying for many generations. No case is on record of a variable organism

ceasing to vary under cultivation. Our oldest cultivated plants, such as wheat, still yield new varieties: our oldest domesticated animals are still capable of rapid improvement ormodification.





As far as I am able to judge, after long attending to the subject, the conditions of life appear to act in two

ways,—directly on the whole organisation or on certain parts alone, and indirectly by affecting the

reproductive system. With respect to the direct action, we must bear in mind that in every case, as

Professor Weismann has lately insisted, and as I have incidentally shown in my work on 'Variation under

Domestication,' there are two factors: namely, the nature of the organism, and the nature of the

conditions. The former seems to be much the more important; for nearly similarvariations sometimes

arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variationsarise under conditions which appear to be nearly uniform. The effects on the offspring are either definite

or indefinite. Theymay be considered as definitewhen all or nearly all the offspring of individuals

exposed to certain conditions during several generations are modified in the same manner. It is extremely

difficult to come to any conclusion in regard to the extent of the changes which have been thus definitely

induced. There can, however, be little doubt about many slight changes,—such as size from the amount

of food, colour from the nature of the food, thickness of the skin and hair from climate, &c. Each of the

endless variations which we see in the plumage of our fowls must have had some efficient cause; and if

the same cause were to act uniformly during a long series of generations on many individuals, all probably

would be modified in the same manner. Such facts as the complex and extraordinary out-growths which

invariably follow from the insertion of a minute drop of poison by a gall-producing insect, show us whatsingular modifications might result in the case of plants from a chemical change in the nature of the sap.

Indefinite variability is a muchmore common result of changed conditions than definite variability, and

has probably played a more important part in the formation of our domestic races. We see indefinite

variability in the endless slightpeculiarities which distinguish the individuals of the same species, and which

cannot be accounted for by inheritance from either parent or from some more remote ancestor. Even

strongly-marked differencesoccasionally appear in the young of the same litter, and in seedlings from the

same seed-capsule. At long intervals of time, out of millions of individuals reared in the same country and

fed on nearly the same food, deviations of structure so strongly pronounced as to deserve to be called

monstrosities arise; but monstrosities cannot be separated by any distinct line from slighter variations. All

such changes of structure,whether extremely slight or stronglymarked,which appear amongst manyindividuals living together,may be considered as the indefinite effects of the conditions of life on each

individual organism, in nearly the samemanner as the chill affects different men in an indefinitemanner,

according to their state of bodyor constitution, causing coughs or colds, rheumatism, or inflammation of

various organs.

With respect to what I have called the indirect action of changed conditions, namely, through the

reproductive system being affected, we may infer that variability is thus induced, partly from the fact of

this systembeing extremely sensitive to any change in the conditions, and partly from the similarity, as

Kölreuter and others have remarked, between thevariabilitywhich follows from the crossing of distinct

species, and that which may be observed with plants and animals when reared under new or unnaturalconditions.Many facts clearly showhow eminently susceptible the reproductive system is to very slight

changes in the surrounding conditions. Nothing is more easy than to tame an animal, and few things more

difficult than to get it to breed freely under confinement, even when the male and female unite. How many

animals there are which will not breed, though kept in an almost free state in their native country! This is

generally, but erroneously, attributed to vitiated instincts. Many cultivatedplants display the utmost

vigour, and yet rarely or never seed! In some few cases it has been discovered that a very trifling change,

such as a little more or less water at some particular period of growth, will determine whether or not a

plant will produce seeds. I cannot here give the details which I have collected and elsewhere published

on this curious subject; but to show how singular the laws are which determine the reproduction of

animalsunder confinement, I maymention that carnivorous animals, even from the tropics, breed in this

countrypretty freely under confinement, with the exception of the plantigrades or bear family,which

seldom produce young; whereas carnivorous birds, with the rarest exceptions, hardly ever lay fertile

eggs. Many exotic plants have pollen utterly worthless, in the same condition as in the most sterile





hybrids. When, on the one hand, we see domesticated animals and plants, though often weak and sickly,

breeding freelyunder confinement; andwhen, on the other hand,we see individuals, though taken young

from a state of nature perfectly tamed, long-lived and healthy (of which I could give numerous instances),

yet having their reproductive system so seriously affected by unperceived causes as to fail to act, we

need not be surprised at this system, when it does act under confinement, acting irregularly, and

producing offspring somewhat unlike their parents. I may add, that as some organisms breed freely under

the most unnatural conditions (for instance, rabbits and ferrets kept in hutches), showing that their reproductive organs are not easily affected; sowill some animals andplants withstand domestication or

cultivation, and vary very slightly—perhaps hardly more than in a state of nature.

Some naturalists have maintained that all variations are connected with the act of sexual reproduction;

but this is certainly an error; for I have given in another work a long list of "sporting plants," as they are

called by gardeners;—that is, of plants which have suddenly produced a single bud with a new and

sometimes widely different character from that of the other buds on the same plant. These bud variations,

as they may be named, can be propagated by grafts, offsets, &c., and sometimes by seed. They occur

rarely under nature, but are far from rare under culture. As a single bud out of the many thousands,

produced year after year on the same tree under uniform conditions, has been known suddenly to assumea new character; and as buds on distinct trees, growing under different conditions, have sometimes

yielded nearly the same variety—for instance, buds on peach-trees producing nectarines, and buds on

common roses producing moss-roses—we clearly see that the nature of the conditions is of subordinate

importance in comparison with the nature of the organism in determining each particular formof

variation;—perhaps of not more importance than the nature of the spark, by which a mass of combustible

matter is ignited, has in determining the nature of the flames.

Effects of Habit and of the Use or Disuse of Parts;Correlated Variation; Inheritance.

Changed habits produce an inherited effect, as in the period of the flowering of plants when transportedfrom one climate to another. With animals the increased use or disuse of parts has had a more marked

influence; thus I find in the domestic duck that the bones of the wing weigh less and the bones of the leg

more, in proportion to the whole skeleton, than do the same bones in the wild-duck; and this change may

be safely attributed to the domestic duck flying much less, and walking more, than its wild parents. The

great and inherited development of the udders in cows and goats in countries where they are habitually

milked, in comparison with these organs in other countries, is probably another instance of the effects of

use. Not one of our domestic animals can be named which has not in some country drooping ears; and

the view which has been suggested that the drooping is due to disuse of the muscles of the ear, from the

animals being seldommuch alarmed, seemsprobable.

Many laws regulate variation, some few of which can be dimly seen, and will hereafter be briefly

discussed. I will here only allude to what may be called correlated variation. Important changes in the

embryo or larvawill probably entail changes in the mature animal. Inmonstrosities, the correlations

between quite distinct parts are very curious; and many instances are given in Isidore Geoffroy St.

Hilaire's great work on this subject. Breeders believe that long limbs are almost always accompanied by

an elongated head. Some instances of correlation are quite whimsical: thus cats which are entirely white

and have blue eyes are generally deaf; but it has been lately stated by Mr. Tait that this is confined to the

males. Colour and constitutional peculiarities go together, ofwhichmany remarkable cases couldbe

given amongst animals and plants. From facts collected by Heusinger, it appears that white sheep and

pigs are injured by certain plants, whilst dark-coloured individuals escape: ProfessorWymanhas recently

communicated to me a good illustration of this fact; on asking some farmers in Virginia how it was that all

their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes), which coloured

their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the





"crackers" (i.e. Virginia squatters) added, "we select the black members of a litter for raising, as they

alone have a good chance of living." Hairless dogs have imperfect teeth; long-haired and coarse-haired

animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin

between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.

Hence ifman goeson selecting, and thus augmenting, any peculiarity, hewill almost certainly modify

unintentionallyotherparts of the structure, owing to the mysterious laws of correlation.

The results of the various, unknown, or but dimly understood laws of variation are infinitely complex and

diversified. It is well worth while carefully to study the several treatises on some of our old cultivated

plants, as on the hyacinth, potato, even the dahlia, &c.; and it is really surprising to note the endless

points of structure and constitution inwhich the varieties and sub-varieties differ slightly fromeach other.

The whole organisation seems to have become plastic, and departs in a slight degree from that of the

parental type.

Any variation which is not inherited is unimportant for us. But the number and diversity of inheritable

deviations of structure, both those of slight and those of considerable physiological importance, are

endless. Dr. Prosper Lucas's treatise, in two large volumes, is the fullest and the best on this subject. No breeder doubts how strong is the tendency to inheritance; that like produces like is his fundamental belief:

doubts have been thrown on this principle only by theoretical writers. When any deviation of structure

often appears, and we see it in the father and child, we cannot tell whether it may not be due to the same

cause having acted on both; but when amongst individuals, apparently exposed to the same conditions,

any very rare deviation, due to some extraordinary combination of circumstances, appears in the

parent—say, once amongst several million individuals—and it reappears in the child, the mere doctrine of

chances almost compels us to attribute its reappearance to inheritance. Every one must have heard of

cases of albinism, prickly skin, hairy bodies, &c., appearing in several members of the same family. If

strange and rare deviations of structure are really inherited, less strange and commoner deviations may be

freely admitted to be inheritable. Perhaps the correct way of viewing the whole subject would be, to look

at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly.

The laws governing inheritance are for the most part unknown. No one can say why the same peculiarity

in different individualsof the same species, or in different species, is sometimes inherited and sometimes

not so; why the child often reverts in certain characters to its grandfather or grandmother or more remote

ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more

commonly but not exclusively to the like sex. It is a fact of some importance to us, that peculiarities

appearing in the males of our domestic breeds are often transmitted, either exclusively or in a much

greater degree, to the males alone. A much more important rule, which I think may be trusted, is that, at

whatever period of life a peculiarity first appears, it tends to reappear in the offspring at a corresponding

age, though sometimes earlier. Inmany cases this could not be otherwise; thus the inherited peculiaritiesin the horns of cattle could appear only in the offspring when nearly mature; peculiarities in the silkworm

are known to appear at the corresponding caterpillar or cocoon stage. But hereditary diseases and some

other facts make me believe that the rule has a wider extension, and that, when there is no apparent

reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the

offspring at the same period at which it first appeared in the parent. I believe this rule to be of the highest

importance in explaining the laws of embryology. These remarks are of course con fined to the first

appearance of the peculiarity, and not to the primary cause which may have acted on the ovules or on

the male element; in nearly the same manner as the increased length of the horns in the offspring from a

short-horned cow by a long-horned bull, though appearing late in life, is clearly due to the male element.

Having alluded to the subject of reversion, I may here refer to a statement often made by

naturalists—namely, that our domestic varieties, when run wild, gradually but invariably revert in

character to their aboriginal stocks. Hence it has been argued that no deductions can be drawn from





domestic races to species in a state of nature. I have in vain endeavoured to discover on what decisive

facts the above statement has so often and so boldly been made. There would be great difficulty in

proving its truth: we may safely conclude that very many of the most strongly marked domestic varieties

could not possibly live in a wild state. In many cases we do not know what the aboriginal stock was, and

so could not tell whether or not nearly perfect reversion had ensued. It would be necessary, in order to

prevent the effects of intercrossing, that only a single variety should have been turned loose in its new

home. Nevertheless, as our varieties certainly do occasionally revert in some of their characters toancestral forms, it seems to me not improbable that if we could succeed in naturalising, or were to

cultivate, during many generations, the several races, for instance, of the cabbage, in very poor soil (in

which case, however, some effect would have to be attributed to thedefinite action of the poor soil), that

they would, to a large extent, or even wholly, revert to the wild aboriginal stock. Whether or not the

experiment would succeed, is not of great importance for our line of argument; for by the experiment

itself the conditions of life are changed. If it could be shown that our domestic varieties manifested a

strong tendency to reversion,—that is, to lose their acquired characters, whilst kept under the same

conditions, and whilst kept in a considerable body, so that free intercrossing might check, by blending

together, any slight deviations in their structure, in such case, I grant that we could deduce nothing from

domestic varieties in regard to species. But there is not a shadow of evidence in favour of this view: toassert that we could not breed our cart and race-horses, long and short-horned cattle, and poultry of

various breeds, and esculent vegetables, for an unlimited number of generations, would be opposed to all

experience.

Character of Domestic Varieties; difficulty of distinguishing between Varieties and Species;

origin of Domestic Varieties from one or more Species.

When we look to the hereditary varieties or races of our domestic animals and plants, and compare

them with closely allied species, we generally perceive in each domestic race, as already remarked, less

uniformity of character than in true species. Domestic races often have a somewhat monstrous character; by which I mean, that, although differing from each other, and from other species of the same genus, in

several trifling respects, they often differ in an extreme degree in some one part, both when compared

one with another, and more especially when compared with the species under nature to which they are

nearest allied. With these exceptions (and with that of the perfect fertility of varieties when crossed,—a

subject hereafter to be discussed), domestic races of the same species differ from each other in the same

manner as do the closely-allied species of the same genus in a state of nature, but the differences in most

cases are less in degree. This must be admitted as true, for the domestic races of many animals and plants

have been ranked by some competent judges as the descendants of aboriginally distinct species, and by

other competent judges as mere varieties. If any well-marked distinction existed between a domestic race

and a species, this source of doubt would not so perpetually recur. It has often been stated that domesticraces do not differ from each other in character of generic value. It can be shown that this statement is

not correct; but naturalists differ much in determining what characters are of generic value; all such

valuations being at present empirical. When it is explained how genera originate under nature, it will be

seen that we have no right to expect often to find a generic amount of difference in our domesticated

races.

In attempting to estimate the amount of structural difference between allied domestic races, we are soon

involved in doubt, from not knowing whether they are descended from one or several parent species.

This point, if it could be cleared up, would be interesting; if, for instance, it could be shown that the

greyhound, bloodhound, terrier, spaniel, andbull-dog, whichwe all knowpropagate their kind truly,

were the offspring of any single species, then such facts would have great weight in making us doubt

about the immutability of the manyclosely allied natural species—for instance,of the many

foxes—inhabiting different quarters of the world. I do not believe, as we shall presently see, that the





whole amount of difference between the several breeds of the dog has been produced under

domestication; I believe that a small part of the difference is due to their being descended from distinct

species. In the case of strongly marked races of some other domesticated species, there is presumptive

or even strong evidence, that all are descended from a single wild stock.

It has often been assumed that man has chosen for domestication animals and plants having an

extraordinary inherent tendency to vary, and likewise to withstand diverse climates. I do not dispute thatthese capacities have added largely to the value of most of our domesticated productions: but how could

a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generations,

and whether it would endure other climates? Has the little variability of the ass and goose, or the small

power of endurance of warmth by the reindeer, or of cold by the common camel, prevented their

domestication? I cannot doubt that if other animals and plants, equal in number to our domesticated

productions, and belonging to equally diverse classes and countries, were taken from a state of nature,

and could be made to breed for an equal number of generations under domestication, they would on an

average vary as largely as the parent species of our existing domesticated productions have varied.

In the case of most of our anciently domesticated animals and plants, it is not possible to come to anydefinite conclusion, whether they are descended from one or several wild species.The argumentmainly

relied on by those who believe in the multiple origin of our domestic animals is, that we find in the most

ancient times, on the monuments of Egypt, and in the lake-habitations ofSwitzerland, much diversity in

the breeds; and that some of these ancient breeds closely resemble, or are even identical with, those still

existing.But this only throws far backwards the historyof civilisation, and shows that animalswere

domesticated at much earlier period than has hitherto been supposed.The lake-inhabitants of Switzerland

cultivated several kinds of wheat and barley, the pea, the poppy for oil, and flax; and they possessed

several domesticated animals. They also carried on commerce with other nations.All this clearly shows,

as Heer has remarked, that they had at this early age progressed considerably in civilisation; and this

again implies a long continued previous periodof less advanced civilisation, duringwhich the

domesticated animals, kept by different tribes in different districts,might have varied andgiven rise todistinct races. Since the discovery of flint tools in the superficial formations of many parts of the world, all

geologists believe that barbarian man existed at an enormously remote period; and we know that at the

present day there is hardly a tribe so barbarous, as not to have domesticated at least the dog.

The origin of most of our domestic animals will probably for ever remain vague. But I may here state,

that, looking to the domestic dogs of the whole world, I have, after a laborious collection of all known

facts, come to the conclusion that several wild species of Canidæ have been tamed, and that their blood,

in some cases mingled together, flows in the veins of our domestic breeds. In regard to sheep and goats I

can form no decided opinion. From facts communicated to me by Mr. Blyth, on the habits, voice,

constitution, and structure of the humped Indian cattle, it is almost certain that they are descended from adifferent aboriginal stock from our European cattle; and some competent judges believe that these latter

have had two or three wild progenitors,—whether or not these deserve to be called species. This

conclusion, as well as that of the specific distinction between the humped and common cattle, may,

indeed, be looked upon as established by the admirable researches of Professor Rütimeyer. With respect

to horses, from reasons which I cannot here give, I am doubtfully inclined to believe, in opposition to

several authors, that all the races belong to the same species. Having kept nearly all the English breeds of

the fowl alive, having bred and crossed them, and examined their skeletons, it appears to me almost

certain that all are the descendants of the wild Indian fowl, Gallus bankiva; and this is the conclusion of

Mr. Blyth, and of others who have studied this bird in India. In regard to ducks and rabbits, some breeds

of which differ much from each other, the evidence is clear that they are all descended from the common

wild duck and rabbit.

The doctrine of the origin of our several domestic races from several aboriginal stocks, has been carried





to an absurd extreme by some authors. They believe that every race which breeds true, let the distinctive

characters be ever so slight, has had its wild prototype. At this rate there must have existed at least a

score of species of wild cattle, as many sheep, and several goats, in Europe alone, and several even

within Great Britain. One author believes that there formerly existed eleven wild species of sheep peculiar

to Great Britain! When we bear in mind that Britain has now not one peculiar mammal, and France but

few distinct from those of Germany, and so with Hungary, Spain, &c., but that each of these kingdoms

possesses several peculiar breeds of cattle, sheep, &c., we must admit that many domestic breeds musthave originated in Europe; for whence otherwise could they have been derived? So it is in India. Even in

the case of the breeds of the domestic dog throughout the world, which I admit are descended from

several wild species, it cannot be doubted that there has been an immense amount of inherited variation;

for who will believe that animals closely resembling the Italian greyhound, the bloodhound, the bull-dog,

pug-dog, or Blenheim spaniel, &c.—so unlike all wild Canidæ—ever existed in a state of nature? It has

often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal

species; but by crossing we can only get forms in some degree intermediate between their parents; and if

we account for our several domestic races by this process, we must admit the former existence of the

most extreme forms, as the Italian greyhound, bloodhound, bull-dog, &c., in the wild state. Moreover,

the possibility of making distinct races by crossing has been greatly exaggerated. Many cases are onrecord, showing that a race may be modified by occasional crosses, if aided by the careful selection of

the individuals which present the desired character; but to obtain a race intermediate between two quite

distinct races, would be very difficult. Sir J. Sebright expressly experimented with this object and failed.

The offspring from the first cross between two pure breeds is tolerably and sometimes (as I have found

with pigeons) quiteuniform in character, and everything seems simple enough; but when these mongrels

are crossed one with another for several generations, hardly two of them are alike, and then the difficulty

of the task becomes manifest.

Breeds of the Domestic Pigeon, their Differences and Origin.

Believing that it is always best to study some special group, I have, after deliberation, taken up domestic

pigeons. I have kept every breed which I could purchase or obtain, and have been most kindly favoured

with skins from several quarters of the world, more especially by the Hon. W. Elliot from India, and by

the Hon. C. Murray from Persia. Many treatises in different languages have been published on pigeons,

and some of them are very important, as being of considerable antiquity. I have associated with several

eminent fanciers, and have been permitted to join two of the London Pigeon Clubs. The diversity of the

breeds is something astonishing. Compare the English carrier and the short-faced tumbler, and see the

wonderful difference in their beaks, entailing corresponding differences in their skulls. The carrier,more

especially the male bird, is also remarkable from the wonderful development of the carunculated skin

about the head; and this is accompanied by greatly elongated eyelids, very large external orifices to thenostrils, and a wide gape of mouth. The short-faced tumbler has a beak in outline almost like that of a

finch; and the common tumbler has the singular inherited habit of flying at a great height in a compact

flock, and tumbling in the air head over heels. The runt is a bird of great size, with long massive beak and

large feet; some of the sub-breeds of runts have very long necks, others very long wings and tails, others

singularly short tails. The barb is allied to the carrier, but, instead of a long beak has a very short and

broad one. The pouter has a much elongated body, wings, and legs; and its enormously developed crop,

which it glories in inflating, may well excite astonishment and even laughter. The turbit has a short and

conical beak, with a line of reversed feathers down the breast; and it has the habit of continually

expanding slightly, the upper part of the esophagus. The Jacobin has the feathers so much reversed along

the back of the neck that they form a hood; and it has, proportionally to its size, elongated wing and tail

feathers. The trumpeter and laugher, as their names express, utter a very different coo from the other

breeds. The fantail has thirty or even forty tail-feathers, instead of twelve or fourteen—the normal number

in all the members of the great pigeon family: these feathers are kept expanded, and are carried so erect,





that in good birds the head and tail touch: the oil-gland is quite aborted. Several other less distinct breeds

might be specified.

In the skeletons of the several breeds, the development of the bones of the face in length and breadth

and curvature differs enormously. The shape, as well as the breadth and length of the ramus of the lower

jaw, varies in a highly remarkable manner. The caudal and sacral vertebræ vary in number; as does the

number of the ribs, together with their relative breadth and the presence of processes. The size and shapeof the apertures in the sternum are highly variable; so is the degree of divergence and relative size of the

two arms of the furcula. The proportional width of the gape of mouth, the proportional length of the

eyelids, of the orifice of the nostrils, of the tongue (not always in strict correlation with the length of beak),

the size of the crop and of the upper part of the æsophagus; the development and abortion of the

oil-gland; the number of the primary wing and caudal feathers; the relative length of the wing and tail to

each other and to the body; the relative length of the leg and foot; the number of scutellæ on the toes, the

development of skin between the toes, are all points of structure which are variable. The period at which

the perfect plumage is acquired varies, as does the state of the down with which the nestling birds are

clothed when hatched. The shape and size of the eggs vary. The manner of flight, and in some breeds the

voice and disposition, differ remarkably. Lastly, in certain breeds, the males and females have come todiffer in a slight degree from each other.

Altogether at least a score of pigeons might be chosen, which, if shown to an ornithologist, and he were

told that they were wild birds, would certainly be ranked by him as well-defined species. Moreover, I do

not believe that any ornithologist would in this case place the English carrier, the short-faced tumbler, the

runt, the barb, pouter, and fantail in the same genus; more especially as in each of these breeds several

truly-inherited sub-breeds, or species, as he would call them, could be-shown him.

Great as are the differences between the breeds of the pigeon, I am fully convinced that the common

opinion of naturalists is correct, namely, that all are descended from the rock-pigeon (Columba livia),

including under this term several geographical races or sub-species, which differ from each other in themost trifling respects. As several of the reasons which have led me to this belief are in some degree

applicable in other cases, I will here briefly give them. If the several breeds are not varieties, and have not

proceeded from the rock-pigeon, they must have descended from at least seven or eight aboriginal

stocks; for it is impossible to make the present domestic breeds by the crossing of any lesser number:

how, for instance, could a pouter be produced by crossing two breeds unless one of the parent-stocks

possessed the characteristic enormous crop? The supposed aboriginal stocks must all have been

rock-pigeons, that is, they did not breed or willingly perch on trees. But besides C. livia, with its

geographical sub-species, only two or three other species of rock-pigeons are known; and these have

not any of the characters of the domestic breeds. Hence the supposed aboriginal stocks must either still

exist in the countries where they were originally domesticated, and yet be unknown to ornithologists; andthis, considering their size, habits, and remarkable characters, seems improbable; or they must have

become extinct in the wild state. But birds breeding on precipices, and good fliers, are unlikely to be

exterminated; and the common rock-pigeon, which has the same habits with the domestic breeds, has not

been exterminated even on several of the smaller British islets, or on the shores of the Mediterranean.

Hence the supposed extermination of so many species having similar habits with the rock-pigeon seems a

very rash assumption. Moreover, the several above-named domesticated breeds have been transported

to all parts of the world, and, therefore, some of them must have been carried back again into their native

country; but not one has become wild or feral, though the dovecot-pigeon, which is the rock-pigeon in a

very slightly altered state, has become feral in several places. Again, all recent experience shows that it is

difficult to get wild animals to breed freely under domestication; yet on the hypothesis of the multiple

origin of our pigeons, it must be assumed that at least seven or eight species were so thoroughly

domesticated in ancient times byhalf-civilisedman, as to bequite prolific under confinement.





An argument of great weight, and applicable in several other cases, is, that the above-specified breeds,

thoughagreeing generallywith the wild rock-pigeon in constitution, habits, voice, colouring, and inmost

parts of their structure, yet are certainly highly abnormal in other parts; we may look in vain through the

whole great family of Columbidæ for a beak like that of the English carrier, or that of the short-faced

tumbler, or barb; for reversed feathers like those of the Jacobin; for a crop like that of the pouter; for

tail-feathers like those of the fantail. Hence it must be assumed not only that half-civilised man succeeded

in thoroughly domesticating several species, but that he intentionallyor by chance picked outextraordinarily abnormal species; and further, that these very species have since all become extinct or

unknown. Somany strange contigencies are improbable in the highest degree.

Some facts in regard to the colouring of pigeons well deserve consideration. The rock-pigeon is of a

slatyblue, with white loins; but the Indian sub-species, C. intermedia of Strickland, has this part bluish.

The tail has a terminal dark bar, with the outer feathers externally edged at the base with white. The

wings have two black bars. Some semi-domestic breeds, and some truly wild breeds, have, besides the

two black bars, the wings chequered with black. These several marks do not occur together in any other

species of the whole family. Now, in every one of the domestic breeds, taking thoroughly well-bred

birds, all the above marks, even to the white edging of the outer tail-feathers, sometimes concur perfectlydeveloped. Moreover, when birds belonging to two or more distinct breeds are crossed, none of which

are blue or have any of the above-specified marks, the mongrel offspring are very apt suddenly to

acquire these characters. To give one instance out of several which I have observed:—I crossed some

white fantails, which breed very true, with some black barbs—and it so happens that blue varieties of

barbs are so rare that I never heard of an instance in England; and the mongrels were black, brown, and

mottled. I also crossed a barb with a spot, which is a white bird with a red tail and red spot on the

forehead, and which notoriously breeds very true; the mongrels were dusky and mottled. I then crossed

one of the mongrel barb-fantails with a mongrel barb-spot, and they produced a bird of as beautiful a

blue colour, with the white loins, double black wing-bar, and barred and white-edged tail-feathers, as

any wild-rock pigeon! We can understand these facts, on the well-known principle of reversion to

ancestral characters, if all the domestic breeds are descended from the rock-pigeon. But if we deny this,wemustmake one of the two following highly improbable suppositions. Either, first, that all the several

imaginedaboriginal stocks were coloured and marked like the rock-pigeon, although no other existing

species is thus coloured and marked, so that in each separate breed there might be a tendency to revert

to the very same colours and markings. Or, secondly, that each breed, even the purest, has within a

dozen, or at most within a score, of generations, been crossed by the rock-pigeon: I say within a dozen

or twenty generations, for no instance is known of crossed descendants reverting to an ancestor of

foreign blood, removed by a greater number of generations. In a breed which has been crossed only

once, the tendency to revert to any character derived from such a cross will naturally become less and

less, as in each succeeding generation there will be less of the foreign blood; but when there has been no

cross, and there is a tendency in the breed to revert to a character which was lost during some former generation, this tendency, for all that we can see to the contrary, may be transmitted undiminished for an

indefinite number of generations. These twodistinct cases of reversion are often confounded together by

those who have written on inheritance.

Lastly, the hybrids or mongrels from between all the breeds of the pigeon are perfectly fertile, as I can

state from my own observations, purposely made, on the most distinct breeds. Now, hardly any cases

have been ascertainedwith certainty of hybrids from two quitedistinct species of animals beingperfectly

fertile. Some authors believe that long-continued domestication eliminates this strong tendency to sterility

in species. From the history of the dog, and of some other domestic animals, this conclusion is probably

quite correct, if applied to species closely related to each other. But to extend it so far as to suppose that

species, aboriginally as distinct as carriers, tumblers, pouters, and fantails now are, should yieldoffspring

perfectly fertileinter se , would be rash in the extreme.





From these several reasons, namely,—the improbability ofmanhaving formerly made seven or eight

supposed species of pigeons to breed freely under domestication;—these supposed species being quite

unknown in a wild state, and their not having become anywhere feral;—these species presenting certain

very abnormal characters, as compared with all other Columbidæ, though so like the rock-pigeon in most

respects;—the occasional re-appearance of the blue colour and various black marks in all the breeds,

bothwhen kept pure andwhen crossed;—and lastly, the mongrel offspring being perfectly fertile;—from

these several reasons taken together, we may safely conclude that all our domestic breeds are descendedfrom the rock-pigeon or Columba livia with its geographical subspecies.

In favour of this view, I may add, firstly, that the wild C. livia has been found capable of domestication in

Europe and in India; and that it agrees in habits and in a great number of points of structure with all the

domestic breeds. Secondly, that, although an English carrier or a short-faced tumbler differs immensely in

certain characters from the rock-pigeon, yet that, by comparing the several sub-breeds of these two

races, more especially those brought from distant countries, we can make, between them and the

rock-pigeon, an almost perfect series; so we can in some other cases, but not with all the breeds. Thirdly,

those characters which are mainly distinctive of each breed are in each eminently variable, for instance the

wattle and length of beak of the carrier, the shortness of that of the tumbler, and the number of tail-feathers in the fantail; and the explanation of this fact will be obvious when we treat of Selection.

Fourthly, pigeons have been watched and tended with the utmost care, and loved by many people. They

have been domesticated for thousands of years in several quarters of the world; the earliest known

record of pigeons is in the fifth Ægyptian dynasty, about 3000 B.C., as was pointed out to me by

Professor Lepsius; but Mr. Birch informs me that pigeons are given in a bill of fare in the previous

dynasty. In the time of the Romans, as we hear from Pliny, immense prices were given for pigeons; "nay,

they are come to this pass, that they can reckon up their pedigree and race." Pigeons were much valued

by Akber Khan in India, about the year 1600; never less than 20,000 pigeons were taken with the court.

"The monarchs of Iran and Turan sent him some very rare birds;" and, continues the courtly historian,

"His Majesty by crossing the breeds, which method was never practised before, has improved them

astonishingly." About this same period the Dutch were as eager about pigeons as were the old Romans.The paramount importance of these considerations in explaining the immense amount of variationwhich

pigeons have undergone, will likewise be obvious when we treat of Selection. We shall then, also, see

how it is that the several breeds so often have a somewhat monstrous character. It is also a most

favourable circumstance for the production of distinct breeds, that male and female pigeons can be easily

mated for life; and thus different breeds can be kept together in the same aviary.

I have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because

when I first kept pigeons and watched the several kinds, well knowing how truly they breed, I felt fully as

much difficulty in believing that since they had been domesticated they had all proceeded from a common

parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches,or other groups of birds, in nature. One circumstance has struck me much; namely, that nearly all the

breeders of the various domestic animals and the cultivators of plants, with whom I have conversed, or

whose treatises I have read, are firmly convinced that the several breeds to which each has attended, are

descended from so many aboriginally distinct species. Ask, as I have asked, a celebrated raiser of

Hereford cattle, whether his cattle might not have descended from Long-horns, or both from a common

parent-stock, and he will laugh you to scorn. I have never met a pigeon, or poultry, or duck, or rabbit

fancier, who was not fully convinced that each main breed was descended from a distinct species. Van

Mons, in his treatise on pears and apples, shows how utterly he disbelieves that the several sorts, for

instance a Ribston-pippin or Codlin-apple, could ever have proceeded from the seeds of the same tree.

Innumerable other examples could be given.The explanation, I think, is simple: from long-continued

study they are strongly impressed with the differences between the several races; and though they well

know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they

ignore all generalarguments, and refuse to sum up in their minds slight differences accumulated during





many successive generations. May not those naturalists who, knowing far less of the laws of inheritance

than does the breeder, and knowing no more than he does of the intermediate links in the long lines of

descent, yet admit that many of our domestic races are descended from the same parents—may they not

learn a lesson of caution, when they deride the idea of species in a state of nature being lineal

descendants of other species?

Principles of Selection anciently followed, and their Effects.

Let us now briefly consider the steps by which domestic races have been produced, either from one or

from several allied species. Some effect may be attributed to the direct and definite action of the external

conditions of life, and some to habit; but he would be a bold man who would account by such agencies

for the differences between a dray- and race-horse, a greyhound and bloodhound, a carrier and tumbler

pigeon. One of the most remarkable features in our domesticated races is that we see in them adaptation,

not indeed to the animal's or plant's own good, but to man's use or fancy. Some variations useful to him

have probably arisen suddenly, or by one step; many botanists, for instance, believe that the fuller's

teasel, with its hooks, which cannot be rivalled by any mechanical contrivance, is only a variety of thewild Dipsacus; and this amount of change may have suddenly arisen in a seedling. So it has probably

been with the turnspit dog; and this is known to have been the case with the ancon sheep. But when we

compare the dray-horse and race-horse, the dromedary and camel, the various breeds of sheep fitted

either for cultivated land or mountain pasture, with the wool of one breed good for one purpose, and that

of another breed for another purpose; when we compare the many breeds of dogs, each good for man in

different ways; when we compare the game-cock, so pertinacious in battle, with other breeds so little

quarrelsome, with "everlasting layers" which never desire to sit, and with the bantam so small and elegant;

when we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most

useful to man at different seasons and for different purposes, or so beautiful in his eyes, we must, I think,

look further than to mere variability. We cannot suppose that all the breeds were suddenly produced as

perfect and as useful as we now see them; indeed, in many cases, we know that this has not been their history. The key isman's power of accumulative selection: nature gives successive variations; man adds

them up in certain directions useful to him. In this sense he may be said to have made for himself useful

breeds.

The great power of this principle of selection is not hypothetical. It is certain that several of our eminent

breeders have, even within a single lifetime, modified to a large extent their breeds of cattle and sheep. In

order fully to realise what they have done, it is almost necessary to read several of the many treatises

devoted to this subject, and to inspect the animals. Breeders habitually speak of an animal's organisation

as something plastic, which they can model almost as they please. If I had space I could quote numerous

passages to this effect from highly competent authorities. Youatt, who wasprobably better acquaintedwith the works of agriculturists than almost any other individual, and who was himself a very good judge

of animals, speaks of the principle of selection as "that which enables the agriculturist, not only to modify

the character of his flock, but to change it altogether. It is the magician's wand, by means of which he

may summon into life whatever form and mould he pleases." Lord Somerville, speaking of what breeders

have done for sheep, says:—"It would seem as if they had chalked out upon a wall a form perfect in

itself, and then had given it existence." In Saxony the importance of the principle of selection in regard to

merino sheep is so fully recognised, that men follow it as a trade: the sheep are placed on a table and are

studied, like a picture by a connoisseur; this is done three times at intervals of months, and the sheep are

each time marked and classed, so that the very best may ultimately be selected for breeding.

What English breeders have actually effected is proved by the enormous prices given for animals with a

good pedigree; and these have been exported to almost every quarter of the world. The improvement is

by no means generally due to crossing different breeds; all the best breeders are strongly opposed to this





practice, except sometimes amongst closely allied sub-breeds. And when a cross has been made, the

closest selection is far more indispensable even than in ordinary cases. If selection consisted merely in

separating some very distinct variety, and breeding from it, the principle would be so obvious as hardly to

be worth notice; but its importance consists in the great effect produced by the accumulation in one

direction, during successivegenerations, of differences absolutely inappreciable by an uneducated

eye—differences which I for one have vainly attempted to appreciate. Not one man in a thousand has

accuracy of eye and judgment sufficient to become an eminent breeder. If gifted with these qualities, andhe studies his subject for years, and devotes his lifetime to it with indomitable perseverance, he will

succeed, and may make great improvements; if he wants any of these qualities, he will assuredly fail. Few

would readily believe in the natural capacity and years of practice requisite to become even a skilful

pigeon-fancier.

The same principles are followed by horticulturists; but the variations are here often more abrupt. No

one supposes that our choicest productions have been produced by a single variation from the aboriginal

stock. We have proofs that this has not been so in several cases in which exact records have been kept;

thus, to give a very trifling instance, the steadily-increasing size of the common gooseberry may be

quoted. We see an astonishing improvement in many florists' flowers, when the flowers of the present dayare compared with drawings made only twenty or thirty years ago. When a race of plants is once pretty

well established, the seed-raisers do not pick out the best plants, but merely go over their seed-beds, and

pull up the "rogues," as they call the plants that deviate from the proper standard. With animals this kind

of selection is, in fact, likewise followed; for hardly any one is so careless as to breed from his worst

animals.

In regard to plants, there is another means of observing the accumulated effects of selection—namely, by

comparing the diversity of flowers in the different varieties of the same species in the flower-garden; the

diversity of leaves, pods, or tubers, or whatever part is valued, in the kitchen garden, in comparison with

the flowers of the same varieties; and the diversity of fruit of the same species in the orchard, in

comparison with the leaves and flowers of the same set of varieties. See how different the leaves of thecabbage are, and how extremely alike the flowers; how unlike the flowers of the heartsease are, and how

alike the leaves; how much the fruit of the different kinds of gooseberries differ in size, colour, shape, and

hairiness, and yet the flowers present very slight differences. It is not that the varieties which differ largely

in some one point do not differ at all in other points; this is hardly ever,—I speak after careful

observation,—perhaps never, the case. The law of correlated variation, the importance of which should

never be overlooked, will ensure some differences; but, as a general rule, it cannot be doubted that the

continued selection of slight variations, either in the leaves, the flowers, or the fruit, will produce races

differing fromeach other chiefly in these characters.

It may be objected that the principle of selection has been reduced to methodical practice for scarcelymore than three-quarters of a century; it has certainly been more attended to of late years, and many

treatises have been published on the subject; and the result has been, in a corresponding degree, rapid

and important. But it is very far from true that the principle is a modern discovery. I could give several

references to works of high antiquity, in which the full importance of the principle is acknowledged. In

rude and barbarous periods of English history choice animals were often imported, and laws were passed

to prevent their exportation: the destruction of horses under a certain size was ordered, and this may be

compared to the "roguing" of plants bynurserymen.The principle of selection I find distinctly given in an

ancient Chinese encyclopædia. Explicit rules are laid down by some of the Roman classical writers. From

passages in Genesis, it is clear that the colour of domestic animals was at that early period attended to.

Savages now sometimes cross their dogs with wild canine animals, to improve the breed, and they

formerly did so, as is attested by passages in Pliny. The savages in South Africa match their draught cattle

by colour, as do some of the Esquimaux their teams of dogs. Livingstone states that good domestic

breeds are highly valued by the negroes in the interior of Africa who have not associated with Europeans.





Some of these facts do not show actual selection, but they show that the breeding of domestic animals

was carefully attended to in ancient times, and is now attended to by the lowest savages. It would,

indeed, have been a strange fact, had attention not been paid to breeding, for the inheritance of good and

bad qualities is so obvious.

Unconscious Selection.

At the present time, eminent breeders try by methodical selection, with a distinct object in view, to make

a new strain or sub-breed, superior to anything of the kind in the country. But, for our purpose, a form of

Selection, which may be called Unconscious, and which results from every one trying to possess and

breed from the best individual animals, is more important. Thus, a man who intends keeping pointers

naturally tries to get as good dogs as he can, and afterwards breeds from his own best dogs, but he has

no wish or expectation of permanently altering the breed. Nevertheless we may infer that this process,

continued during centuries, would improve and modify any breed, in the same way as Bakewell, Collins,

&c., by this very same process, only carried on more methodically, did greatly modify, even during their

lifetimes, the forms and qualities of their cattle. Slow and insensible changes of this kind can never berecognised unless actual measurements or careful drawings of the breeds in question have been made

long ago, which may serve for comparison. In some cases, however, unchanged, or but little changed

individuals of the same breed exist in less civilised districts, where the breed has been less improved.

There is reason to believe that King Charles's spaniel has been unconsciously modified to a large extent

since the time of that monarch. Some highly competent authorities are convinced that the setter is directly

derived from the spaniel, and has probably been slowly altered from it. It is known that the English

pointer has been greatly changed within the last century, and in this case the change has, it is believed,

been chiefly effected by crosses with the foxhound; but what concerns us is, that the change has been

effected unconsciously and gradually, and yet so effectually, that, though theold Spanish pointer certainly

came from Spain, Mr. Borrow has not seen, as I am informed by him, any native dog in Spain like our

pointer.

By a similar process of selection, and by careful training, English racehorses have come to surpass in

fleetness and size the parent Arabs, so that the latter, by the regulations for the Goodwood Races, are

favoured in the weights which they carry. Lord Spencer and others have shown how the cattle of England

have increased in weight and in early maturity, compared with the stock formerly kept in this country. By

comparing the accounts given in various old treatises of the former and present state of carrier and

tumbler pigeons in Britain, India, and Persia, we can trace the stages through which they have insensibly

passed, and come to differ so greatly from the rock-pigeon.

Youatt gives an excellent illustration of the effects of a course of selection, which may be considered asunconscious, in so far that the breeders could never have expected, or even wished, to produce the result

which ensued—namely, the production of two distinct strains. The two flocks of Leicester sheep kept by

Mr. Buckley and Mr. Burgess, as Mr. Youatt remarks, "have been purely bred from the original stock of

Mr. Bakewell for upwards of fifty years. There is not a suspicion existing in the mind of any one at all

acquainted with the subject, that the owner of either of them has deviated in any one instance from the

pure blood of Mr. Bakewell's flock, and yet the difference between the sheep possessed by these two

gentlemen is so great that they have the appearance of being quite different varieties."

If there exist savages so barbarous as never to think of the inherited character of the offspring of their

domestic animals, yet any one animal particularly useful to them, for any special purpose, would be

carefully preserved during famines and other accidents, to which savages are so liable, and such choice

animals would thus generally leave more offspring than the inferior ones; so that in this case there would

be a kind of unconscious selection going on.We see the value set on animals even by the barbarians of





Tierra del Fuego, by their killing and devouring their old women, in times of dearth, as of less value than

their dogs.

In plants the same gradual process of improvement, through the occasional preservation of the best

individuals, whether or not sufficiently distinct to be ranked at their first appearance as distinct varieties,

and whether or not two or more species or races have become blended together by crossing, may plainly

be recognised in the increased size and beauty which we now see in the varieties of the heartsease, rose, pelargonium, dahlia, and other plants, when compared with the older varieties or with their parent-stocks.

No one would ever expect to get a first-rate heartsease or dahlia from the seed of a wild plant. No one

would expect to raise a first-rate melting pear from the seed of the wild pear, though he might succeed

from a poor seedling growing wild, if it had come from a garden-stock. The pear though cultivated in

classical times, appears, from Pliny's description, to have been a fruit of very inferior quality. I have seen

great surprise expressed in horticulturalworks at thewonderful skill of gardeners, in having produced

such splendid results from such poor materials; but the art has been simple, and, as far as the final result is

concerned, has been followed almost unconsciously. It has consisted in always cultivating the best known

variety, sowing its seeds, and, when a slightly better variety chanced to appear, selecting it, and so

onwards. But the gardeners of the classical period, who cultivated the best pears which they could procure, never thought what splendid fruit we should eat; though we owe our excellent fruit in some small

degree, to their having naturally chosen and preserved the best varieties they could anywhere find.

A large amount of change, thus slowly and unconsciously accumulated, explains, as I believe, the

well-known fact, that in a number of cases we cannot recognise, and therefore do not know, the wild

parent-stocks of the plants which have been longest cultivated in our flower and kitchen gardens. If it has

taken centuries or thousands of years to improve or modify most of our plants up to their present

standard of usefulness to man, we can understand how it is that neither Australia, the Cape of Good

Hope, nor any other region inhabited by quite uncivilised man, has afforded us a single plant worth

culture. It is not that these countries, so rich in species, do not by a strange chance possess the aboriginal

stocks of any useful plants, but that the native plants have not been improved by continued selection up toa standardof perfection comparablewith that acquired by the plants in countries anciently civilised.

In regard to the domestic animals kept by uncivilised man, it should not be overlooked that they almost

always have to struggle for their own food, at least during certain seasons. And in two countries very

differently circumstanced, individuals of the same species, having slightly different constitutions or

structure, would often succeed better in the one country than in the other; and thus by a process of

"natural selection," aswill hereafter bemore fully explained, two sub-breeds might be formed. This,

perhaps, partly explains why the varieties kept by savages, as has been remarked by some authors, have

more of the character of true species than the varieties kept in civilised countries.

On the view here given of the important part which selection by man has played, it becomes at once

obvious, how it is that our domestic races show adaptation in their structure or in their habits to man's

wants or fancies. We can, I think, further understand the frequently abnormal character of our domestic

races, and likewise their differences being so great in external characters, and relatively so slight in

internal parts or organs. Man can hardly select, or only with much difficulty, any deviation of structure

excepting such as is externally visible; and indeed he rarely cares for what is internal. He can never act by

selection, excepting on variations which are first given to him in some slight degree by nature. No man

would ever try to make a fantail till he saw a pigeon with a tail developed in some slight degree in an

unusual manner, or a pouter till he saw a pigeon with a crop of somewhat unusual size; and the more

abnormal or unusual any character was when it first appeared, the more likely it would be to catch his

attention. But to use such an expression as trying to make a fantail, is, I have no doubt, in most cases,

utterly incorrect. The man who first selected a pigeon with a slightly larger tail, never dreamed what the

descendants of that pigeonwould become through long-continued, partly unconscious and partly





methodical, selection. Perhaps theparent-bird of all fantails hadonly fourteen tail-feathers somewhat

expanded, like the present Java fantail, or like individuals of other and distinct breeds, in which as many

as seventeen tail-feathers have been counted. Perhaps the first pouter-pigeon did not inflate its crop much

more than the turbit now does the upper part of its æsophagus,—a habit which is disregarded by all

fanciers, as it is not one of the points of the breed.

Nor let it be thought that some great deviation of structure would be necessary to catch the fancier's eye:he perceives extremely small differences, and it is in human nature to value any novelty, however slight, in

one's own possession. Nor must the value which would formerly have been set on any slight differences

in the individuals of the same species, be judged of by the value which is now set on them, after several

breeds have fairlybeen established. It is known that with pigeons many slight variations now occasionally

appear, but these are rejected as faults or deviations from the standard of perfection in each breed. The

common goose has not given rise to any marked varieties; hence the Toulouse and the common breed,

which differ only in colour, that most fleeting of characters, have lately been exhibited as distinct at our

poultry-shows.

These views appear to explain what has sometimes been noticed—namely, that we know hardlyanything about the origin or history of any of our domestic breeds. But, in fact, a breed, like a dialect of a

language, can hardly be said to have a distinct origin. A man preserves and breeds from an individual with

some slight deviation of structure, or takes more care than usual in matching his best animals, and thus

improves them, and the improved animals slowly spread in the immediate neighbourhood.But theywill as

yet hardly have a distinct name, and from being only slightly valued, their history will have been

disregarded. When further improved by the same slow and gradual process, they will spread more

widely, and will be recognised as something distinct and valuable, and will then probably first receive a

provincial name. In semi-civilised countries, with little free communication, the spreading of a new

sub-breed would be a slow process. As soon as the points of value are once acknowledged, the

principle, as I have called it, of unconscious selection will always tend,—perhaps more at one period than

at another, as the breed rises or falls in fashion,—perhaps more in one district than in another, accordingto the state of civilisation of the inhabitants,—slowly to add to the characteristic features of the breed,

whatever they may be. But the chance will be infinitely small of any record having been preserved of such

slow, varying, and insensible changes.

Circumstances favourable to Man's Power of Selection.

I will now say a few words on the circumstances, favourable, or the reverse, to man's power of

selection. A highdegree of variability is obviously favourable, as freely giving the materials for selection to

work on; not that mere individual differences are not amply sufficient, with extreme care, to allow of theaccumulation of a large amount ofmodification in almost any desireddirection.But as variations

manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be

much increased by a large number of individuals being kept. Hence, number is of the highest importance

for success. On this principle Marshall formerly remarked, with respect to the sheep of parts of

Yorkshire, "as they generally belong to poor people, and are mostlyin small lots , they never can be

improved." On the other hand, nurserymen, from keeping large stocks of the same plant, are generally far

more successful than amateurs in raisingnew and valuable varieties. A large number of individualsof an

animal or plant can be reared only where the conditions for its propagation are favourable. When the

individuals are scanty, all will be allowed to breed, whatever their quality may be, and this will effectually

prevent selection. But probably the most important element is that the animal or plant should be so highly

valued by man, that the closest attention is paid to even the slightest deviations in its qualities or structure.

Unless such attention be paid nothing can be affected. I have seen it gravely remarked, that it was most

fortunate that the strawberry began to vary just when gardeners began to attend to this plant. No doubt





the strawberry had always varied since it was cultivated, but the slight varieties had been neglected. As

soon, however, as gardeners picked out individual plants with slightly larger, earlier, or better fruit, and

raised seedlings from them, and again picked out the best seedlings and bred from them, then (with some

aid by crossing distinct species) those many admirable varieties of the strawberry were raised which have

appeared during the last half-century.

With animals, facility in preventing crosses is an important element in the formation of new races,—atleast, in a country which is already stocked with other races. In this respect enclosure of the land plays a

part. Wandering savages or the inhabitants of open plains rarely possess more than one breed of the

same species. Pigeons can be mated for life, and this is a great convenience to the fancier, for thus many

races may be improved and kept true, though mingled in the same aviary; and this circumstance must

have largely favoured the formation of new breeds. Pigeons, I may add, can be propagated in great

numbers and at a very quick rate, and inferior birds may be freely rejected, as when killed they serve for

food. On the other hand, cats, from their nocturnal rambling habits, cannot be easily matched, and,

although so much valued by women and children, we rarely see a distinct breed long kept up; such

breeds as we do sometimes see are almost always imported from some other country. Although I do not

doubt that some domestic animals vary less than others, yet the rarity or absence of distinct breeds of thecat, the donkey, peacock, goose, &c., may be attributed in main part to selection not having been

brought into play: in cats, from the difficulty in pairing them; in donkeys, from only a few being kept by

poor people, and little attention paid to their breeding; for recently in certain parts of Spain and of the

United States this animal has been surprisingly modified and improved by careful selection: in peacocks,

from not being very easily reared and a large stock not kept: in geese, from being valuable only for two

purposes, food and feathers, and more especially from no pleasure having been felt in the display of

distinct breeds; but the goose, under the conditions to which it is exposed when domesticated, seems to

have a singularly inflexible organisation, though it has varied to a slight extent, as I have elsewhere

described.

Some authors have maintained that the amount of variation in our domestic productions is soon reached,and can never afterwards be exceeded. It would be somewhat rash to assert that the limit has been

attained in any one case; for almost all our animals and plants have been greatly improved in many ways

within a recent period; and this implies variation. It would be equally rash to assert that characters now

increased to their utmost limit, could not, after remaining fixed for many centuries, again vary under new

conditions of life. No doubt, as Mr.Wallace has remarked with much truth, a limit will be at last reached.

For instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by

the friction to be overcome, the weight of body to be carried, and the power of contraction in the

muscular fibres. But what concerns us is that the domestic varieties of the same species differ from each

other in almost every character, which man has attended to and selected, more than do the distinct

species of the same genera. Isidore Geoffroy St. Hilaire has proved this in regard to size, and so it is withcolour and probably with the length of hair. With respect to fleetness, which depends on many bodily

characters, Eclipse was far fleeter, and a dray-horse is incomparably stronger than any two natural

species belonging to the same genus. So with plants, the seeds of the different varieties of the bean or

maize probably differ more in size, than do the seeds of the distinct species in any one genus in the same

two families. The same remark holds good in regard to the fruit of the several varieties of the plum, and

still more strongly with the melon, as well as in many other analogous cases.

To sum up on the origin of our domestic races of animals and plants. Changed conditions of life are of

the highest importance in causing variability, both byacting directly on the organisation, and indirectlyby

affecting the reproductive system. It is not probable that variability is an inherent and necessary

contingent, under all circumstances. The greater or less force of inheritance and reversiondetermine

whether variations shall endure.Variability is governed bymanyunknown laws, ofwhichcorrelated

growth is probably the most important. Something, but how much we do not know, may be attributed to





the definite action of the conditions of life. Some, perhaps a great, effect may be attributed to the

increased use or disuse of parts. The final result is thus rendered infinitely complex. In some cases the

intercrossing of aboriginally distinct species appears to have played an important part in the origin of our

breeds. When several breeds have once been formed in any country, their occasional intercrossing, with

the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance

of crossing has been much exaggerated, both in regard to animals and to those plants which are

propagated by seed With plants which are temporarily propagated by cuttings, buds, &c., theimportance of crossing is immense; for the cultivator may here disregard the extreme variability both of

hybrids and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little

importance to us, for their endurance is only temporary. Over all these causes of Change, the

accumulative action of Selection,whether appliedmethodicallyand quickly, or unconsciously and slowly

but more efficiently seems to have been the predominant Power.

|Go to Contents |

Chapter X

On the Imperfection of the Geological

Record.

On the absence of intermediate varieties at the present day—On the nature of extinct intermediate

varieties; on their number—On the lapse of time, as inferred from the rate of denudation and of

deposition—On the lapse of time as estimated by years—On the poorness of our palæontological

collections—On the intermittence of geological formations—On the denudation of granitic areas—On the

absence of intermediate varieties in any one formation—On the sudden appearance of groups of

species—On their sudden appearance in the lowest known fossiliferous strata—Antiquity of thehabitable

earth.

IN the sixth chapter I enumerated the chief objections which might be justly urged against the views

maintained in this volume. Most of them have now been discussed. One, namely the distinctness of

specific forms, and their not being blended together by innumerable transitional links, is a very obvious

difficulty. I assigned reasons why such links do not commonly occur at the present day under the

circumstances apparently most favourable for their presence, namely, on an extensive and continuous

area with graduated physical conditions. I endeavoured to show, that the life of each species depends in

a more important manner on the presence of other already defined organic forms, than on climate, and,

therefore, that the really governing conditions of life do not graduate away quite insensibly like heat or

moisture. I endeavoured, also, to show that intermediate varieties, from existing in lesser numbers thanthe forms which they connect, will generally be beaten out and exterminated during the course of further

modification and improvement. The main cause, however, of innumerable intermediate links not now





occurring everywhere throughout nature, depends on the very process of natural selection, throughwhich

new varieties continually take the places of and supplant their parent-forms. But just in proportion as this

process of extermination has acted on an enormous scale, so must the number of intermediate varieties,

whichhave formerly existed, be truly enormous. Why then is not everygeological formation and every

stratum full of such intermediate links? Geology assuredly does not reveal any such finely-graduated

organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against

the theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.

In the first place, it should always be borne in mind what sort of intermediate forms must, on the theory,

have formerly existed. I have found it difficult, when looking at any two species, to avoid picturing to

myself formsdirectly intermediate between them. But this is a wholly false view; we should always look

for forms intermediate between each species and a common but unknown progenitor; and the progenitor

will generally have differed in some respects from all its modified descendants. To give a simple

illustration: the fantail and pouter pigeons are both descended from the rock-pigeon; if we possessed all

the intermediate varieties which have ever existed, we should have an extremely close series between

both and the rock-pigeon; but we should have no varieties directly intermediate between the fantail and

pouter; none, for instance, combining a tail somewhat expanded with a crop somewhat enlarged, thecharacteristic features of these two breeds. These two breeds, moreover, have become so much

modified, that, if we had no historical or indirect evidence regarding their origin, it would not have been

possible to have determined, from a mere comparison of their structure with that of the rock-pigeon, C.

livia, whether they had descended from this species or from some other allied form, such as C.ænas.

So, with natural species, if we look to forms very distinct, for instance to the horse and tapir, we have no

reason to suppose that links directly intermediate between them ever existed, but between each and an

unknown commonparent. The commonparentwill have had in itswhole organisation muchgeneral

resemblance to the tapir and to the horse; but in some points of structure may have differed considerably

from both, even perhaps more than they differ from each other. Hence, in all such cases, we should be

unable to recognise the parent-form of any two or more species, even if we closely compared thestructure of the parent with that of its modified descendants, unless at the same time we had a nearly

perfect chain of the intermediate links.

It is just possible by the theory, that one of two living forms might have descended from the other; for

instance, a horse from a tapir; and in this casedirect intermediate links will have existed between them.

But such a case would imply that one form had remained for a very long period unaltered, whilst its

descendants had undergone a vast amount of change; and the principle of competition between organism

and organism, between child and parent, will render this a very rare event; for in all cases the new and

improved forms of life tend to supplant the old and unimproved forms.

By the theory of natural selection all living species have been connected with the parent-species of each

genus, by differences not greater than we see between the natural and domestic varieties of the same

species at the present day; and these parent-species, now generally extinct, have in their turn been

similarly connected with more ancient forms; and so on backwards, always converging to the common

ancestor of each great class. So that the number of intermediate and transitional links, between all living

and extinct species, must have been inconceivably great. But assuredly, if this theory be true, such have

lived upon the earth.

On the Lapse of Time, as inferred from the rate of Deposition and extent of Denudation.

Independently of our not finding fossil remains of such infinitely numerous connecting links, itmay be





objected that time cannot have sufficed for so great an amount of organic change, all changes having been

effected slowly. It is hardly possible for me to recall to the reader who is not a practical geologist, the

facts leading the mind feebly to comprehend the lapse of time. He who can read Sir Charles Lyell's grand

work on the Principles of Geology, which the future historian will recognise as having produced a

revolution in natural science, and yet does not admit how vast have been the past periods of time, may at

once close this volume. Not that it suffices to study the Principles of Geology, or to read special treatises

by different observers on separate formations, and to mark how each author attempts to give aninadequate idea of the duration of each formation, or even of each stratum. We can best gain some idea

of past time by knowing the agencies at work, and learning how deeply the surface of the land has been

denuded, and how much sediment has been deposited. As Lyell has well remarked, the extent and

thickness of our sedimentary formations are the result and the measure of the denudation which the

earth's crust has elsewhere undergone. Therefore a man should examine for himself the great piles of

superimposed strata, and watch the rivulets bringing down mud, and the waves wearing away the

sea-cliffs, in order to comprehend something about the duration of past time, the monuments of which we

see all around us.

It is good to wander along the coast, when formed of moderately hard rocks, and mark the process of degradation. The tides in most cases reach the cliffs only for a short time twice a day, and the waves eat

into them only when they are charged with sand or pebbles; for there is good evidence that pure water

effects nothing in wearing away rock. At last the base of the cliff is undermined, huge fragments fall down,

and these, remaining fixed, have to be worn away atom by atom, until after being reduced in size they can

be rolled about by the waves, and then they are more quickly ground into pebbles, sand, or mud. But

how often do we see along the bases of retreating cliffs rounded boulders, all thickly clothed by marine

productions, showing how little they are abraded and how seldom they are rolled about! Moreover, if we

follow for a few miles any line of rocky cliff, which is undergoing degradation, we find that it is only here

and there, along a short length or round a promontory, that the cliffs are at the present time suffering. The

appearance of the surface and the vegetation show that elsewhere years have elapsed since the waters

washed their base.

We have, however, recently learnt from the observations of Ramsay, in the van of many excellent

observers—of Jukes, Geikie, Croll, and others, that subaerial degradation is a much more important

agency than coast-action, or the power of the waves. The whole surface of the land is exposed to the

chemical action of the air and of the rain-water with its dissolved carbonic acid, and in colder countries to

frost; the disintegrated matter is carried down even gentle slopes during heavy rain, and to a greater

extent than might be supposed, especially in arid districts, by the wind; it is then transported by the

streams and rivers, which when rapid deepen their channels, and triturate the fragments. On a rainy day,

even in a gently undulating country, we see the effects of subaerial degradation in the muddy rills which

flow down every slope. Messrs. Ramsay and Whitaker have shown, and the observation is a moststriking one, that the great lines of escarpment in the Wealden district and those ranging across England,

which formerly were looked at as ancient seacoasts, cannot have been thus formed, for each line is

composed of one and the same formation, whilst our sea-cliffs are everywhere formed by the intersection

of various formations. This being the case, we are compelled to admit that the escarpments owe their

origin in chief part to the rocks of which they are composed having resisted subaerial denudation better

than the surrounding surface; this surface consequentlyhas been gradually lowered,with the lines of

harder rock left projecting. Nothing impresses the mind with the vast duration of time, according to our

ideas of time,more forcibly than the conviction thus gained that subaerial agencies whichapparently have

so little power, and which seem to work so slowly, have produced great results.

When thus impressed with the slow rate at which the land is worn away through subaerial and littoral

action, it is good, in order to appreciate the past duration of time, to consider, on the one hand, the

masses of rock which have been removed over many extensive areas, and on the other hand the





thickness of our sedimentary formations. I remember having beenmuch struckwhen viewing volcanic

islands, which have been worn by the waves and pared all round into perpendicular cliffs of one or two

thousand feet in height; for the gentle slope of the lava-streams, due to their formerly liquid state, showed

at a glance how far the hard, rocky beds had once extended into the open ocean. The same story is told

still more plainly by faults,—those great cracks along which the strata have been upheaved on one side,

or thrown down on the other, to the height or depth of thousands of feet; for since the crust cracked, and

it makes no great difference whether the upheaval was sudden, or, as most geologists now believe, wasslow and effected by many starts, the surface of the land has been so completely planed down that no

trace of these vast dislocations is externally visible. The Craven fault, for instance, extends for upwards of

30 miles, and along this line the vertical displacement of the strata varies from 600 to 3000 feet.

Professor Ramsay has published an account of a downthrow in Anglesea of 2300 feet; and he informs

me that he fully believes that there is one in Merionethshire of 12,000 feet; yet in these cases there is

nothing on the surface of the land to show such prodigious movements; the pile of rocks on either side of

the crack having been smoothly swept away.

On the other hand, in all parts of the world the piles of sedimentary strata are of wonderful thickness. In

the Cordillera I estimated one mass of conglomerate at ten thousand feet; and although conglomerateshave probably been accumulated at a quicker rate than finer sediments, yet from being formed of worn

and rounded pebbles, each of which bears the stamp of time, they are good to show how slowly the

mass must have been heaped together. Professor Ramsay has given me the maximum thickness, from

actual measurement inmost cases, of the successive formations indifferent parts of Great Britain; and

this is the result:—

Feet.

Palæozoic strata (not including igneous beds)……………………….…….57,154

Secondary strata ……………………………………………………..……13,190

Tertiary strata……………………………………………………………….2,240

—making altogether 72,584 feet; that is, very nearly thirteen and three-quarters British miles. Some of

the formations, which are represented in England by thin beds, are thousands of feet in thickness on the

Continent. Moreover, between each successive formation, we have, in the opinion of most geologists,

blank periods of enormous length. So that the lofty pile of sedimentary rocks in Britain gives but an

inadequate idea of the time whichhas elapsed during their accumulation. The consideration of thesevarious facts impresses the mind almost in the same manner as does the vain endeavour to grapple with

the idea of eternity.

Nevertheless this impression is partly false. Mr. Croll, in an interesting paper, remarks that we do not err

"in forming too great a conception of the length of "geological periods," but in estimating them by years.

When geologists look at large and complicated phenomena, and then at the figures representing several

million years, the two produce a totally different effect on the mind, and the figures are at once

pronounced too small. In regard to subaerial denudation, Mr. Croll shows, by calculating the known

amount of sediment annually brought down by certain rivers, relatively to their areas of drainage, that

1000 feet of solid rock, as it became gradually disintegrated, would thus be removed from the mean level

of the whole area in the course of six million years. This seems an astonishing result, and some

considerations lead to the suspicion that it may be too large, but even if halved or quartered it is still very

surprising. Few of us, however, knowwhat a million really means:Mr.Croll gives the following





illustration: take a narrow strip of paper, 83 feet 4 inches in length, and stretch it along the wall of a large

hall; then mark off at one end the tenth of an inch. This tenth of an inch will represent one hundred years,

and the entire strip a million years. But let it be borne in mind, in relation to the subject of this work, what

a hundred years implies, represented as it is by a measure utterly insignificant in a hall of the above

dimensions. Several eminent breeders, during a single lifetime, have so largelymodified someof the

higher animals which propagate their kind much more slowly than most of the lower animals, that they

have formed what well deserves to be called a new sub-breed. Few men have attended with due care toany one strain for more than half a century, so that a hundred years represents the work of two breeders

in succession. It is not to be supposed that species in a state of nature ever change so quickly as

domestic animals under the guidance of methodical selection. The comparison would be in every way

fairerwith the effects which follow from unconscious selection, that is the preservationof the most useful

or beautiful animals, with no intention ofmodifying the breed; but by this process of unconscious

selection, various breeds have been sensibly changed in the course of two or three centuries.

Species, however, probably change much more slowly, and within the same country only a few change

at the same time. This slowness follows from all the inhabitants of the same country being already so well

adapted to each other, that new places in the polity of nature do not occur until after long intervals, due tothe occurrence of physical changes of some kind, or through the immigration of new forms. Moreover

variations or individual differences of the right nature,bywhichsomeof the inhabitantsmight be better

fitted to their new places under the altered circumstances, would not always occur at once. Unfortunately

we have no means of determining, according to the standard of years, how long a period it takes to

modify a species; but to the subject of time we must return.

On the Poorness of Palæontological Collections.

Now let us turn to our richest geological museums, and what a paltry display we behold! That our collections are imperfect is admitted by every one. The remark of that admirable palæontologist, Edward

Forbes, should never be forgotten, namely, that very many fossil species are known and named from

single and often broken specimens, or from a few specimens collected on some one spot. Only a small

portion of the surface of the earth has been geologically explored, and no part with sufficient care, as the

important discoveries made every year in Europe prove. No organism wholly soft can be preserved.

Shells and bones decay and disappear when left on the bottom of the sea, where sediment is not

accumulating. We probably take a quite erroneous view, when we assume that sediment is being

deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil

remains. Throughout an enormously large proportion of the ocean, the bright blue tint of the water

bespeaks its purity. The many cases on record of a formation conformably covered, after an immenseinterval of time, by another and later formation, without the underlyingbed having suffered in the interval

any wear and tear, seem explicable only on the view of the bottom of the sea not rarely lying for ages in

an unaltered condition. The remains which do become embedded, if in sand or gravel, will, when the

beds are upraised, generally be dissolved by the percolation of rain-water charged with carbonic acid.

Some of the many kinds of animals which live on the beach between high and low water mark seem to be

rarely preserved. For instance, the several species of the Chthamalinæ (a sub-family of sessile cirripedes)

coat the rocks all over the world in infinite numbers: they are all strictly littoral, with the exception of a

single Mediterranean species, which inhabits deep water, and this has been found fossil in Sicily, whereas

not one other species has hitherto been found in any tertiary formation: yet it is known that the genus

Chthamalus existedduring the Chalkperiod. Lastly, many great deposits requiring a vast length of time

for their accumulation, are entirelydestitute of organic remains,without our being able to assign any

reason: one of the most striking instances is that of the Flysch formation, which consists of shale and

sandstone, several thousand, occasionally even six thousand feet in thickness, andextending for at least





300 miles from Vienna to Switzerland; and although this great mass has been most carefully searched, no

fossils, except a few vegetable remains, have been found.

With respect to the terrestrial productions which lived during the Secondary and Palæozoic periods, it is

superfluous to state that our evidence is fragmentary in an extreme degree. For instance, until recently not

a land-shell was known belonging to either of these vast periods, with the exception of one species

discovered by Sir C. Lyell and Dr. Dawson in the carboniferous strata of North America; but nowland-shells have been found in the lias. In regard to mammiferous remains, a glance at the historical table

published in Lyell's Manual will bring home the truth, how accidental and rare is their preservation, far

better than pages of detail. Nor is their rarity surprising, when we remember how large a proportion of

the bones of tertiary mammals have been discovered either in caves or in lacustrine deposits; and that not

a cave or true lacustrine bed is known belonging to the age of our secondary or palæozoic formations.

But the imperfection in the geological record largely results from another and more important cause than

any of the foregoing; namely, from the several formations being separated from each other by wide

intervals of time. This doctrine has been emphatically admitted bymanygeologists and palæntologists,

who, like E. Forbes, entirely disbelieve in the change of species. When we see the formations tabulated inwritten works, or when we follow them in nature, it is difficult to avoid believing that they are closely

consecutive. But we know, for instance, from Sir R. Murchison's great work on Russia, what wide gaps

there are in that country between the superimposed formations; so it is in North America, and in many

other parts of the world. The most skilful geologist, if his attention had been confined exclusively to these

large territories, would never have suspected that, during the periods which were blank and barren in his

own country, great piles of sediment, charged with new and peculiar forms of life, had elsewhere been

accumulated. And if, in each separate territory, hardly any idea can be formed of the length of time which

has elapsed between the consecutive formations, we may infer that this could nowhere be ascertained.

The frequent and great changes in the mineralogical composition of consecutive formations, generally

implying great changes in the geographyof the surrounding lands, whence the sedimentwasderived,

accord with the belief of vast intervals of time having elapsed between each formation.

We can, I think, see why the geological formations of each region are almost invariably intermittent; that

is, have not followed each other in close sequence. Scarcely any fact struck me more when examining

many hundred miles of the South American coasts, which have been upraised several hundred feet within

the recent period, than the absence of any recent deposits sufficiently extensive to last for even a short

geological period. Along the whole west coast, which is inhabited by a peculiar marine fauna, tertiary

beds are so poorly developed, that no record of several successive and peculiar marine faunas will

probably be preserved to a distant age. A little reflection will explain why, along the rising coast of the

western side of South America, no extensive formations with recent or tertiary remains can anywhere be

found, though the supply of sediment must for ages have been great, from the enormous degradation of the coast-rocks and from muddy streams entering the sea. The explanation, no doubt, is, that the littoral

and sub-littoral deposits are continually worn away, as soon as they are brought up by the slow and

gradual rising of the land within the grinding action of the coast-waves.

We may, I think, conclude that sediment must be accumulated in extremely thick, solid, or extensive

masses, in order to withstand the incessant action of the waves, when first upraised and during successive

oscillations of level as well as the subsequent subaerial degradation. Such thick andextensive

accumulations of sediment may be formed in two ways; either in profound depths of the sea, in which

case the bottom will not be inhabited by so many and such varied forms of life, as the more shallow seas;

and the mass when upraised will give an imperfect record of the organisms which existed in the

neighbourhood during the period of its accumulation. Or, sediment may be deposited to any thickness

and extent over a shallow bottom, if it continue slowly to subside. In this latter case, as long as the rate of

subsidence and the supply of sediment nearly balance each other, the sea will remain shallow and





favourable for many and varied forms, and thus a rich fossiliferous formation, thickenough, when

upraised, to resist a large amount of denudation, may be formed.

I am convinced that nearly all our ancient formations, which are throughout the greater part of their

thicknessrich in fossils , have thus been formed during subsidence. Since publishing my views on this

subject in 1845, I have watched the progress of Geology, and have been surprised to note how author

after author, in treating of this or that great formation, has come to the conclusion that it was accumulatedduring subsidence. I may add, that the only ancient tertiary formation on the west coast of South

America, which has been bulky enough to resist such degradation as it has as yet suffered, but which will

hardly last to a distant geological age, was deposited during a downward oscillation of level, and thus

gained considerable thickness.

All geological facts tell us plainly that each area has undergone numerous slow oscillations of level, and

apparently theseoscillations have affectedwide spaces. Consequently, formations rich in fossils and

sufficiently thick and extensive to resist subsequent degradation,will have been formedover wide spaces

during periods of subsidence, but only where the supply of sediment was sufficient to keep the sea

shallow and to embed and preserve the remains before they had time to decay. On the other hand, aslong as the bed of the sea remains stationary,thick deposits cannot have been accumulated in the shallow

parts, which are the most favourable to life. Still less can this have happened during the alternate periods

of elevation; or, to speak more accurately, the beds which were then accumulated will generally have

been destroyed by being upraised and brought within the limits of the coast-action.

These remarks apply chiefly to littoral and sub-littoral deposits. In the case of an extensive and shallow

sea, such as that within a large part of the Malay Archipelago, where the depth varies from 30 or 40 to

60 fathoms, a widely extended formation might be formed during a period of elevation, and yet not suffer

excessively from denudation during its slow upheaval; but the thickness of the formation could not be

great, for owing to the elevator movement it would be less than the depth in which it was formed; nor

would the deposit be much consolidated, nor be capped by overlying formations, so that it would run agood chance of being worn away by atmospheric degradation and by the action of the sea during

subsequent oscillations of level. It has, however, been suggested by Mr. Hopkins, that if one part of the

area, after rising and before being denuded, subsided, the deposit formed during the rising movement,

though not thick, might afterwards become protected by fresh accumulations, and thus be preserved for a

long period.

Mr. Hopkins also expresses his belief that sedimentary beds of considerable horizontal extent have

rarely been completely destroyed. But all geologists, excepting the few who believe that our present

metamorphic schists and plutonic rocks once formed the primordial nucleus of the globe, will admit that

these latter rocks have been stript of their covering to an enormous extent. For it is scarcely possible thatsuch rocks could have been solidified and crystallized whilst uncovered; but if the metamorphicaction

occurred at profound depths of the ocean, the former protecting mantle of rock may not have been very

thick.Admitting then that gneiss, mica-schist, granite, diorite, &c., were once necessarily covered up,

how can we account for the naked and extensive areas of such rocks in many parts of the world, except

on the belief that they have subsequently been completely denuded of all overlying strata? That such

extensive areas do exist cannot be doubted: the granitic region of Parime is described by Humboldt as

being at least nineteen times as large as Switzerland. South of the Amazon, Boué colours an area

composed of rocks of this nature as equal to that of Spain, France, Italy, part of Germany, and the

British Islands, all conjoined.This region has not been carefully explored, but from the concurrent

testimony of travellers, the granitic area is very large: thus, Von Eschwege gives a detailed section of

these rocks, stretching from Rio de Janeiro for 260 geographical miles inland in a straight line; and I

travelled for 150 miles in another direction, and saw nothing but granitic rocks. Numerous specimens,

collected along the whole coast from near Rio Janeiro to the mouth of the Plata, a distance of 1100





geographical miles, were examined by me, and they all belonged to this class. Inland, along the whole

northern bank of the Plata I saw, besides modern tertiary beds, only one small patch of slightly

metamorphosed rock, which alone could have formed a part of the original capping of the granitic series.

Turning to a well-known region, namely, to the United States and Canada, as shown in Professor H. D.

Rogers's beautiful map, I have estimated the areas by cutting out and weighing the paper, and I find that

the metamorphic (excluding "the semi-metamorphic") and granitic rocksexceed, in the proportion of 19

to 12.5, the whole of the newer Palæozoic formations. In many regions the metamorphic and graniticrocks would be found much more widely extended than they appear to be, if all the sedimentary beds

were removed which rest unconformably on them, and which could not have formed part of the original

mantle under which they were crystallized. Hence it is probable that in some parts of the world whole

formations have been completely denuded, with not a wreck left behind.

One remark is here worth a passing notice. During periods of elevation the area of the land and of the

adjoining shoal parts of the sea will be increased, and new stations will often be formed:—all

circumstances favourable, as previously explained, for the formationof new varieties and species; but

during such periods there will generally be a blank in the geological record. On the other hand, during

subsidence, the inhabited area and number of inhabitants will decrease (excepting on the shores of acontinentwhen first broken up into an archipelago), andconsequently during subsidence, though there

will be much extinction, few new varieties or species will be formed; and it is during these very periods of

subsidence, that the deposits which are richest in fossils have been accumulated.

On the Absence of Numerous Intermediate Varieties in any Single Formation.

From these several considerations, it cannot be doubted that the geological record, viewed as a whole,

is extremely imperfect; but if we confine our attention to any one formation, it becomes much more

difficult to understand why we do not therein find closely graduated varieties between the allied specieswhich lived at its commencement and at its close. Several cases are on record of the same species

presenting varieties in the upper and lower parts of the same formation; thus, Trautschold gives a number

of instances with Ammonites; and Hilgendorf has described a most curious case of ten graduated forms

of Planorbis multiformis in the successive beds of a fresh-water formation in Switzerland.Although each

formation has indisputably required a vast number of years for its deposition, several reasons can be

given why each should not commonly include a graduated series of links between the species which lived

at its commencement and close; but I cannot assign due proportional weight to the following

considerations.

Although each formation may mark a very long lapse of years, each probably is short compared with the period requisite to change one species into another. I am aware that two palæntologists, whose opinions

are worthy of much deference, namely Bronn andWoodward, have concluded that the average duration

of each formation is twice or thrice as long as the average duration of specific forms. But insuperable

difficulties, as it seems to me, prevent us from coming to any just conclusion on this head.When we see a

species first appearing in the middle of any formation, it would be rash in the extreme to infer that it had

not elsewhere previously existed. So again when we find a species disappearing before the last layers

have been deposited, it would be equally rash to suppose that it then became extinct. We forget how

small the area of Europe is compared with the rest of the world; nor have the several stages of the same

formation throughout Europe been correlated with perfect accuracy.

We may safely infer that with marine animals of all kinds there has been a large amount of migration due

to climatal and other changes; and when we see a species first appearing in any formation, the probability

is that it only then first immigrated into that area. It is well-known, for instance, that several species





appear somewhat earlier in the palæozoic beds of North America than in those of Europe; time having

apparently been required for their migration from the American to the European seas. In examining the

latest deposits in various quarters of the world, it has everywhere been noted, that some few still existing

species are common in the deposit, but have become extinct in the immediately surrounding sea; or,

conversely, that some are now abundant in the neighbouring sea, but are rare or absent in this particular

deposit. It is an excellent lesson to reflect on the ascertained amount of migration of the inhabitants of

Europe during the glacial epoch, which forms only a part of one whole geological period; and likewise toreflect on the changes of level, on the extreme change of climate, and on the great lapse of time, all

included within this same glacial period. Yet it may be doubted whether, in any quarter of the world,

sedimentary deposits,in-including fossil remains , have gone on accumulating within the same area

during the whole of this period. It is not, for instance, probable that sediment was deposited during the

whole of the glacial period near the mouth of the Mississippi, within that limit of depth at which marine

animals can best flourish: for we know that great geographical changes occurred in other parts of

America during this space of time. When such beds as were deposited in shallow water near the mouth

of the Mississippi during some part of the glacial period shall have been upraised, organic remains will

probably first appear and disappear at different levels, owing to the migrations of species and to

geographical changes. And in the distant future, a geologist, examining these beds, would be tempted toconclude that the average duration of life of the embedded fossils had been less than that of the glacial

period, instead of having been really far greater, that is, extending from before the glacial epoch to the

present day.

In order to get a perfect gradation between two forms in the upper and lower parts of the same

formation, the deposit must havegone oncontinuously accumulating during a long period, sufficient for

the slow process of modification; hence the deposit must be a very thick one; and the species undergoing

change must have lived in the same district throughout the whole time. But we have seen that a thick

formation, fossiliferous throughout its entire thickness, canaccumulate only during a period of subsidence;

and to keep the depth approximately the same, which is necessary that the same marine species may live

on the same space, the supply of sediment must nearly counterbalance the amount of subsidence. But thissame movement of subsidence will tend to submerge the area whence the sediment is derived, and thus

diminish the supply, whilst the downwardmovement continues. In fact, this nearly exact balancing

between the supply of sediment and the amount of subsidence is probably a rare contingency; for it has

been observed by more than one palæontologist, that very thick deposits are usually barren of organic

remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile of formations in any country, has

generally been intermittent in its accumulation. When we see, as is so often the case, a formation

composed of beds of widelydifferent mineralogical composition, we may reasonably suspect that the

process of deposition has been more or less interrupted. Nor will the closest inspection of a formationgive us any idea of the length of time which its deposition may have consumed. Many instances could be

given of beds only a few feet in thickness, representing formations, which are elsewhere thousands of feet

in thickness, and which must have required an enormous period for their accumulation; yet no one

ignorant of this fact would have even suspected the vast lapse of time represented by the thinner

formation. Many cases could be given of the lower beds of a formation having been upraised, denuded,

submerged, and then re-covered by the upper beds of the same formation,—facts, showing what wide,

yet easily overlooked, intervals have occurred in its accumulation. In other cases we have the plainest

evidence in great fossilised trees, still standing upright as they grew, of many long intervals of time and

changes of level during the process of deposition, which would not have been suspected, had not the

trees been preserved: thus Sir C. Lyell and Dr. Dawson found carboniferous beds 1400 feet thick in

Nova Scotia, with ancient root-bearing strata, one above the other at no less than sixty-eight different

levels. Hence, when the same species occurs at the bottom, middle, and top of a formation, the

probability is that it has not lived on the same spot during the whole period of deposition, but has





disappeared and reappeared, perhaps many times, during the same geological period. Consequently if it

were to undergo a considerable amount of modification during the deposition of any one geological

formation, a section would not include all the fine intermediate gradations which must on our theory have

existed, but abrupt, though perhaps slight, changes of form.

It is all-important to remember that naturalists have no golden rule by which to distinguish species and

varieties; they grant some little variability to each species, but when they meet with a somewhat greater amount of difference between any two forms, they rank both as species, unless they are enabled to

connect them together by the closest intermediate gradations; and this, from the reasons just assigned, we

can seldom hope to effect in any one geological section. Supposing B and C to be two species, and a

third, A, to be found in an older and underlying bed; even if A were strictly intermediate between B and

C, it would simply be ranked as a third and distinct species, unless at the same time it could be closely

connected by intermediate varieties with either one or both forms. Nor should it be forgotten, as before

explained, that A might be the actual progenitor of B and C, and yet would not necessarily be strictly

intermediate between them in all respects. So that we might obtain the parent-species and its several

modified descendants from the lower and upper beds of the same formation, and unless we obtained

numerous transitional gradations, we should not recognise their blood-relationship, and shouldconsequently rank them as distinct species.

It is notorious onwhat excessively slight differencesmany palæontologists have founded their species;

and they do this the more readily if the specimens come from different sub-stages of the same formation.

Some experienced conchologists are now sinking many of the very fine species of D'Orbigny and others

into the rank of varieties; and on this view we do find the kind of evidence of change which on the theory

we ought to find. Look again at the later tertiary deposits, which include many shells believed by the

majorityof naturalists to be identicalwith existing species; but some excellent naturalists, asAgassiz and

Pictet,maintain that all these tertiary species are specifically distinct, though the distinction is admitted to

be very slight; so that here, unless we believe that these eminent naturalists have been misled by their

imaginations, and that these late tertiary species really present no difference whatever from their livingrepresentatives, or unless we admit, in opposition to the judgment of most naturalists, that these tertiary

species are all truly distinct from the recent, we have evidence of the frequent occurrence of slight

modifications of the kind required. If we look to rather wider intervals of time, namely, to distinct but

consecutive stages of the same great formation,we find that theembedded fossils, though universally

ranked as specifically different, yet are far more closely related to each other than are the species found

in more widely separated formations; so that here again we have undoubted evidence of change in the

direction required by the theory; but to this latter subject I shall return in the following chapter.

With animals and plants that propagate rapidly and do not wander much, there is reason to suspect, as

we have formerly seen, that their varieties are generally at first local; and that such local varieties do notspread widely and supplant their parent-forms until they have been modified and perfected in some

considerable degree. According to this view, the chance of discovering in a formation in any one country

all the early stages of transition between any two forms, is small, for the successive changes are supposed

to have been local or confined to some one spot. Most marine animals have a wide range; and we have

seen that with plants it is those which have the widest range, that oftenest present varieties; so that, with

shells and other marine animals, it is probable that those which had the widest range, far exceeding the

limitsof the known geological formations in Europe, have oftenest given rise, first to localvarieties and

ultimately to new species; and this again would greatly lessen the chance of our being able to trace the

stages of transition in any one geological formation.

It is a more important consideration, leading to the same result, as lately insisted on by Dr. Falconer,

namely, that theperiod during which each species underwent modification, though long asmeasuredby

years, was probably short in comparison with that duringwhich it remainedwithout undergoing any





change.

It should not be forgotten, that at the present day, with perfect specimens for examination, two forms

can seldom be connected by intermediate varieties, and thus proved to be the same species, until many

specimens are collected from many places; and with fossil species this can rarely be done. We shall,

perhaps, best perceive the improbability of our being enabled to connect species by numerous, fine,

intermediate, fossil links, by asking ourselveswhether, for instance, geologists at some future period will be able to prove that our different breeds of cattle, sheep, horses, and dogs are descended from a single

stock or from several aboriginal stocks; or, again, whether certain sea-shells inhabiting the shores of

North America, which are ranked by some conchologists as distinct species from their European

representatives, and by other conchologists as only varieties, are really varieties, or are, as it is called,

specifically distinct. This could be effected by the future geologist only byhis discovering in a fossil state

numerous intermediate gradations; and such success is improbable in the highest degree.

It has been asserted over and over again, by writers who believe in the immutability of species, that

geology yields no linking forms. This assertion, as we shall see in the next chapter, is certainly erroneous.

As Sir J. Lubbock has remarked, "Every species is a link "between other allied forms." If we take agenus having a score of species, recent and extinct, and destroy four-fifths of them, no one doubts that

the remainder will stand much more distinct from each other. If the extreme forms in the genus happen to

have been thus destroyed, the genus itself will stand more distinct from other allied genera. What

geological research has not revealed, is the former existence of infinitely numerous gradations, as line as

existing varieties, connecting together nearly all existing and extinct species. But this ought not to be

expected; yet this has been repeatedly advanced as a most serious objection against my views.

It may be worth while to sum up the foregoing remarks on the causes of the imperfection of the

geological record under an imaginary illustration. The MalayArchipelago is about the size of Europe from

the North Cape to the Mediterranean, and from Britain to Russia; and therefore equals all the geological

formations which have been examined with any accuracy, excepting those of the United States of America. I fully agree with Mr. Godwin-Austen, that the present condition of the Malay Archipelago,

with its numerous large islands separated by wide and shallow seas, probably represents the former state

of Europe, whilst most of our formations were accumulating. The Malay Archipelago is one of the richest

regions in organic beings; yet if all the species were to be collected which have ever lived there, how

imperfectlywould they represent the natural history of the world!

But we have every reason to believe that the terrestrial productions of the archipelago would be

preserved in an extremely imperfect manner in the formations which we suppose to be there

accumulating. Not many of the strictly littoral animals, or of those which lived on naked submarine rocks,

would be embedded; and those embedded in gravel or sand would not endure to a distant epoch.Wherever sediment did not accumulate on the bed of the sea, or where it did not accumulate at a

sufficient rate to protect organic bodies from decay, no remains could be preserved.

Formations rich in fossils of many kinds, and of thickness sufficient to last to an age as distant in futurity

as the secondary formations lie in the past, would generally be formed in the archipelago only during

periods of subsidence. These periods of subsidence would be separated from each other by immense

intervals of time, duringwhich the area would be either stationary or rising;whilst rising, the fossiliferous

formations on the steeper shores would be destroyed, almost as soon as accumulated, by the incessant

coastaction, as we now see on the shores of South America. Even throughout the extensive and shallow

seas within the archipelago, sedimentary beds could hardly be accumulated of great thickness during the

periods of elevation, or become capped and protected by subsequent deposits, so as to have a good

chance of enduring to a very distant future. During the periods of subsidence, there would probably be

much extinction of life; during the periodsof elevation, therewould bemuch variation, but the geological





record would then be less perfect.

It may be doubted whether the duration of any one great period of subsidence over the whole or part of

the archipelago, togetherwith a contemporaneous accumulation of sediment, wouldexceed the average

duration of the same specific forms; and these contingencies are indispensable for the preservation of all

the transitional gradations between any two or more species. If such gradations were not all fully

preserved, transitional varieties would merely appear as so many new, though closely allied species. It isalso probable that each great period of subsidence would be interrupted by oscillations of level, and that

slight climatal changes would interveneduring such lengthyperiods; and in these cases the inhabitants of

thearchipelagowould migrate, andno closely consecutive recordof their modifications could be

preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range thousands of miles beyond its

confines; and analogyplainly leads to the belief that it would be chiefly these far-ranging species, though

only some of them, which would oftenest produce new varieties; and the varieties would at first be local

or confined to one place, but if possessed of any decided advantage, or when further modified and

improved, they would slowly spread and supplant their parent-forms. When such varieties returned totheir ancient homes, as they would differ from their former state in a nearly uniform, though perhaps

extremely slight degree, and as they would be found embedded in slightly different sub-stages of the same

formation, they would, according to the principles followed by many palæontologists, be ranked as new

anddistinct species.

If then there be some degree of truth in these remarks, we have no right to expect to find, in our

geological formations, an infinite number of those fine transitional forms which, on our theory, have

connected all the past and present species of the same group into one long and branching chain of life.

We ought only to look for a few links, and such assuredly we do find—some more distantly, some more

closely, related to each other; and these links, let them be ever so close, if found in different stages of the

same formation, would, by many paleontologists, be ranked as distinct species. But I do not pretend thatI should ever have suspected how poor was the record in the best preserved geological sections, had not

the absenceof innumerable transitional links between the specieswhich lived at the commencement and

close of each formation, pressed so hardly on my theory.

On the sudden Appearance of whole Groups of allied Species.

The abrupt manner in which whole groups of species suddenly appear in certain formations, has been

urged by several palæeontologists—for instance, by Agassiz, Pictet, andSedgwick—as a fatal objectionto the belief in the transmutation of species. If numerous species, belonging to the same genera or

families, have really started into life at once, the fact would be fatal to the theory of evolution through

natural selection. For the development by this means of a group of forms, all of which are descended

from some one progenitor, must have been an extremely slow process; and the progenitors must have

lived long before theirmodifieddescendants. But we continuallyoverrate the perfection of thegeological

record, and falsely infer, because certain genera or families have not been found beneath a certain stage,

that they did not exist before that stage. In all cases positive palæontological evidence may be implicitly

trusted; negative evidence is worthless, as experience has so often shown. We continually forget how

large the world is, compared with the area over which our geological formations have been carefully

examined; we forget that groups of species may elsewhere have long existed, and have slowly multiplied,

before they invaded the ancient archipelagoes of Europe and the United States. We do not make due

allowance for the intervals of time whichhave elapsed between our consecutive formations,—longer

perhaps in many cases than the time required for the accumulation of each formation. These intervals will





have given time for the multiplication of species from some one parent-form: and in the succeeding

formation, such groups or species will appear as if suddenly created.

I may here recall a remark formerly made, namely, that it might require a long succession of ages to

adapt an organism to some new and peculiar line of life, for instance, to fly through the air; and

consequently that the transitional formswould often long remain confined to someone region; but that,

when this adaptation had once been effected, and a few species had thus acquired a great advantageover other organisms, a comparatively short time would be necessary to produce many divergent forms,

which would spread rapidly and widely, throughout the world. Professor Pictet, in his excellent Review of

this work, in commenting on early transitional forms, and taking birds as an illustration, cannot see how

the successive modifications of the anterior limbs of a supposed prototype could possibly have been of

any advantage. But look at the penguins of the Southern Ocean; have not these birds their front limbs in

this precise intermediate state of "neither true "arms nor true wings"? Yet these birds hold their place

victoriously in the battle for life; for they exist in infinite numbers and of many kinds. I do not suppose that

we here see the real transitional grades through which the wings of birds have passed; but what special

difficulty is there in believing that it might profit the modified descendants of the penguin, first to become

enabled to flap along the surface of the sea like the logger-headed duck, and ultimately to rise from itssurface and glide through the air?

I will now give a few examples to illustrate the foregoing remarks, and to show how liable we are to

error in supposing that whole groups of species have suddenly been produced. Even in so short an

interval as that between the first and second editions of Pictet's great work on Palæontology, published in

1844-46 and in 1853-57, the conclusions on the first appearance and disappearance of several groups

of animals have been considerably modified; and a third edition would require still further changes. I may

recall the well-known fact that in geological treatises, published not many years ago, mammals were

always spoken of as having abruptly come in at the commencement of the tertiary series. And now one of

the richest known accumulations of fossil mammals belongs to the middle of the secondary series; and

true mammals have been discovered in the new red sandstone at nearly the commencement of this greatseries. Cuvier used to urge that no monkey occurred in any tertiary stratum; but now extinct species have

been discovered in India, South America and in Europe, as far back as the miocene stage. Had it not

been for the rare accident of the preservation of footsteps in the new red sandstone of the United States,

who would have ventured to suppose that no less than at least thirty different bird-like animals, some of

gigantic size, existed during that period? Not a fragment of bone has been discovered in these beds. Not

long ago, palæontologistsmaintained that thewhole class of birds came suddenly into existence during

the eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived

during the deposition of the upper greensand; and still more recently, that strange bird, the Archeopteryx,

with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two

free claws, has been discovered in the oolitic slates of Solenhofen. Hardly any recent discovery showsmore forcibly than this, how little we as yet know of the former inhabitants of the world.

I may give another instance, which, from having passed under my own eyes, has much struck me. In a

memoir on Fossil Sessile Cirripedes, I stated that, from the large number of existing and extinct tertiary

species; from the extraordinary abundance of the individuals of many species all over the world, from the

Arctic regions to the equator, inhabiting various zones of depths from the upper tidal limits to 50 fathoms;

from the perfect manner in which specimens are preserved in the oldest tertiary beds; from the ease with

which even fragment of a valve can be recognised; from all these circumstances, I inferred that, had

sessile cirripedes existed during the secondary periods, they would certainly have been preserved and

discovered; and as not one species had then been discovered in beds of this age, I concluded that this

great group had been suddenly developed at the commencement of the tertiary series. This was a sore

trouble to me, adding as I then thought one more instance of the abrupt appearance of a great group of

species. But my work had hardly been published, when a skilful palæontologist, M. Bosquet, sent me a





drawing of a perfect specimen of an unmistakable sessile cirripede, which he had himself extracted from

the chalk of Belgium. And, as if to make the case as striking as possible, this cirripede was a Chthamalus,

a very common, large, and ubiquitous genus, of which not one species has as yet been found even in any

tertiary stratum. Still more recently, a Pyrgoma, a member of a distinct sub-family of sessile cirripedes,

has been discovered by Mr. Woodward in the upper chalk; so that we now have abundant evidence of

the existence of this group of animals during the secondary period.

The case most frequently insisted on by palæontologists of the apparently sudden appearance of a whole

group of species, is that of the teleostean fishes, low down, according to Agassiz, in the Chalk period.

This group includes the large majority of existing species. But certain Jurassic and Triassic forms are now

commonly admitted to be teleostean; and even some palæozoic forms have thus been classed by one

high authority. If the teleosteans had really appeared suddenly in the northern hemisphere at the

commencement of the chalk formation the fact would have been highly remarkable; but it would not have

formed an insuperable difficulty, unless it could likewise have been shown that at the same period the

species were suddenly and simultaneously developed in other quarters of the world. It is almost

superfluous to remark that hardly any fossil-fish are known from south of the equator; and by running

through Pictet's Palæontology it will be seen that very few species are known from several formations inEurope. Some few families of fish now have a confined range; the teleostean fishes might formerly have

had a similarly confined range, and after having been largely developed in some one sea, have spread

widely. Nor have we any right to suppose that the seas of the world have always been so freely open

from south to north as they are at present. Even at this day, if the Malay Archipelago were converted into

land, the tropical parts of the Indian Ocean would form a large and perfectly enclosed basin, in which any

great group of marine animals might be multiplied; and here they would remain confined, until some of the

species became adapted to a cooler climate, and were enable to double the Southern capes of Africa or

Australia, and thus reach other and distant seas.

From these considerations, from our ignorance of the geology of other countries beyond the confines of

Europe and the United States, and from the revolution in our palæontological knowledge effected by thediscoveries of the last dozen years, it seems to me to be about as rash to dogmatize on the succession of

organic forms throughout the world, as it would be for a naturalist to land for five minutes on a barren

point in Australia, and then to discuss the number and range of its productions.

On the sudden Appearance of Groups of allied Species in the lowest known Fossiliferous Strata.

There is another and allied difficulty, which is much more serious. I allude to the manner in which species

belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest knownfossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the

same group are descended from a single progenitor, apply with equal force to the earliest known species.

For instance, it cannot be doubted that all the Cambrian and Silurian trilobites are descended from some

one crustacean, which must have lived long before the Cambrian age, and which probably differed

greatly from any known animal. Some of the most ancient animals, as the Nautilus, Lingula, &c., do not

differ much from living species; and it cannot on our theory be supposed, that these old species were the

progenitors of all the species belonging to the same groups which have subsequently appeared, for they

are not in any degree intermediate in character.

Consequently, if the theory be true, it is indisputable that before the lowest Cambrian stratum was

deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the

Cambrian age to the present day; and that during these vast periods the world swarmed with living

creatures. Here we encounter a formidable objection; for it seems doubtful whether the earth, in a fit state





for the habitation of living creatures, has lasted long enough. Sir W. Thompson concludes that the

consolidation of the crust can hardly have occurred less than 20 or more than 400 million years ago, but

probably not less than 98 or more than 200 million years. These very wide limits show how doubtful the

data are; and other elements may have hereafter to be introduced into the problem. Mr. Croll estimates

that about 60 million years have elapsed since the Cambrian period, but this, judging from the small

amount of organic change since the commencement of the Glacial epoch, appears a very short time for

the many and great mutations of life, which have certainly occurred since the Cambrian formation; and the previous 140 million years can hardly be considered as sufficient for the development of the varied forms

of life which already existed during the Cambrian period. It is, however, probable, as Sir William

Thompson insists, that the world at a very early period was subjected to more rapid and violent changes

in its physical conditions than those now occurring; and such changes would have tended to induce

changes at a corresponding rate in the organisms which then existed.

To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest

periods prior to the Cambrian system, I can give no satisfactory answer. Several eminent geologists, with

Sir R. Murchison at their head, were until recently convinced that we beheld in the organic remains of the

lowest Silurian stratum the first dawn of life. Other highly competent judges, as Lyell and E. Forbes, havedisputed this conclusion. We should not forget that only a small portion of the world is known with

accuracy. Not very long ago M. Barrande added another and lower stage, abounding with new and

peculiar species, beneath the then known Silurian system; and now, still lower down in the Lower

Cambrian formation, Mr. Hicks has found in SouthWales beds rich in trilobites, and containing various

molluscs and annelids. The presence of phosphatic nodules and bituminous matter, even in some of the

lowest azoic rocks, probably indicates life at these periods; and the existence of the Eozoon in the

Laurentian formation of Canada is generally admitted. There are three great series of strata beneath the

Silurian system in Canada, in the lowest of which the Eozoon is found. Sir W. Logan states that their

"united thickness may possibly far surpass that of all the succeeding rocks, from the base of the

palæozoic series to the present time. We are thus carried back to a period so remote, that the

appearance of the so-called Primordial fauna (of Barrande) may by some be considered as acomparatively modern event." The Eozoon belongs to the most lowly organised of all classes of animals,

but is highly organised for its class; it existed in countless numbers, and, as Dr. Dawson has remarked,

certainly preyed on other minute organic beings, which must have lived in great numbers. Thus the words,

which I wrote in 1859, about the existence of living beings long before the Cambrian period, and which

are almost the same with those since used by Sir W. Logan, have proved true. Nevertheless, the difficulty

of assigning any good reason for the absence of vast piles of strata rich in fossils beneath the Cambrian

system is very great. It does not seem probable that the most ancient beds have been quite worn away

by denudation, or that their fossils have been wholly obliterated by metamorphic action, for if this had

been the case we should have found only small remnants of the formations next succeeding them in age,

and thesewould always have existed in a partially metamorphosedcondition. But thedescriptions whichwe possess of the Silurian deposits over immense territories in Russia and in North America, do not

support the view, that the older a formation is, the more invariably it has suffered extreme denudation and

metamorphism.

The case at present must remain inexplicable; and may be truly urged as a valid argument against the

views here entertained. To show that it may hereafter receive some explanation, I will give the following

hypothesis. From the nature of the organic remains which do not appear to have inhabited profound

depths, in the several formations of Europe and of the United States; and from the amount of sediment,

miles in thickness, of which the formations are composed, we may infer that from first to last large islands

or tracts of land, whence the sediment was derived, occurred in the neighbourhood of the now existing

continents of Europe and North America. This same view has since been maintained by Agassiz and

others. But we do not know what was the state of things in the intervals between the several successive

formations; whether Europe and the United States during these intervals existed as dry land, or as a





submarine surface near land, on which sediment was not deposited, or as the bed of an open and

unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the land, we see them studded with

many islands; but hardly one truly oceanic island (with the exception of New Zealand, if this can be called

a truly oceanic island) is as yet known to afford even a remnant of any palæozoic or secondary

formation. Hence we may perhaps infer, that during the palæozoic and secondary periods, neither continents nor continental islands existed where our oceans now extend; for had they existed, palæozoic

and secondary formations would in all probability have been accumulated from sediment derived from

their wear and tear; and these would have been at least partially upheaved by the oscillations of level,

which must have intervened during these enormously longperiods. If thenwemay inferanything from

these facts, we may infer that, where our oceans now extend, oceans have extended from the remotest

period of which we have any record; and on the other hand, that where continents now exist, large tracts

of land have existed, subjected no doubt to great oscillations of level, since the Cambrian period. The

coloured map appended to my volume on Coral Reefs, led me to conclude that the great oceans are still

mainlyareas of subsidence, the great archipelagoes still areas of oscillations of level, and the continents

areas of elevation. But we have no reason to assume that things have thus remained from the beginning of the world. Our continents seem to have been formed by a preponderance, during many oscillations of

level, of the force of elevation; but may not the areas of preponderant movement have changed in the

lapse of ages? At a period long antecedent to the Cambrian epoch, continents may have existed where

oceans are now spread out; and clear and open oceans may have existed where our continents now

stand. Nor should we be justified in assuming that if, for instance, the bed of the Pacific Ocean were now

converted into a continentwe should there find sedimentary formations in a recognisable condition older

than the Cambrian strata, supposing such to have been formerly deposited; for it might well happen that

strata which had subsided some miles nearer to the centre of the earth, and which had been pressed on

by an enormousweight of superincumbent water, might have undergone farmore metamorphic action

than strata which have always remained nearer to the surface. The immense areas in some parts of the

world, for instance in South America, of naked metamorphic rocks, which must have been heated under great pressure, have always seemed to me to require some special explanation; and we may perhaps

believe that we see in these large areas, the many formations long anterior to the Cambrian epoch in a

completelymetamorphosed and denuded condition.

The several difficultieshere discussed, namely—that, thoughwe find in our geological formationsmany

links between the specieswhichnow exist and which formerly existed, wedonot find infinitely numerous

fine transitional forms closely joining themall together;—the suddenmanner inwhich several groupsof

species first appear in our European formations;—the almost entire absence, as at present known, of

formations rich in fossils beneath the Cambrian strata,—are all undoubtedly of themost serious nature.

We see this in the fact that the most eminent palæontologists, namely, Cuvier, Agassiz, Barrande, Pictet,Falconer, E. Forbes, &c., and all our greatest geologists, as Lyell, Murchison, Sedgwick, &c., have

unanimously, often vehemently,maintained the immutabilityof species.ButSirCharles Lyell now gives

the support of his high authority to the opposite side; and most geologists and palæontologists are much

shaken in their former belief. Those who believe that the geological record is in any degree perfect, will

undoubtedly at once reject the theory. For my part, following out Lyell's metaphor, I look at the

geological record as a history of the world imperfectly kept, and written in a changing dialect; of this

history we possess the last volume alone, relating only to two or three countries. Of this volume, only

here and there a short chapter has been preserved; and of each page, only here and there a few lines.

Each word of the slowly-changing language, more or less different in the successive chapters, may

represent the forms of life, which are entombed in our consecutive formations, and which falsely appear

to have been abruptly introduced. On this view, the difficulties above discussed are greatly diminished, or

even disappear.





|Go to Contents |

Chapter XI

On the Geological Succession of Organic

Beings.

On the slow and successive appearance of new species—On their different rates of change—Species

once lost do not reappear—Groups of species follow the same general rules in their appearance and

disappearance as do single species—Onextinction—On simultaneous changes in the formsof life

throughout the world—On the affinities of extinct species to each other and to living species—On the

state of development of ancient forms—On the succession of the same types within the same

areas—Summary of preceding and present chapter.

LET us now see whether the several facts and laws relating to the geological succession of organic

beings accord best with the common view of the immutability of species, or with that of their slow and

gradual modification, through variation andnatural selection.

New species have appeared very slowly, one after another, both on the land and in the waters. Lyell has

shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages;

and every year tends to fill up the blanks between the stages, and to make the proportion between the

lost and existing forms more gradual. In some of the most recent beds, though undoubtedly of high

antiquity if measured by years, only one or two species are extinct, and only one or two are new, having

appeared there for the first time, either locally, or, as far as we know, on the face of the earth. The

secondary formations are more broken; but, as Bronn has remarked, neither the appearance nor

disappearance of the many species embedded in each formation has been simultaneous.

Species belonging to different genera and classes have not changed at the same rate, or in the same

degree. In the older tertiary beds a few living shells may still be found in the midst of a multitude of extinct

forms. Falconer has given a striking instance of a similar fact, for an existing crocodile is associated with

many lostmammals and reptiles in the sub-Himalayan deposits. The SilurianLingula differs but little from

the living species of this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have

changed greatly. The productions of the land seem to have changed at a quicker rate than those of the

sea, of which a striking instance has been observed in Switzerland. There is some reason to believe that

organisms high in the scale, change more quickly than those that are low: though there are exceptions to

this rule. The amount of organic change, as Pictet has remarked, is not the same in each successive

so-called formation. Yet if we compare any but the most closely related formations, all the species will befound to have undergone some change. When a species has once disappeared from the face of the earth,

we have no reason to believe that the same identical form ever reappears. The strongest apparent





exception to this latter rule is that of the so-called "colonies" of M. Barrande, which intrude for a period

in the midst of an older formation, and then allow the pre-existing fauna to reappear; but Lyell's

explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems

satisfactory.

These several facts accord well with our theory, which includes no fixed law of development, causing all

the inhabitants of an area to change abruptly, or simultaneously, or to an equal degree. The process of modification must be slow, and will generally effect only a few species at the same time; for the variability

of each species is independent of that of all others. Whether such variations or individual differences as

may arise will be accumulated through natural selection in a greater or less degree, thus causing a greater

or less amount of permanentmodification, will dependonmany complex contingencies—on the

variations being of a beneficial nature,on the freedomof intercrossing, on the slowly changing physical

conditions of the country, on the immigration of new colonists, and on the nature of the other inhabitants

with which the varying species come into competition. Hence it is by no means surprising that one species

should retain the same identical form much longer than others; or, if changing, should change in a less

degree.We find similar relations between the existing inhabitants of distinct countries; for instance, the

land-shells and coleopterous insects of Madeira have come to differ considerably from their nearest allieson the continent of Europe, whereas the marine shells and birds have remained unaltered. We can

perhaps understand the apparently quicker rate of change in terrestrial and in more highly organised

productions compared with marine and lower productions, by the more complex relations of the higher

beings to their organic and inorganic conditions of life, as explained in a former chapter. When many of

the inhabitants of any area have become modified and improved, we can understand, on the principle of

competition, and from the all-important relationsof organism to organism in the struggle for life, that any

form which did not become in some degree modified and improved, would be liable to extermination.

Hence we see why all the species in the same region do at last, if we look to long enough intervals of

time, become modified, for otherwise they wouldbecomeextinct.

In members of the same class the average amount of change, during long and equal periods of time, may, perhaps, be nearly the same; but as the accumulation of enduring formations, rich in fossils, depends on

great masses of sediment being deposited on subsiding areas, our formations have been almost

necessarilyaccumulated at wide and irregularly intermittent intervals of time; consequently the amount of

organic change exhibited by the fossils embedded in consecutive formations is not equal.Each formation,

on this view, does not mark a new and complete act of creation, but only an occasional scene, taken

almost at hazard, in an ever slowly changing drama.

We can clearly understand why a species when once lost should never reappear, even if the very same

conditions of life, organic and inorganic, should recur. For though the offspring of one species might be

adapted (and no doubt this has occurred in innumerable instances) to fill the place of another species inthe economy of nature, and thus supplant it; yet the two forms—the old and the new—would not be

identically the same; for bothwould almost certainly inherit different characters from their distinct

progenitors; and organisms already differing wouldvary in a different manner. For instance, it is possible,

if all our fantail pigeons were destroyed, that fanciers might make a new breed hardly distinguishable from

the present breed; but if the parent rock-pigeon were likewise destroyed, and under nature we have

every reason to believe that parent-forms are generally supplanted and exterminated by their improved

offspring, it is incredible that a fantail, identical with the existing breed, could be raised from any other

species of pigeon, or even from any other well-established race of the domestic pigeon, for the

successive variationswould almost certainlybe in some degreedifferent, and the newly-formed variety

would probably inherit from its progenitor some characteristic differences.

Groups of species, that is, genera and families, follow the same general rules in their appearance and

disappearance as do single species, changing more or less quickly, and in a greater or lesser degree. A





group, when it has once disappeared, never reappears; that is, its existence, as long as it lasts, is

continuous. I am aware that there are some apparent exceptions to this rule, but the exceptions are

surprisingly few, so few that E. Forbes, Pictet, and Woodward (though all strongly opposed to such

views as I maintain) admit its truth; and the rule strictly accords with the theory. For all the species of the

same group, however long it may have lasted, are the modified descendants one from the other, and all

from a commonprogenitor. In the genus Lingula, for instance, the specieswhichhave successively

appeared at all ages must have been connected by an unbroken series of generations, from the lowestSilurian stratum to the present day.

We have seen in the last chapter that whole groups of species sometimes falsely appear to have been

abruptly developed; and I have attempted to give an explanation of this fact, which if true would be fatal

to my views. But such cases are certainly exceptional; the general rule being a gradual increase in

number, until the group reaches its maximum, and then, sooner or later, a gradual decrease. If the number

of the species included within a genus, or the number of the genera within a family, be represented by a

vertical line of varying thickness, ascending through the successive geological formations, inwhich the

species are found, the line will sometimes falsely appear to begin at its lower end, not in a sharp point, but

abruptly; it then gradually thickens upwards, often keeping of equal thickness for a space, and ultimatelythins out in the upper beds, marking the decrease and final extinction of the species. This gradual increase

in number of the species of a group is strictly conformable with the theory, for the species of the same

genus, and the genera of the same family, can increase only slowly and progressively; the process of

modification and the production of a number of allied forms necessarily being a slow and gradual

process,—one species first giving rise to two or three varieties, these being slowly converted into

species, which in their turn produce by equally slow steps other varieties and species, and so on, like the

branching of a great tree from a single stem, till the group becomes large.

On Extinction.

We have as yet only spoken incidentally of the disappearance of species and of groups of species. On

the theory of natural selection, the extinction of old forms and the production of new and improved forms

are intimately connected together. The old notion of all the inhabitants of the earth having been swept

away by catastrophes at successive periods is very generally given up, even by those geologists, as Elie

de Beaumont, Murchison, Barrande, &c., whose general views would naturally lead them to this

conclusion. On the contrary, we have every reason to believe, from the study of the tertiary formations,

that species and groups of species gradually disappear, one after another, first from one spot, then from

another, and finally from the world. In some few cases however, as by the breaking of an isthmus and the

consequent irruption of a multitude of new inhabitants into an adjoining sea, or by the final subsidence of an island, the process of extinction may have been rapid. Both single species and whole groups of

species last for very unequal periods; some groups, as we have seen, have endured from the earliest

known dawn of life to the present day; some have disappeared before the close of the palæozoic period.

No fixed law seems to determine the length of time during which any single species or any single genus

endures. There is reason to believe that the extinction of a whole group of species is generally a slower

process than their production: if their appearance and disappearance be represented, as before, by a

vertical line of varying thickness the line is found to taper more gradually at its upper end, which marks

the progress of extermination, than at its lower end, which marks the first appearance and the early

increase in number of the species. In some cases, however, the extermination of whole groups, as of

ammonites, towards the close of the secondary period, has been wonderfully sudden.

The extinction of species has been involved in the most gratuitous mystery. Some authors have even

supposed that, as the individual has a definite length of life, so have species a definite duration. No one





can have marvelled more than I have done at the extinction of species. When I found in La Plata the

tooth of a horse embedded with the remains of Mastodon, Megatherium, Toxodon, and other extinct

monsters, which all co-existed with still living shells at a very late geological period, I was filled with

astonishment; for, seeing that the horse, since its introduction by the Spaniards into South America, has

run wild over the whole country and has increased in numbers at an unparalleled rate, I asked myself

what could so recently have exterminated the former horse under conditions of life apparently so

favourable. But my astonishment wasgroundless. Professor Owen soon perceived that the tooth, thoughso like that of the existing horse, belonged to an extinct species. Had this horse been still living, but in

some degree rare, no naturalist would have felt the least surprise at its rarity; for rarity is the attribute of a

vast number of species of all classes, in all countries. If we ask ourselves why this or that species is rare,

we answer that something is unfavourable in its conditions of life; but what that something is we can

hardly ever tell. On the supposition of the fossil horse still existing as a rare species, we might have felt

certain, from the analogy of all other mammals, even of the slow-breeding elephant, and from the history

of the naturalisation of the domestic horse in South America, that under more favourable conditions it

would in a very few years have stocked the whole continent. But we could not have told what the

unfavourable conditions were which checked its increase, whether some one or several contingencies,

and at what period of the horse's life, and in what degree they severally acted. If the conditions had goneon, however slowly, becoming less and less favourable, we assuredly should not have perceived the fact,

yet the fossil horse would certainly have become rarer and rarer, and finally extinct;—its place being

seized on by some more successful competitor.

It is most difficult always to remember that the increase of every creature is constantly being checked by

unperceived hostile agencies; and that these same unperceived agencies are amply sufficient to cause

rarity, and finally extinction. So little is this subject understood, that I have heard surprise repeatedly

expressed at such great monsters as the Mastodon and the more ancient Dinosaurians having become

extinct; as if mere bodily strength gave victory in the battle of life. Mere size, on the contrary, would in

some cases determine, as has been remarked by Owen, quicker extermination from the greater amount

of requisite food. Before man inhabited India or Africa, some cause must have checked the continuedincrease of the existing elephant. A highly capable judge, Dr. Falconer, believes that it is chiefly insects

which, from incessantly harassing and weakening the elephant in India, check its increase; and this was

Bruce's conclusion with respect to the African elephant in Abyssinia. It is certain that insects and

blood-sucking bats determine the existence of the larger naturalised quadrupeds in several parts of S.

America.

We see in many cases in the more recent tertiary formations, that rarity precedes extinction; and we

know that this has been the progress of events with those animals which have been exterminated, either

locally or wholly, through man's agency. I may repeat what I published in 1845, namely, that to admit that

species generally become rare before they become extinct—to feel no surprise at the rarity of a species,and yet to marvel greatly when the species ceases to exist, is much the same as to admit that sickness in

the individual is the forerunner of death—to feel no surprise at sickness, but, when the sick man dies, to

wonder and to suspect that he died by some deed of violence.

The theory of natural selection is grounded on the belief that each new variety and ultimately each new

species, is produced and maintained by having some advantage over those with which it comes into

competition; and the consequent extinction of the less-favoured forms almost inevitably follows. It is the

same with our domestic productions; when a new and slightly improved variety has been raised, it at first

supplants the less improvedvarieties in the same neighbourhood;when much improved it is transported

far and near, like our short-horn cattle, and takes the place of other breeds in other countries. Thus the

appearance of new forms and the disappearance of old forms, both those naturally and those artificially

produced, are bound together. In flourishing groups, the number of new specific forms which have been

produced within a given time has at some periods probably been greater than the number of the old





specific forms which have been exterminated; but we know that species have not gone on indefinitely

increasing, at least during the later geological epochs, so that, looking to later times, we may believe that

the production of new forms has caused the extinction of about the same number of old forms.

The competition will generally bemost severe, as formerly explained and illustratedby examples,

between the forms which are most like each other in all respects. Hence the improved and modified

descendants of a species will generally cause the extermination of the parent-species; and if many newforms have been developed from any one species, the nearest allies of that species,i.e . the species of the

same genus, will be the most liable to extermination. Thus, as I believe, a number of new species

descended from one species, that is a new genus, comes to supplant an old genus, belonging to the same

family. But it must often have happened that a new species belonging to some one group has seized on

the place occupied by a species belonging to a distinct group, and thus have caused its extermination. If

many allied forms be developed from the successful intruder, many will have to yield their places; and it

will generally be the allied forms,which will suffer fromsome inherited inferiority in common. But whether

it be species belonging to the same or to a distinct class, which have yielded their places to other

modified and improved species, a few of the sufferers may often be preserved for a long time, from being

fitted to some peculiar line of life, or from inhabiting some distant and isolated station, where they willhave escaped severe competition. For instance, some species of Trigonia, a great genus of shells in the

secondary formations, survive in the Australian seas; and a few members of the great and almost extinct

group of Ganoid fishes still inhabit our fresh waters. Therefore the utter extinction of a group is generally,

as we have seen, a slower process than its production.

With respect to the apparently sudden extermination of whole families or orders, as of Trilobites at the

close of the palæozoic period and of Ammonites at the close of the secondary period, we must

remember what has been already said on the probable wide intervals of time between our consecutive

formations; and in these intervals there may have been much slow extermination. Moreover, when, by

sudden immigration or by unusually rapid development, many species of a new group have taken

possession of an area, many of the older species will have been exterminated in a correspondingly rapidmanner; and the forms which thus yield their places will commonly be allied, for they will partake of the

same inferiority in common.

Thus, as it seems to me, the manner in which single species and whole groups of species become extinct

accords well with the theory of natural selection. We need not marvel at extinction; if we must marvel, let

it be at our own presumption in imagining for a moment that we understand the many complex

contingencies on which the existence of each species depends. If we forget for an instant that each

species tends to increase inordinately, and that some check is always in action, yet seldom perceived by

us, the whole economy of nature will be utterly obscured. Whenever we can precisely say why this

species is more abundant in individuals than that; why this species and not another can be naturalised in agiven country; then, and not until then, we may justly feel surprise why we cannot account for the

extinction of any particular species or group of species.

On the Forms of Life changing almost simultaneously throughout the World .

Scarcely any palæontological discovery is more striking than the fact that the forms of life change almost

simultaneously throughout the world. Thus our EuropeanChalk formation canbe recognised inmany

distant regions, under the most different climates, where not a fragment of the mineral chalk itself can be

found; namely in North America, in equatorial South America, in Tierra del Fuego, at the Cape of Good

Hope, and in the peninsula of India. For at these distant points, the organic remains in certain beds

present an unmistakeable resemblance to those of the Chalk. It is not that the same species are met with;





for in some cases not one species is identically the same, but they belong to the same families, genera,

and sections of genera, and sometimes are similarly characterised in such trifling points asmere superficial

sculpture. Moreover, other forms, which are not found in the Chalk of Europe, but which occur in the

formations either above or below, occur in the same order at these distant points of the world. In the

several successivepalæozoic formations of Russia, Western Europe, andNorthAmerica, a similar

parallelism in the forms of life has been observed by several authors; so it is, according to Lyell, with the

European and North American tertiary deposits. Even if the few fossil species which are common to theOld and NewWorlds were kept wholly out of view, the general parallelism in the successive forms of

life, in the palæozoic and tertiary stages, would still be manifest, and the several formations could be

easily correlated.

These observations, however, relate to the marine inhabitants of the world: we have not sufficient data to

judge whether the productions of the land and of fresh water at distant points change in the same parallel

manner.We maydoubt whether they have thus changed: if theMegatherium, Mylodon, Macrauchenia,

and Toxodon had been brought to Europe from La Plata, without any information in regard to their

geological position, no one wouldhave suspected that they had co-existedwith sea-shells all still living;

but as these anomalous monsters co-existed with the Mastodon and Horse, it might at least have beeninferred that they had lived during one of the later tertiary stages.

When the marine forms of life are spoken of as having changed simultaneously throughout the world, it

must not be supposed that this expression relates to the same year, or to the same century, or even that it

has a very strict geological sense; for if all the marine animals now living in Europe, and all those that lived

in Europe during the pleistocene period (a very remote period as measured by years, including the whole

glacial epoch) were compared with those now existing in South America or in Australia, the most skilful

naturalist would hardly be able to say whether the present or the pleistocene inhabitants of Europe

resembledmost closely those of the southern hemisphere. So, again, several highly competent observers

maintain that the existing productions of the United States are more closely related to those which lived in

Europe during certain late tertiary stages, than to the present inhabitants of Europe; and if this be so, it isevident that fossiliferous beds now deposited on the shores of North America would hereafter be liable

to be classed with somewhat older European beds. Nevertheless, looking to a remotely future epoch,

there can be little doubt that all the more modernmarine formations, namely, the upper pliocene, the

pleistocene and strictly modern beds of Europe, North and South America, and Australia, from

containing fossil remains in somedegree allied, and fromnot including those forms which are found only

in the older underlying deposits, would be correctly ranked as simultaneous in a geological sense.

The fact of the forms of life changing simultaneously, in the above large sense, at distant parts of the

world, has greatly struck these admirable observers, MM. de Verneuil and d'Archiac. After referring to

the parallelism of the palæozoic forms of life in various parts of Europe, they add, "If, struck by thisstrange sequence, we turn our attention to North America, and there discover a series of analogous

phenomena, it will appear certain that all these modifications of species, their extinction, and the

introduction of new ones, cannot be owing to mere changes in marine currents or other causes more or

less local and temporary, but depend on general laws which govern the whole animal kingdom." M.

Barrande has made forcible remarks to precisely the same effect. It is, indeed, quite futile to look to

changes of currents, climate, or other physical conditions, as the cause of these great mutations in the

forms of life throughout the world, under the most different climates. We must, as Barrande has

remarked, look to some special law. We shall see this more clearly when we treat of the present

distribution of organic beings, and find how slight is the relation between the physical conditions of

various countries and the nature of their inhabitants.

This great fact of the parallel succession of the forms of life throughout the world, is explicable on the

theory of natural selection. New species are formed by having some advantage over older forms; and the





forms, which are already dominant, or have some advantage over the other forms in their own country,

give birth to the greatest number of new varieties or incipient species. We have distinct evidence on this

head, in the plants which are dominant, that is, which are commonest and most widely diffused,

producing the greatest number of new varieties. It is also natural that the dominant, varying, and

far-spreading species, which have already invaded to a certain extent the territories of other species,

should be those which would have the best chance of spreading still further, and of giving rise in new

countries to other new varieties and species. The process of diffusion would often be very slow,depending on climatal and geographical changes, on strange accidents, and on the gradual acclimatisation

of new species to the various climates through which they might have to pass, but in the course of time

the dominant forms would generallysucceed in spreading and would ultimately prevail. The diffusion

would, it is probable, be slower with the terrestrial inhabitants of distinct continents than with the marine

inhabitants of the continuous sea. We might therefore expect to find, as we do find, a less strict degree of

parallelism in the succession of the productions of the land than with those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense, simultaneous, succession of the same

forms of life throughout the world, accords well with the principle of new species having been formed by

dominant species spreadingwidely and varying; the new species thus produced being themselvesdominant, owing to their having had some advantage over their already dominant parents, as well as over

other species, and again spreading, varying, and producing new forms. The old forms which are beaten

and which yield their places to the new and victorious forms, will generally be allied in groups, from

inheriting some inferiority in common; and therefore, as new and improved groups spread throughout the

world, old groups disappear from the world; and the succession of forms everywhere tends to

correspond both in their first appearance and final disappearance.

There is one other remark connected with this subject worth making. I have given my reasons for

believing that most of our great formations, rich in fossils, were deposited during periods of subsidence;

and that blank intervals of vast duration, as far as fossils are concerned, occurred during the periods

when the bed of the sea was either stationary or rising, and likewise when sediment was not throwndown quickly enough to embed and preserve organic remains. During these long and blank intervals I

suppose that the inhabitants of each regionunderwent a considerable amount of modification and

extinction, and that there was much migration from other parts of the world. As we have reason to

believe that large areas are affected by the same movement, it is probable that strictly contemporaneous

formations have often been accumulated over very wide spaces in the same quarter of the world; but we

are very far from having any right to conclude that this has invariably been the case, and that large areas

have invariably been affected by the same movements. When two formations have been deposited in two

regions during nearly, but not exactly, the same period, we should find in both, from the causes explained

in the foregoing paragraphs, the same general succession in the forms of life; but the species would not

exactly correspond; for there will have been a little more time in the one region than in the other for modification, extinction, and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his admirable Memoirs on the

eocene deposits of England and France, is able to draw a close general parallelism between the

successive stages in the two countries; but when he compares certain stages in England with those in

France, although he finds in both a curious accordance in the numbers of the species belonging to the

same genera, yet the species themselves differ in a manner very difficult to account for, considering the

proximity of the two areas,—unless, indeed, it be assumed that an isthmus separated two seas inhabited

by distinct, but contemporaneous, faunas. Lyell has made similar observations on some of the later

tertiary formations. Barrande, also, shows that there is a striking general parallelism in the successive

Silurian deposits ofBohemia and Scandinavia; nevertheless he finds a surprising amount of difference in

the species. If the several formations in these regions have not been deposited during the same exact

periods,—a formation in one region often corresponding with a blank interval in the other,—and if in both





regions the species have gone on slowly changing during the accumulation of the several formations and

during the long intervals of time between them; in this case the several formations in the two regions could

be arranged in the same order, in accordance with the general succession of the forms of life, and the

order would falsely appear to be strictly parallel; nevertheless the species would not be all the same in the

apparently corresponding stages in the two regions.

On the Affinities of Extinct Species to each other, and to Living Forms.

Let us now look to the mutual affinities of extinct and living species. All fall into a few grand classes; and

this fact is at once explained on the principle of descent. The more ancient any form is, the more, as a

general rule, it differs from living forms. But, as Buckland long ago remarked, extinct species can all be

classed either in still existing groups, or between them. That the extinct forms of life help to fill up the

intervals between existing genera, families, and orders, is certainly true; but as this statement has often

been ignored or even denied, it may be well to make some remarks on this subject, and to give some

instances. If we confine our attention either to the living or to the extinct species of the same class, theseries is far less perfect than if we combine both into one general system. In the writings of Professor

Owen we continually meet with the expression of generalised forms, as applied to extinct animals; and in

the writings of Agassiz, of prophetic or synthetic types; and these terms imply that such forms are in fact

intermediate or connecting links. Anotherdistinguished palæontologist, M.Gaudry,has shown in the

most striking manner that many of the fossil mammals discovered by him in Attica serve to break down

the intervals between existing genera. Cuvier ranked the Ruminants and Pachyderms as two of the most

distinct orders of mammals: but so many fossil links have been disentombed that Owen has had to alter

the whole classification, and has placed certain pachyderms in the same sub-order with ruminants; for

example, he dissolves by gradations the apparently wide interval between the pig and the camel. The

Ungulata or hoofed quadrupeds are now divided into the even-toed or odd-toed divisions; but the

Macrauchenia of S. America connects to a certain extent these two grand divisions. No one will denythat the Hipparion is intermediate between the existing horse and certain older ungulate forms. What a

wonderful connecting link in the chain of mammals is the Typotherium from S. America, as the name

given to it by Professor Gervais expresses, and which cannot be placed in any existing order. The Sirenia

form a very distinct group of mammals, and one of the most remarkable peculiarities in the existing

dugong and lamentin is the entire absence of hind limbs without even a rudiment being left; but the extinct

Halitherium had, according to Professor Flower, an ossified thigh-bone "articulated to a well-defined

acetabulum in the pelvis," and it thus makes some approach to ordinary hoofed quadrupeds, to which the

Sirenia are in other respects allied. The cetaceans or whales are widely different from all other mammals,

but the tertiary Zeuglodon and Squalodon, which have been placed by some naturalists in an order by

themselves, are considered by Professor Huxley to be undoubtedly cetaceans, "and to constituteconnecting linkswith the aquatic carnivora."

Even the wide interval between birds and reptiles has been shown by the naturalist just quoted to be

partially bridged over in the most unexpected manner, on the one hand, by the ostrich and extinct

Archeopteryx, and on the other hand, by the Compsognathus, one of the Dinosaurians—that group

which includes the most gigantic of all terrestrial reptiles.Turning to the Invertebrata, Barrande asserts,

and a higher authority could not be named, that he is every day taught that, although palæozoic animals

can certainly be classed under existing groups, yet that at this ancient period the groups were not so

distinctly separated from each other as they now are.

Some writers have objected to any extinct species, or group of species, being considered as

intermediate between any two living species, or groups of species. If by this term it is meant that an

extinct form is directly intermediate in all its characters between two living forms or groups, the objection





is probably valid. But in a natural classificationmany fossil species certainly stand between living species,

and some extinct genera between living genera, even betweengenera belonging to distinct families. The

most common case, especially with respect to very distinct groups, such as fish and reptiles, seems to be,

that, supposing them to be distinguished at the present day by a score of characters, the ancient members

are separated by a somewhat lesser number of characters; so that the two groups formerly made a

somewhat nearer approach to each other than they now do.

It is a common belief that the more ancient a form is, by so much the more it tends to connect by some

of its characters groups now widely separated from each other. This remark no doubt must be restricted

to those groups which have undergone much change in the course of geological ages; and it would be

difficult to prove the truth of the proposition, for every now and then even a living animal, as the

Lepidosiren, is discovered having affinities directed towards very distinct groups. Yet if we compare the

older Reptiles and Batrachians, the older Fish, the older Cephalopods, and the eocene Mammals, with

the more recent members of the same classes, we must admit that there is truth in the remark.

Let us see how far these several facts and inferences accord with the theory of descent with

modification. As the subject is somewhat complex, I must request the reader to turn to the diagram in thefourth chapter. We may suppose that the numbered letters in italics represent genera; and the dotted lines

diverging from them the species in each genus. The diagram is much too simple, too few genera and too

fewspecies being given, but this is unimportant for us. The horizontal lines may represent successive

geological formations, and all the forms beneath the uppermost line may be considered as extinct. The

three existing genera a14, q14, p14, will form a small family; b14and f14 a closely allied family or

sub-family; ando14,e14, m14, a third family. These three families, together with the many extinct genera

on the several lines of descent diverging from the parent-form (A) will form an order, for all will have

inherited something in common from their ancient progenitor. On the principle of the continued tendency

to divergence of character, which was formerly illustrated by this diagram, the more recent any form is,

the more it will generally differ from its ancient progenitor. Hence we can understand the rule that the

most ancient fossils differ most from existing forms.We must not, however, assume that divergence of character is a necessary contingency; it depends solely on the descendants from a species being thus

enabled to seize on many and different places in the economy of nature. Therefore it is quite possible, as

we have seen in the case of some Silurian forms, that a species might go on being slightly modified in

relation to its slightly altered conditions of life, and yet retain throughout a vast period the same general

characteristics. This is represented in the diagram by the letter F14.

All the many forms, extinct and recent, descended from (A), make, as before remarked, one order; and

this order, from the continued effects of extinction and divergence of character, has become divided into

several sub-families and families, some of which are supposed to have perished at different periods, and

some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct forms supposed to be imbedded in the

successive formations, were discovered at several points low down in the series, the three existing

families on the uppermost line would be rendered less distinct from each other. If, for instance, the genera

a1, a5, a10, f8, m3, m6, m9, were disinterred, these three families would be so closely linked together

that they probably would have to be united into one great family, in nearly the same manner as has

occurred with ruminants and certain pachyderms. Yet he who objected to consider as intermediate the

extinct genera,which thus link together the livinggenera of three families, would be partly justified, for

they are intermediate, not directly, but only by a long and circuitous course through many widely different

forms. If many extinct forms were to be discovered above one of the middle horizontal lines or geological

formations—for instance, above No. VI.—but none from beneath this line, then only two of the families

(those on the left hand, a14, &c., and b14, &c.) would have to be united into one; and there would

remain two families, which would be less distinct from each other than they were before the discovery of





the fossils. So again if the three families formed of eight genera (a14to m14), on the uppermost line, be

supposed to differ from each other by half-a-dozen important characters, then the families which existed

at the period marked VI. would certainly have differed from each other by a less number of characters;

for they would at this early stage of descent have diverged in a less degree from their common

progenitor. Thus it comes that ancient and extinct genera are often in a greater or less degree intermediate

in character between their modified descendants, or between their collateral relations.

Under nature the process will be far more complicated than is represented in the diagram; for the groups

will havebeenmore numerous; theywill have endured for extremely unequal lengths of time, and will

have been modified in various degrees. As we possess only the last volume of the geological record, and

that in a very broken condition, we have no right to expect, except in rare cases, to fill up the wide

intervals in the natural system, and thus to unite distinct families or orders. All that we have a right to

expect is, that those groupswhichhave, within known geological periods, undergone muchmodification,

should in the older formations make some slight approach to each other; so that the older members

should differ less from each other in some of their characters than do the existing members of the same

groups; and this by the concurrent evidence of our best palæontologists is frequently the case.

Thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the

extinct forms of life to each other and to living forms, are explained in a satisfactory manner. And they are

wholly inexplicable on any other view.

On this same theory, it is evident that the fauna during any one great period in the earth's history will be

intermediate in general character between that which preceded and that which succeeded it. Thus the

species which lived at the sixth great stage of descent in the diagram are the modified offspring of those

which lived at the fifth stage, and are the parents of those which became still more modified at the seventh

stage; hence they could hardly fail to be nearly intermediate in character between the forms of life above

and below. We must, however, allow for the entire extinction of some preceding forms, and in any one

region for the immigration of new forms from other regions, and for a large amount of modification duringthe long and blank intervals between the successive formations. Subject to these allowances, the fauna of

each geological periodundoubtedly is intermediate in character, between the precedingand succeeding

faunas. I need give only one instance, namely, the manner in which the fossils of the Devonian system,

when this system was first discovered, were at once recognised by palæontologists as intermediate in

character between those of the overlying carboniferous, and underlyingSilurian systems. But each fauna

is not necessarily exactly intermediate, as unequal intervals of time have elapsed between consecutive

formations.

It is no real objection to the truth of the statement that the fauna of each period as a whole is nearly

intermediate in character between the precedingand succeeding faunas, that certain genera offer exceptions to the rule. For instance, the species of mastodons and elephants, when arranged by Dr.

Falconer in two series,—in the first place according to their mutual affinities, and in the second place

according to their periods of existence,—do not accord in arrangement. The species extreme in character

are not the oldest or the most recent; nor are those which are intermediate in character, intermediate in

age. But supposing for an instant, in this and other such cases, that the record of the first appearance and

disappearance of the species was complete, which is far from the case, we have no reason to believe that

forms successivelyproduced necessarily endure for corresponding lengths of time.A very ancient form

mayoccasionally have lastedmuch longer than a form elsewhere subsequently produced, especially in the

case of terrestrial productions inhabiting separated districts. To compare small things with great; if the

principal living and extinct races of the domestic pigeonwere arranged in serial affinity, this arrangement

would not closely accord with the order in time of their production, and even less with the order of their

disappearance; for the parent rock-pigeon still lives; and many varieties between the rock-pigeon and the

carrier have become extinct; and carriers which are extreme in the important character of length of beak





originated earlier than short-beaked tumblers, which are at the opposite end of the series in this respect.

Closely connected with the statement, that the organic remains from an intermediate formation are in

some degree intermediate in character, is the fact, insisted on by all paleontologists, that fossils from two

consecutive formations are far more closely related to each other, than are the fossils from two remote

formations. Pictet gives as a well-known instance, the general resemblance of the organic remains from

the several stages of the Chalk formation, though the species are distinct in each stage. This fact alone,from its generality, seems to have shaken Professor Pictet in his belief in the immutability of species. He

who is acquainted with the distribution of existing species over the globe, will not attempt to account for

the close resemblance of distinct species in closely consecutive formations, by the physical conditions of

the ancient areas having remained nearly the same. Let it be remembered that the forms of life, at least

those inhabiting the sea, have changedalmost simultaneously throughout the world, and therefore under

the most different climatesand conditions.Consider the prodigious vicissitudes of climate during the

pleistocene period, which includes the whole glacial epoch, and note how little the specific forms of the

inhabitants of the sea have been affected.

On the theory of descent, the full meaning of the fossil remains from closely consecutive formations beingclosely related, though ranked as distinct species, is obvious. As the accumulation of each formation has

often been interrupted, and as long blank intervals have intervened between successive formations, we

ought not to expect to find, as I attempted to show in the last chapter, in any one or in any two

formations, all the intermediate varieties between the species which appeared at the commencement and

close of these periods: but we ought to find after intervals, very long as measured by years, but only

moderately long as measured geologically, closely allied forms, or, as they have been called by some

authors, representative species; and these assuredly we do find. We find, in short, such evidence of the

slow and scarcely sensible mutations of specific forms, as we have the right to expect.

On the State of Development of Ancient compared with Living Forms.

We have seen in the fourth chapter that the degree of differentiation and specialisation of the parts in

organic beings, when arrived at maturity, is the best standard, as yet suggested, of their degree of

perfection or highness. We have also seen that, as the specialisation of parts is an advantage to each

being, so natural selection will tend to render the organisation of each being more specialised and perfect,

and in this sense higher; not but that it may leave many creatures with simple and unimproved structures

fitted for simple conditions of life, and in some cases will even degrade or simplify the organisation, yet

leaving such degraded beings better fitted for their new walks of life. In another and more general

manner, new species become superior to their predecessors; for they have to beat in the struggle for lifeall the older forms, with which they come into close competition. We may therefore conclude that if under

a nearly similar climate the eocene inhabitants of the world could be put into competition with the existing

inhabitants, the former would be beaten and exterminated by the latter, as would the secondary by the

eocene, and the palæozoic by the secondary forms. So that by this fundamental test of victory in the

battle for life, as well as by the standard of the specialisation of organs, modern forms ought, on the

theory of natural selection, to stand higher than ancient forms. Is this the case? A large majority of

palæontologists would answer in the affirmative; and it seems that this answer must be admitted as true,

though difficult of proof.

It is no valid objection to this conclusion, that certain Brachiopods have been but slightly modified from

an extremely remote geological epoch; and that certain land and fresh-water shells have remained nearly

the same, from the time when, as far as is known, they first appeared. It is not an insuperable difficulty

that Foraminifera have not, as insisted on by Dr. Carpenter, progressed in organisation since even the









and Cautley's discoveries, that Northern India was formerly more closely related in its mammals to Africa

than it is at the present time. Analogous facts could be given in relation to the distribution of marine

animals.

On the theory of descent with modification, the great law of the long enduring, but not immutable,

succession of the same types within the same areas, is at once explained; for the inhabitants of each

quarter of the world will obviously tend to leave in that quarter, during the next succeeding period of time,closely allied though in some degree modified descendants. If the inhabitants of one continent formerly

differed greatly from those of another continent, sowill their modified descendants still differ in nearly the

same manner and degree. But after very long intervals of time, and after great geographical changes,

permittingmuch intermigration, the feebler will yield to the moredominant forms, and there will benothing

immutable in thedistribution of organic beings.

It may be asked in ridicule, whether I suppose that the megatherium and other allied huge monsters,

which formerly lived in South America, have left behind them the sloth, armadillo, and anteater, as their

degenerate descendants. This cannot for an instant be admitted. These huge animals have become wholly

extinct, and have left no progeny. But in the caves of Brazil, there are many extinct species which areclosely allied in size and in all other characters to the species still living in South America; and some of

these fossils may have been the actual progenitors of the living species. It must not be forgotten that, on

our theory, all the species of the same genus are the descendants of some one species; so that, if six

genera, each having eight species, be found in one geological formation, and in a succeeding formation

there be six other allied or representative genera each with the same number of species, then we may

conclude that generally only one species of each of the older genera has left modified descendants, which

constitute the new genera containing the several species; the other seven species of each old genus having

died out and left no progeny. Or, and this will be a far commoner case, two or three species in two or

three alone of the six older genera will be the parents of the new genera: the other species and the other

old genera having become utterly extinct. In failing orders, with the genera and species decreasing in

numbers as is the case with the Edentata of South America, still fewer genera and species will leavemodified blood-descendants.

Summary of the preceding and present Chapters.

I have attempted to show that the geological record is extremely imperfect; that only a small portion of

the globe has been geologically explored with care; that only certain classes of organic beings have been

largely preserved in a fossil state; that the number both of specimens and of species, preserved in our

museums, is absolutely as nothingcompared with the number of generationswhichmust have passedaway even during a single formation; that, owing to subsidence being almost necessary for the

accumulation of deposits rich in fossil species of many kinds, and thick enough to outlast future

degradation, great intervals of time must have elapsed between most of our successive formations; that

there has probably been more extinction during the periods of subsidence, and more variation during the

periods of elevation, and during the latter the record will have been less perfectly kept; that each single

formation has not been continuously deposited; that the duration of each formation is probably short

compared with the average duration of specific forms; that migration has played an important part in the

first appearance of new forms in any one area and formation; that widely ranging species are those which

have varied most frequently, and have oftenest given rise to new species; that varieties have at first been

local; and lastly, although each speciesmust have passed through numerous transitional stages, it is

probable that the periods, during which each underwentmodification, though many and long asmeasured

by years, have been short in comparison with the periods during which each remained in an unchanged

condition. These causes, taken conjointly, will to a large extent explain why-though we do find many





links—wedonot find interminable varieties, connecting together all extinct and existing forms by the

finest graduated steps. It should also be constantly borne in mind that any linking variety between two

forms, which might be found, would be ranked, unless the whole chain could be perfectly restored, as a

new and distinct species; for it is not pretended that we have any sure criterion by which species and

varieties canbe discriminated.

He who rejects this view of the imperfection of the geological record, will rightly reject the whole theory.For he may ask in vain where are the numberless transitional links which must formerly have connected

the closely allied or representative species, found in the successive stages of the same great formation?

He may disbelieve in the immense intervals of time which must have elapsed between our consecutive

formations; he may overlook how important a part migration has played, when the formations of any one

great region, as those of Europe, are considered; he may urge the apparent, but often falsely apparent,

sudden coming in of whole groups of species. He may ask where are the remains of those infinitely

numerous organisms which must have existed long before the Cambrian system was deposited? We now

know that at least one animal did then exist; but I can answer this last question only by supposing that

where our oceans now extend they have extended for an enormous period, and where our oscillating

continents now stand they have stood since the commencement of the Cambrian system; but that, long before that epoch, the world presented a widely different aspect; and that the older continents formed of

formations older than any known to us, exist now only as remnants in a metamorphosed condition, or lie

still buried under the ocean.

Passing from these difficulties, the other great leading facts in paleontology agreeadmirablywith the

theory of descent with modification throughvariation andnatural selection.We can thus understandhow

it is that new species come in slowly and, successively; how species of different classes do not

necessarily change together, or at the same rate, or in the same degree; yet in the long run that all undergo

modification to some extent. The extinction of old forms is the almost inevitable consequence of the

production of new forms. We can understand why, when a species has once disappeared, it never

reappears. Groups of species increase in numbers slowly, and endure for unequal periods of time; for the process ofmodification is necessarily slow, and depends onmany complex contingencies. The dominant

species belonging to large anddominant groups tend to leavemany modifieddescendants, which form

new sub-groups and groups. As these are formed, the species of the less vigorous groups, from their

inferiority inherited from a common progenitor, tend to become extinct together, and to leave no modified

offspring on the face of the earth. But the utter extinction of a whole group of species has sometimes been

a slow process, from the survival of a few descendants, lingering in protected and isolated situations.

When a group has once wholly disappeared, it does not reappear; for the link of generation has been

broken.

We can understand how it is that dominant forms which spread widely and yield the greatest number of varieties tend to people the world with allied, but modified, descendants; and these will generally succeed

in displacing the groupswhich are their inferiors in the struggle for existence.Hence, after long intervals of

time, the productions of the world appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and recent, make together a few grand

classes. We can understand, from the continued tendency to divergence of character, why the more

ancient a form is, the more it generally differs from those now living; why ancient and extinct forms often

tend to fill up gaps between existing forms, sometimes blending twogroups, previously classed as

distinct, into one; but more commonly bringing them only a little closer together. The more ancient a form

is, the more often it stands in some degree intermediate between groups now distinct; for the more

ancient a form is, the more nearly it will be related to, and consequently resemble, the common

progenitor of groups, since become widelydivergent.Extinct forms are seldom directly intermediate

between existing forms; but are intermediate only by a long and circuitous course through other extinct





and different forms. We can clearly see why the organic remains of closely consecutive formations are

closely allied; for they are closely linked together by generation. We can clearly see why the remains of

an intermediate formation are intermediate in character.

The inhabitants of the world at each successive period in its history have beaten their predecessors in the

race for life, and are, in so far, higher in the scale, and their structure has generally become more

specialised; and this may account for the common belief held by so many paleontologists, thatorganisation on the whole has progressed. Extinct and ancient animals resemble to a certain extent the

embryos of the more recent animals belonging to the same classes, and this wonderful fact receives a

simple explanation according to our views. The succession of the same types of structure within the same

areas during the later geological periods ceases to be mysterious, and is intelligible on the principle of

inheritance.

If then the geological record be as imperfect as many believe, and it may at least be asserted that the

record cannot be proved to be much more perfect, the main objections to the theory of natural selection

are greatly diminished or disappear. On the other hand, all the chief laws of paleontology plainly

proclaim, as it seems to me, that species have been produced by ordinary generation: old forms having been supplanted by new and improved forms of life, the products of Variation and the Survival of the

Fittest.

|Go to Contents |

Chapter XII

Geographical Distribution.

Present distribution cannot be accounted for by differences in physical conditions—Importanceof

barriers—Affinity of the productions of the same continent—Centres of creation—Meansof dispersal by

changes of climate and of the level of the land, and by occasional means—Dispersal during the Glacial period—Alternate Glacial periods in the North and South.

IN considering the distribution of organic beings over the face of the globe, the first great fact which

strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be

wholly accounted for by climatal and other physical conditions. Of late, almost every author who has

studied the subject has come to this conclusion. The case of America alone would almost suffice to prove

its truth; for if we exclude the arctic and northern temperate parts, all authors agree that one of the most

fundamental divisions in geographical distribution is that between the New and OldWorlds; yet if we

travel over the vast American continent, from the central parts of the United States to its extremesouthernpoint, we meet with themost diversified conditions; humiddistricts, arid deserts, lofty

mountains, grassy plains, forests, marshes, lakes, and great rivers, under almost every temperature. There





is hardly a climate or condition in the Old World which cannot be paralleled in the New—at least as

closely as the same species generally require. No doubt small areas can be pointed out in the Old World

hotter than any in the NewWorld; but these are not inhabited by a fauna different from that of the

surrounding districts; for it is rare to find a group of organisms confined to a small area, of which the

conditions arepeculiar in only a slight degree. Notwithstanding this general parallelism in the conditions of

the Old and New Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in Australia, South Africa, and western

SouthAmerica, between latitudes25°and 35°,we shall find parts extremely similar in all their conditions,

yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or, again, we may

compare the productions of South America south of lat. 35°with those north of 25°, which consequently

are separated by a space of ten degrees of latitude, and are exposed to considerably different conditions;

yet they are incomparably more closely related to each other than they are to the productions of Australia

or Africa under nearly the same climate. Analogous facts could be given with respect to the inhabitants of

the sea.

A second great fact which strikes us in our general review is, that barriers of any kind, or obstacles tofree migration, are related in a close and important manner to the differences between the productions of

various regions. We see this in the great difference in nearly all the terrestrial productions of the New and

OldWorlds, excepting in the northern parts, where the land almost joins, and where, under a slightly

different climate, there might have been free migration for the northern temperate forms, as there now is

for the strictly arctic productions. We see the same fact in the great difference between the inhabitants of

Australia, Africa, and South America under the same latitude; for these countries are almost as much

isolated from each other as is possible. On each continent, also, we see the same fact; for on the

opposite sides of lofty and continuous mountain-ranges, of great deserts and even of large rivers, we find

different productions; though as mountain-chains, deserts, &c., are not as impassable, or likely to have

endured so long, as the oceans separating continents, the differences are very inferior in degree to those

characteristic of distinct continents.

Turning to the sea, we find the same law. The marine inhabitants of the eastern and western shores of

South America are very distinct, with extremely few shells, crustacea, or echinodermata in common; but

Dr. Günther has recently shown that about thirty per cent. of the fishes are the same on the opposite sides

of the isthmus of Panama; and this fact has led naturalists to believe that the isthmus was formerly open.

Westward of the shores of America, a wide space of open ocean extends, with not an island as a

halting-place for emigrants; here we have a barrier of another kind, and as soon as this is passed we meet

in the eastern islands of the Pacific with another and totally distinct fauna. So that three marine faunas

range far northward and southward in parallel lines not far from each other, under corresponding

climates; but from being separated from each other by impassable barriers, either of land or open sea,they are almost wholly distinct. On the other hand, proceeding still farther westward from the eastern

islands of the tropical parts of the Pacific, we encounter no impassable barriers, and we have innumerable

islands as halting-places, or continuous coasts, until, after travelling over a hemisphere, we come to the

shores of Africa; and over this vast space we meet with no well-defined and distinct marine faunas.

Although so few marine animals are common to the above-named three approximate faunas of Eastern

andWestern America and the Eastern Pacific islands, yet many fishes range from the Pacific into the

Indian Ocean, and many shells are common to the eastern islands of the Pacific and the eastern shores of

Africa on almost exactly opposite meridians of longitude.

A third great fact, partly included in the foregoing statement, is the affinity of the productions of the same

continent or of the same sea, though the species themselves are distinct at different points and stations. It

is a lawof the widest generality, andeverycontinentoffers innumerable instances.Nevertheless the

naturalist, in travelling, for instance, from north to south, never fails to be struck by the manner in which





successivegroups of beings, specifically distinct, though nearly related, replace each other.He hears from

closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed,

but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan

are inhabited by one species of Rhea (American ostrich) and northward the plains of La Plata by another

species of the same genus; and not by a true ostrich or emu, like those inhabiting Africa and Australia

under the same latitude. On these same plains of La Plata we see the agouti and bizacha, animals having

nearly the same habits as our hares and rabbits, and belonging to the same order of Rodents, but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera, and we find

an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the

coypu and capybara, rodents of the S. American type. Innumerable other instances could be given. If we

look to the islands off the American shore, however much they may differ in geological structure, the

inhabitants are essentially American, though they may be all peculiar species. We may look back to past

ages, as shown in the last chapter, and we find American types then prevailing on the American continent

and in the American seas. We see in these facts some deep organic bond, throughout space and time,

over the same areas of land and water, independently of physical conditions. The naturalist must be dull

who is not led to enquire what this bond is.

The bond is simply inheritance, that cause which alone, as far as we positively know, produces

organisms quite like each other, or, as we see in the case of varieties, nearly alike. The dissimilarity of the

inhabitants of different regionsmay be attributed tomodification through variation and natural selection,

and probably in a subordinate degree to thedefinite influence of different physical conditions. The

degrees of dissimilarity will depend on the migration of the more dominant forms of life from one region

into another having been more or less effectually prevented, at periods more or less remote;—on the

nature and number of the former immigrants;—and on the action of the inhabitants on each other in

leading to the preservation of differentmodifications; the relation of organism to organism in the struggle

for life being, as I have already often remarked, the most important of all relations. Thus the high

importance of barriers comes into play by checking migration; as does time for the slow process of

modification through natural selection. Widely-ranging species, abounding in individuals,which havealready triumphed over many competitors in their own widely-extended homes, will have the best chance

of seizing on new places, when they spread into new countries. In their new homes they will be exposed

to new conditions, and will frequently undergo further modification and improvement; and thus theywill

become still further victorious, and will produce groups of modified descendants. On this principle of

inheritance with modification we can understand how it is that sections of genera, whole genera, and even

families, are confined to the same areas, as is so commonly and notoriously the case.

There is no evidence, as was remarked in the last chapter, of the existence of any law of necessary

development. As the variability of each species is an independent property, and will be taken advantage

of by natural selection, only so far as it profits each individual in its complex struggle for life, so theamount of modification in different species will be no uniform quantity. If a number of species, after

having long competed with each other in their old home, were to migrate in a body into a new and

afterwards isolated country, theywould be little liable tomodification; for neither migrationnor isolation in

themselves effect anything. These principles come into play only bybringingorganisms into new relations

with each other and in a lesser degree with the surrounding physical conditions. As we have seen in the

last chapter that some forms have retained nearly the same character from an enormously remote

geological period, so certain species have migrated over vast spaces, and have not become greatly or at

all modified.

According to these views, it is obvious that the several species of the same genus, though inhabiting the

most distant quarters of the world, must originally have proceeded from the same source, as they are

descended from the same progenitor. In the case of those species which have undergone during whole

geological periods littlemodification, there is not much difficulty in believing that they havemigrated from





the same region; for during the vast geographical and climatal changeswhichhave supervened since

ancient times, almost any amount of migration is possible. But in many other cases, in which we have

reason to believe that the species of a genus have been produced within comparatively recent times, there

is great difficulty on this head. It is also obvious that the individuals of the same species, though now

inhabiting distant and isolated regions, must have proceeded from one spot, where their parents were first

produced: for, as has been explained, it is incredible that individuals identically the same should have been

produced fromparents specifically distinct.

Single Centres of supposed Creation.—We are thus brought to the question which has been largely

discussed by naturalists, namely, whether species have been created at one or more points of the earth's

surface.Undoubtedly there aremany cases of extreme difficulty in understanding how the same species

could possibly have migrated from some one point to the several distant and isolated points, where now

found. Nevertheless the simplicity of the view that each species was first produced within a single region

captivates the mind. He who rejects it, rejects thevera causa of ordinarygenerationwith subsequent

migration, and calls in the agency of a miracle. It is universally admitted, that in most cases the area

inhabited by a species is continuous; and that when a plant or animal inhabits two points so distant from

each other, or with an interval of such a nature, that the space could not have been easily passed over bymigration, the fact is given as something remarkable and exceptional. The incapacity ofmigrating across a

wide sea is more clear in the case of terrestrial mammals than perhaps with any other organic beings; and,

accordingly, we find no inexplicable instances of the samemammals inhabiting distant points of the world.

No geologist feels any difficulty in Great Britain possessing the same quadrupeds with the rest of Europe,

for they were no doubt once united. But if the same species can be produced at two separate points,

why do we not find a single mammal common to Europe and Australia or South America? The conditions

of life are nearly the same, so that a multitude of European animals and plants have become naturalised in

America and Australia; and some of the aboriginal plants are identically the same at these distant points of

the northern and southern hemispheres? The answer, as I believe, is, that mammals have not been able to

migrate, whereas some plants, from their varied means of dispersal, have migrated across the wide and

broken interspaces. The great and striking influence of barriers of all kinds, is intelligible only on the viewthat the great majority of species have been produced on one side, and have not been able to migrate to

the opposite side. Some few families, many sub-families, very many genera, and a still greater number of

sections of genera, are confined to a single region; and it has been observed by several naturalists that the

most natural genera, or those genera in which the species are most closely related to each other, are

generally confined to the same country, or if they have a wide range that their range is continuous. What a

strange anomaly it would be, if a directly opposite rule were to prevail, when we go down one step lower

in the series, namely, to the individuals of the same species, and these had not been, at least at first,

confined to some one region!

Hence it seems to me, as it has to many other naturalists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of

migration and subsistence under past and present conditions permitted, is the most probable.

Undoubtedly many cases occur, in which we cannot explain how the same species could have passed

from one point to the other. But the geographical and climatal changes which have certainly occurred

within recent geological times,must have rendered discontinuous the formerly continuous range ofmany

species. So that we are reduced to consider whether the exceptions to continuity of range are so

numerous and of so grave a nature, that we ought to give up the belief, rendered probable by general

considerations, that each species has been produced within one are and has migrated thence as far as it

could. It would be hopelessly tedious to discuss all the exceptional cases of the same species, now living

at distant and separated points, nor do I for a moment pretend that any explanation could be offered of

many instances. But, after some preliminary remarks, I will discuss a few of the most striking classes of

facts; namely, the existence of the same species on the summits of distant mountain ranges, and at distant

points in the arctic and antarctic regions; and secondly (in the following chapter), the wide distribution of





freshwater productions; and thirdly, the occurrence of the same terrestrial species on islands and on the

nearest mainland, though separated by hundreds of miles of open sea. If the existence of the same

species at distant and isolated points of the earth's surface, can in many instances be explained on the

viewof each species havingmigrated from a single birthplace; then, considering our ignorance with

respect to former climatal and geographical changes and to the various occasional means of transport, the

belief that a single birthplace is the law, seems to me incomparably the safest.

In discussing this subject, we shall be enabled at the same time to consider a point equally important for

us, namely, whether the several species of a genus which must on our theory all be descended from a

commonprogenitor, can havemigrated, undergoingmodification during their migration, from someone

area. If, when most of the species inhabiting one region are different from those of another region, though

closely allied to them, it can be shown that migration from the one region to the other has probably

occurred at some former period, our general view will be much strengthened; for the explanation is

obvious on theprinciple of descent with modification.A volcanic island, for instance, upheavedand

formed at the distance of a few hundreds of miles from a continent, would probably receive from it in the

course of time a few colonists, and their descendants, though modified, would still be related by

inheritance to the inhabitants of that continent. Cases of this nature are common, and are, as we shallhereafter see, inexplicable on the theory of independent creation. This view of the relation of the species

of one region to those of another, does not differ much from that advanced by Mr. Wallace, who

concludes that "every species has come into existence coincident both in space and time with a

pre-existing closely allied species." And it is now well known that he attributes this coincidence to

descent with modification.

The question of single or multiple centres of creation differs from another though allied

question,—namely, whether all the individuals of the same species are descended from a single pair, or

single hermaphrodite, orwhether, as some authors suppose, from many individuals simultaneously

created. With organic beings which never intercross, if such exist, each species must be descended from

a succession of modified varieties, that have supplanted each other, but have never blended with other individuals or varieties of the same species; so that, at each successive stage of modification, all the

individuals of the same form will be descended from a single parent. But in the great majority of cases,

namely, with all organisms whichhabitually unite for each birth, orwhich occasionally intercross, the

individuals of the same species inhabiting the same area will be kept nearly uniform by intercrossing; so

thatmany individuals will goon simultaneously changing, and the whole amount ofmodification at each

stage will not be due to descent from a single parent. To illustrate what I mean: our English race-horses

differ from the horses of every other breed; but they do not owe their difference and superiority to

descent fromany single pair, but to continued care in the selecting and training ofmany individuals during

each generation.

Before discussing the three classes of facts, which I have selected as presenting the greatest amount of

difficulty on the theory of "single centres of creation," I must say a few words on the means of dispersal.

Means of Dispersal .

Sir C. Lyell and other authors have ably treated this subject. I can give here only the briefest abstract of

the more important facts. Change of climate must have had a powerful influence on migration. A region

now impassable to certain organisms from the nature of its climate, might have been a high road for

migration, when the climate was different. I shall, however, presently have to discuss this branch of the

subject in some detail. Changes of level in the land must also have been highly influential: a narrow

isthmus now separates two marine faunas; submerge it, or let it formerly have been submerged, and the





two faunas will now blend together, or may formerly have blended. Where the sea now extends, land

may at a former period have connected islands or possibly even continents together, and thus have

allowed terrestrial productions to pass from one to the other. No geologist disputes that great mutations

of level have occurred within the period of existing organisms. Edward Forbes insisted that all the islands

in the Atlantic must have been recently connected with Europe or Africa, and Europe likewise with

America. Other authors have thus hypothetically bridged over every ocean, and united almost every

island with some mainland. If indeed the arguments used by Forbes are to be trusted, it must be admittedthat scarcely a single island exists which has not recently been united to some continent. This view cuts

the Gordian knot of the dispersal of the same species to the more distant points, and removes many a

difficulty; but to the best of my judgment we are not authorised in admitting such enormous geographical

changes within the period of existing species. It seems to me that we have abundant evidence of great

oscillations in the level of the land or sea; but not of such vast changes in the position and extension of our

continents, as to have united them within the recent period to each other and to the several intervening

oceanic islands. I freely admit the former existence of many islands, now buried beneath the sea, which

may have served as halting-places for plants and for many animals during their migration. In the

coral-producing oceans such sunken islands are now marked by rings of coral or atolls standing over

them.Whenever it is fully admitted, as it will some day be, that each species has proceeded from a single birthplace, and when in the course of time we know something definite about the means of distribution,

we shall be enabled to speculate with security on the former extension of the land. But I do not believe

that it will ever be proved that within the recent period most of our continents which now stand quite

separate have been continuously, or almost continuously united with each other, and with the many

existing oceanic islands. Several facts in distribution,—suchas the great difference in the marine faunas on

the opposite sides of almost every continent,—the close relation of the tertiary inhabitants of several lands

and even seas to their present inhabitants,—the degree of affinitybetween the mammals inhabiting islands

with those of the nearest continent, being in part determined (as we shall hereafter see) by the depth of

the intervening ocean,—these and other such facts are opposed to the admission of such prodigious

geographical revolutions within the recent period, as are necessary on the view advanced by Forbes and

admitted by his followers. The nature and relative proportions of the inhabitants of oceanic islands arelikewise opposed to the belief of their former continuity with continents. Nor does the almost universally

volcanic composition of such islands favour the admission that they are the wrecks of sunken

continents;—if they had originally existedas continentalmountain ranges, some at least of the islands

would have been formed, like other mountain summits, of granite, metamorphic schists, old fossiliferous

and other rocks, instead of consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental means, but which more properly should be

called occasional means of distribution. I shall here confine myself to plants. In botanical works, this or

that plant is often stated to be ill adapted for wide dissemination; but the greater or less facilities for

transport across the sea may be said to be almost wholly unknown. Until I tried, with Mr. Berkeley's aid,a few experiments, it was not even known how far seeds could resist the injurious action of sea-water.

To my surprise I found that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few

survived an immersion of 137 days. It deserves notice that certain orders were far more injured than

others: nine Leguminosæ were tried, and, with one exception, they resisted the salt-water badly; seven

species of the allied orders, Hydrophyllaceæ and Polemoniaceæ, were all killed by a month's immersion.

For convenience' sake I chiefly tried small seeds without the capsule or fruit; and as all of these sank in a

few days they could not have been floated across wide spaces of the sea, whether or not they were

injured by the salt-water. Afterwards I tried some larger fruits, capsules, &c., and some of these floated

for a long time. It is well known what a difference there is in the buoyancy of green and seasoned timber;

and it occurred to me that floods would often wash into the sea dried plants or branches with

seed-capsules or fruit attached to them. Hence I was led to dry the stems and branches of 94 plants with

ripe fruit, and to place them on sea-water. The majority sank rapidly, but some which, whilst green,

floated for a short time,when dried floated much longer; for instance, ripe hazelnuts sank immediately,





but when dried they floated for 90 days, and afterwards when planted germinated; an asparagus-plant

with ripe berries floated for 23 days, when dried it floated for 85 days, and the seeds afterwards

germinated; the ripe seeds of Helosciadium sank in two days, when dried they floated for above 90 days,

and afterwards germinated. Altogether, out of the 94 dried plants, 18 floated for above 28 days; and

some of the 18 floated for a very much longer period. So that as kinds of seeds germinated after an

immersion of 28 days; and as distinct species with ripe fruit (but not all the same species as in the

foregoing experiment) floated, after being dried, for above 28 days, we may conclude, as far as anything

can be inferred from these scanty facts, that the seeds of kinds of plants of any country might be

floated by sea-currents during 28 days, and would retain their power of germination. In Johnston's

Physical Atlas, the average rate of the several Atlantic currents is 33 miles per diem (some currents

running at the rate of 60 miles per diem); on this average, the seeds of plants belonging to one

country might be floated across 924 miles of sea to another country, and when stranded, if blown by an

inland gale to a favourable spot, would germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in a much better manner, for he

placed the seeds in a box in the actual sea, so that they were alternately wet and exposed to the air likereally floating plants. He tried 98 seeds, mostly different from mine; but he chose many large fruits and

likewise seeds from plants which live near the sea; and this would have favoured both the average length

of their flotation and their resistance to the injurious action of the salt-water. On the other hand, he did

not previously dry the plants or branches with the fruit; and this, as we have seen, would have caused

some of them to have floated much longer. The result was that of his seeds of different kinds floated

for 42 days, and were then capable of germination. But I do not doubt that plants exposed to the waves

would float for a less time than those protected from violent movement as in our experiments. Therefore it

would perhaps be safer to assume that the seeds of about plants of a flora, after having been dried,

could be floated across a space of sea 900 miles in width, and would then germinate. The fact of the

larger fruits often floating longer than the small, is interesting; as plants with large seeds or fruit which, asAlph. de Candolle has shown, generally have restricted ranges, could hardly be transported by any other

means.

Seeds may be occasionally transported in another manner. Drift timber is thrown up on most islands,

even on those in the midst of the widest oceans; and the natives of the coral-islands in the Pacific procure

stones for their tools, solely from the roots of drifted trees, these stones being a valuable royal tax. I find

that when irregularly shaped stones are embedded in the roots of trees, small parcels of earth are

frequently enclosed in their interstices and behind them,—so perfectly that not a particle could be washed

away during the longest transport: out of one small portion of earth thuscompletely enclosed by the roots

of an oak about 50 years old, three dicotyledonous plants germinated: I am certain of the accuracy of thisobservation. Again, I can show that the carcases of birds, when floating on the sea, sometimes escape

being immediately devoured: and many kinds of seeds in the crops of floating birds long retain their

vitality: peas and vetches, for instance, are killed by even a few days' immersion in sea-water; but some

taken out of the crop of a pigeon, which had floated on artificial sea-water for 30 days, to my surprise

nearly all germinated.

Living birds can hardly fail to be highly effective agents in the transportation of seeds. I could give many

facts showing how frequently birds of many kinds are blown by gales to vast distances across the ocean.

We may safely assume that under such circumstances their rate of flight would often be 35 miles an hour;

and some authors have given a far higher estimate. I have never seen an instance of nutritious seeds

passing through the intestines of a bird; but hard seeds of fruit pass uninjured through even the digestiveorgans of a turkey. In the course of two months, I picked up in my garden 12 kinds of seeds, out of the

excrement of small birds, and these seemed perfect, and some of them, which were tried, germinated.





But the following fact is more important: the crops of birds do not secrete gastric juice, and do not, as I

know by trial, injure in the least the germination of seeds; now, after a bird has found and devoured a

large supply of food, it is positively asserted that all the grains do not pass into the gizzard for twelve or

even eighteen hours. A bird in this interval might easily be blown to the distance of 500 miles, and hawks

are known to look out for tired birds, and the contents of their torn crops might thus readily get scattered.

Some hawks and owls bolt their prey whole, and, after an interval of from twelve to twenty hours,

disgorge pellets, which, as I know from experiments made in the Zoological Gardens, include seedscapable of germination. Some seeds of the oat, wheat, millet, canary, hemp, clover, and beet germinated

after having been from twelve to twenty-one hours in the stomachs of different birds of prey; and two

seeds of beet grew after having been thus retained for two days and fourteen hours. Fresh-water fish, I

find, eat seeds of many land and water plants; fish are frequently devoured by birds, and thus the seeds

might be transported from place to place. I forced many kinds of seeds into the stomachs of dead fish,

and then gave their bodies to fishing-eagles, storks, and pelicans; these birds, after an interval of many

hours, either rejected the seeds in pellets or passed them in their excrement; and several of these seeds

retained the power of germination. Certain seeds, however, were always killed by this process.

Locusts are sometimes blown to great distances from the land; I myself caught one 370 miles from thecoast of Africa, and have heard of others caught at greater distances. The Rev. R. T. Lowe informed Sir

C. Lyell that in November 1844 swarms of locusts visited the island of Madeira, They were in countless

numbers, as thick as the flakes of snow in the heaviest snowstorm, and extended upwards as far as could

be seen with a telescope. During two or three days they slowly careered round and round in an immense

ellipse, at least five or six miles in diameter, and at night alighted on the taller trees, which were

completely coated with them. They then disappeared over the sea, as suddenly as they had appeared,

and have not since visited the island. Now, in parts of Natal it is believed by some farmers, though on

insufficient evidence, that injurious seeds are introduced into their grass-land in the dung left by the great

flights of locusts which often visit that country. In consequence of this belief Mr. Weale sent me in a letter

a small packet of the dried pellets, out of which I extracted under the microscope several seeds, and

raised from them seven grass plants, belonging to two species, of two genera. Hence a swarm of locusts,such as that which visitedMadeira, might readily be the means of introducing several kinds of plants into

an island lying far fromthe mainland.

Although the beaks and feet of birds are generally clean, earth sometimes adheres to them: in one case I

removed sixty-one grains, and in another case twenty-two grains of dry argillaceous earth from the foot

of a partridge, and in the earth there was a pebble as large as the seed of a vetch. Here is a better case:

the leg of a woodcock was sent to me by a friend, with a little cake of dry earth attached to the shank,

weighing only nine grains; and this contained a seed of the toad-rush (Juncus bufonius) whichgerminated

and flowered. Mr. Swaysland, of Brighton, who during the last forty years has paid close attention to our

migratory birds, informsme that he has often shot wagtails (Motacillæ),wheatears, andwhinchats(Saxicolæ), on their first arrival on our shores, before they had alighted; and he has several times noticed

little cakes of earth attached to their feet. Many facts could be given showing how generally soil is

charged with seeds. For instance, Prof. Newton sent me the leg of a red-legged partridge (Caccabis

rufa) which had been wounded and could not fly, with a ball of hard earth adhering to it, and weighing six

and a half ounces. The earth had been kept for three years, but when broken, watered and placed under

a bell glass, no less than 82 plants sprung from it: these consisted of 12 monocotyledons, including the

common oat, and at least one kind of grass, and of 70 dicotyledons, which consisted, judging from the

young leaves, of at least three distinct species. With such facts before us, can we doubt that the many

birds which are annually blown by gales across great spaces of ocean, and which annually migrate—for

instance, themillions of quails across the Mediterranean—must occasionally transport a fewseeds

embedded in dirt adhering to their feet or beaks? But I shall have to recur to this subject.

As icebergs are known to be sometimes loaded with earth and stones, and have even carried





brushwood, bones, and the nest of a land-bird, it can hardly be doubted that they must occasionally, as

suggested by Lyell, have transported seeds from one part to another of the arctic and antarctic regions;

and during the Glacial period from one part of the now temperate regions to another. In the Azores, from

the large number of plants common to Europe, in comparison with the species on the other islands of the

Atlantic, which stand nearer to the mainland, and (as remarked by Mr. H. C. Watson) from their

somewhat northern character in comparison with the latitude, I suspected that these islands had been

partly stocked by ice-borne seeds, during the Glacial epoch. At my request Sir C. Lyell wrote to M.Hartung to inquire whether he had observed erratic boulders on these islands, and he answered that he

had found large fragments of granite and other rocks, which do not occur in the archipelago. Hence we

may safely infer that icebergs formerly landed their rocky burthens on the shores of these mid-ocean

islands, and it is at least possible that they may have brought thither some few seeds of northern plants.

Considering that these several means of transport, and that other means, which without doubt remain to

be discovered, have been in action year after year for tens of thousands of years, it would, I think, be a

marvellous fact if many plants had not thus become widely transported. These means of transport are

sometimes called accidental, but this is not strictly correct: the currents of the sea are not accidental, nor

is the direction of prevalent gales of wind. It should be observed that scarcely any means of transportwould carry seeds for very great distances: for seeds do not retain their vitality when exposed for a great

length of time to the action of sea-water; nor could they be long carried in the crops or intestines of birds.

These means, however, would suffice for occasional transport across tracts of sea some hundred miles in

breadth, or from island to island, or from a continent to a neighbouring island, but not from one distant

continent to another. The floras of distant continents would not by such means become mingled; but

would remain as distinct as they now are. The currents, from their course, would never bring seeds from

North America to Britain, though they might and do bring seeds from the West Indies to our western

shores, where, if not killed by their very long immersion in salt water, they could not endure our climate.

Almost every year, one or two land-birds are blown across the whole Atlantic Ocean, from North

America to the western shores of Ireland and England; but seeds could be transported by these rare

wanderers only by one means, namely, by dirt adhering to their feet or beaks, which is in itself a rareaccident. Even in this case, how small would be the chance of a seed falling on favourable soil, and

coming to maturity! But it would be a great error to argue that because a well-stocked island, like Great

Britain, has not, as far as is known (and it would be very difficult to prove this), received within the last

fewcenturies, through occasional means of transport, immigrants from Europe or any other continent,

that a poorly-stocked island, though standing more remote from the mainland, would not receive

colonists by similar means. Out of a hundred kinds of seeds or animals transported to an island, even if

far less well-stocked than Britain, perhaps not more than one would be so well fitted to its new home, as

to become naturalised. But this is no valid argument against what would be effected by occasional means

of transport, during the long lapse of geological time, whilst the island was being upheaved, and before it

had become fully stocked with inhabitants. On almost bare land, with few or no destructive insects or birds living there, nearly every seed which chanced to arrive, if fitted for the climate, would germinate and

survive.

Dispersal during the Glacial Period .

The identity of many plants and animals, on mountain-summits, separated from each other by hundreds

of miles of lowlands, where Alpine species could not possibly exist, is one of the most striking cases

known of the same species living at distant points, without the apparent possibilityof their having

migrated from one point to the other. It is indeed a remarkable fact to see so many plants of the same

species living on the snowy regions of the Alps or Pyrenees, and in the extreme northern parts of Europe;

but it is far more remarkable, that the plants on the White Mountains, in the United States of America,





are all the same with those of Labrador, and nearly all the same, as we hear from Asa Gray, with those

on the loftiest mountains of Europe. Even as long ago as 1747, such facts led Gmelin to conclude that the

same species must have been independently created at many distinct points; and we might have remained

in this same belief, had not Agassiz and others called vivid attention to the Glacial period, which, as we

shall immediately see, affords a simple explanation of these facts. We have evidence of almost every

conceivable kind, organic and inorganic, that, within a very recent geological period, central Europe and

North America suffered under an arctic climate. The ruins of a house burnt by fire do not tell their talemore plainly than do the mountains of Scotland andWales, with their scored flanks, polished surfaces,

and perched boulders, of the icy streams with which their valleys were lately filled. So greatly has the

climate of Europe changed, that in Northern Italy, gigantic moraines, left by old glaciers, are now clothed

by the vine and maize. Throughout a large part of the United States, erratic boulders and scored rocks

plainly reveal a former cold period.

The former influence of the glacial climate on the distribution of the inhabitants of Europe, as explained

by Edward Forbes, is substantially as follows. But we shall follow the changes more readily, by

supposing a new glacial period slowly to come on, and then pass away, as formerly occurred. As the

cold came on, and as each more southern zone became fitted for the inhabitants of the north, these wouldtake the places of the former inhabitants of the temperate regions. The latter, at the same time, would

travel further and further southward, unless they were stopped by barriers, in which case they would

perish. The mountains would become covered with snow and ice, and their former Alpine inhabitants

would descend to the plains. By the time that the cold had reached its maximum, we should have an

arctic fauna and flora, covering the central parts of Europe, as far south as the Alps and Pyrenees, and

even stretching into Spain. The now temperate regions of the United States would likewise be covered

by arctic plants and animals and these would be nearly the same with those of Europe; for the present

circumpolar inhabitants, whichwe suppose to have everywhere travelled southward, are remarkably

uniform round the world.

As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat bythe productions of the more temperate regions. And as the snow melted from the bases of the mountains,

the arctic forms would seize on the cleared and thawed ground, always ascending, as the warmth

increased and the snow still further disappeared, higher and higher, whilst their brethrenwere pursuing

their northern journey. Hence, when the warmth had fully returned, the same species, which had lately

lived together on the European and North American lowlands, would again be found in the arctic regions

of the Old and NewWorlds, and on many isolated mountain-summits far distant from each other.

Thus we can understand the identity of many plants at points so immensely remote as the mountains of

the United States and those of Europe. We can thus also understand the fact that the Alpine plants of

each mountain-range are more especially related to the arctic forms living due north or nearly due northof them: for the first migration when the cold came on, and the re-migration on the returning warmth,

would generally have been due south and north. The Alpine plants, for example, of Scotland, as

remarked by Mr. H. C. Watson, and those of the Pyrenees, as remarked by Ramond, are more

especially allied to the plants of northern Scandinavia; those of the United States to Labrador; those of

the mountains of Siberia to the arctic regions of that country. These views, grounded as they are on the

perfectly well-ascertained occurrence of a former Glacial period, seem to me to explain in so satisfactory

a manner the present distribution of the Alpine and Arctic productions of Europe and America, that when

in other regionswe find the same species on distant mountain-summits, wemay almost conclude, without

other evidence, that a colder climate formerly permitted their migration across the intervening lowlands,

now become too warm for their existence.

As the arctic forms moved first southward and afterwards backwards to the north, in unison with the

changingclimate, theywill not havebeen exposed during their longmigrations to any great diversity of





temperature; and as they all migrated in a body together, their mutual relations will not have been much

disturbed. Hence, in accordance with the principles inculcated in this volume, these forms will not have

been liable to much modification. But with the Alpine productions, left isolated from the moment of the

returning warmth, first at the bases and ultimately on the summits of the mountains, the case will have

been somewhat different; for it is not likely that all the same arctic species will have been left on

mountain-ranges far distant from each other, and have survived there ever since; they will also in all

probability, have becomemingledwith ancient Alpine species, whichmust have existedon the mountains before the commencement of the Glacial epoch, and which during the coldest period will have been

temporarily driven down to the plains; they will, also, have been subsequently exposed to somewhat

different climatal influences. Their mutual relationswill thushavebeen in somedegree disturbed;

consequently they will have been liable to modification; and they have been modified; for if we compare

the present Alpine plants and animals of the several great European mountain-ranges one with another,

though many of the species remain identically the same, some exist as varieties, some as doubtful forms

or sub-species, and some as distinct yet closely allied species representing each other on the several

ranges.

In the foregoing illustration I have assumed that at the commencement of our imaginary Glacial period,the arctic productions were as uniform round the polar regions as they are at the present day. But it is

also necessary to assume that many sub-arctic and some few temperate forms were the same round the

world, for some of the species which now exist on the lower mountain-slopes and on the plains of North

America and Europe are the same; and it may be asked how I account for this degree of uniformity in the

sub-arctic and temperate forms round the world, at the commencement of the real Glacial period. At the

present day, the sub-arctic and northern temperate productions of the Old and New Worlds are

separated from each other by the whole Atlantic Ocean and by the northern part of the Pacific. During

the Glacial period, when the inhabitants of the Old and NewWorlds lived farther southwards than they

do at present, they must have been still more completely separated from each other by wider spaces of

ocean; so that it may well be asked how the same species could then or previously have entered the two

continents. The explanation, I believe, lies in the nature of the climate before the commencement of theGlacial period. At this, the newer Pliocene period, the majority of the inhabitants of the world were

specifically the same as now, and we have good reason to believe that the climate was warmer than at

the present day. Hence we may suppose that the organisms which now live under latitude 60° , lived

during the Pliocene period farther north under the Polar Circle, in latitude 66° -67°; and that the present

arctic productions then lived on the broken land still nearer to the pole. Now, if we look at a terrestrial

globe, we see under the Polar Circle that there is almost continuous land from western Europe, through

Siberia, to eastern America. And this continuity of the circumpolar land, with the consequent freedom

under a more favourable climate for intermigration, will account for the supposed uniformity of the

sub-arctic and temperate productions of the Old and NewWorlds, at a period anterior to the Glacial

epoch.

Believing, from reasons before alluded to, that our continents have long remained in nearly the same

relative position, though subjected to great oscillations of level, I am strongly inclined to extend the above

view, and to infer that during some still earlier and still warmer period, such as the older Pliocene period,

a large number of the same plants and animals inhabited the almost continuous circumpolar land; and that

these plants and animals, both in the Old and NewWorlds, began slowly to migrate southwards as the

climate became less warm, long before the commencement of the Glacial period. We now see, as I

believe, their descendants, mostly in a modified condition, in the central parts of Europe and the United

States. On this view we can understand the relationship with very little identity, between the productions

ofNorthAmerica andEurope,—a relationshipwhich is highly remarkable, considering the distance of the

two areas, and their separation by the whole Atlantic Ocean. We can further understand the singular fact

remarked on by several observers that the productions of Europe and America during the later tertiary

stages were more closely related to each other than they are at the present time; for during these warmer





periods the northern parts of the Old and NewWorlds will have been almost continuously united by land,

serving as a bridge, since rendered impassable by cold, for the intermigration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which

inhabited the New and Old Worlds, migrated south of the Polar Circle, they will have been completely

cut off from each other. This separation, as far as the more temperate productions are concerned, must

have taken place long ages ago. As the plants and animals migrated southward, they will have becomemingled in the one great region with the native American productions, and would have had to compete

with them; and in the other great region, with those of the Old World. Consequently we have here

everything favourable formuch modification,—for farmore modification than with theAlpine

productions, left isolated, within a much more recent period, on the several mountain-ranges and on the

arctic lands of Europe and N. America. Hence it has come, that when we compare the now living

productions of the temperate regions of the New and Old Worlds, we find very few identical species

(though Asa Gray has lately shown that more plants are identical than was formerly supposed), but we

find in every great class many forms, which some naturalists rank as geographical races, and others as

distinct species; and a host of closely allied or representative forms which are ranked by all naturalists as

specifically distinct.

As on the land, so in the waters of the sea, a slow southern migration of a marine fauna, which, during

the Pliocene or even a somewhat earlier period, was nearly uniform along the continuous shores of the

Polar Circle,will account, on the theory ofmodification, for manyclosely allied forms now living in

marine areas completely sundered. Thus, I think, we can understand the presence of some closely allied,

still existing and extinct tertiary forms, on the eastern and western shores of temperate North America;

and the still more striking fact of many closely allied crustaceans (as described in Dana's admirable

work), some fish and other marine animals, inhabiting the Mediterranean and the seas of Japan,—these

two areas being now completely separated by the breadth of a whole continent and by wide spaces of

ocean.

These cases of close relationship in species either now or formerly inhabiting the seas on the eastern and

western shores of North America, the Mediterranean and Japan, and the temperate lands of North

America and Europe, are inexplicable on the theory of creation. We cannot maintain that such species

have been created alike, in correspondence with the nearly similar physical conditions of the areas; for if

we compare, for instance, certain parts of South America with parts of South Africa or Australia, we see

countries closely similar in all their physical conditions,with their inhabitants utterly dissimilar.

Alternate Glacial Periods in the North and South.

But we must return to our more immediate subject. I am convinced that Forbes's view may be largely

extended. In Europe we meet with the plainest evidence of the Glacial period, from the western shores of

Britain to the Oural range, and southward to the Pyrenees. We may infer from the frozen mammals and

nature of the mountain vegetation, that Siberia was similarly affected. In the Lebanon, according to Dr.

Hooker, perpetual snow formerly covered the central axis, and fed glaciers which rolled 400 feet down

the valleys. The same observer has recently found great moraines at a low level on the Atlas range in N.

Africa. Along the Himalaya, at points 900 miles apart, glaciers have left the marks of their former low

descent; and in Sikkim, Dr. Hooker saw maize growing on ancient and gigantic moraines. Southward of

the Asiatic continent, on the opposite side of the equator, we know, from the excellent researches of Dr.

J. Haast and Dr. Hector, that in New Zealand immense glaciers formerly descended to a low level; and

the same plants found by Dr. Hooker on widely separated mountains in this island tell the same story of a

former cold period. From facts communicated to me by the Rev. W. B. Clarke, it appears also that there





are traces of former glacial action on the mountains of the south-eastern corner of Australia.

Looking to America; in the northern half, ice-borne fragments of rock have been observed on the

eastern side of the continent, as far south as lat. 36°-37° , and on the shores of the Pacific, where the

climate is now so different, as far south as lat. 46°. Erratic boulders have, also, been noticed on the

Rocky Mountains. In the Cordillera of South America, nearly under the equator, glaciers once extended

far below their present level. In Central Chile I examined a vast mound of detritus with great boulders,crossing the Portillo valley, which there can hardly be a doubt once formed a huge moraine; andMr. D.

Forbes informs me that he found in various parts of the Cordillera, from lat. 13° to 30° S., at about the

height of 12,000 feet, deeply-furrowed rocks, resembling those with which he was familiar in Norway,

and likewise great masses of detritus, including grooved pebbles. Along this whole space of the

Cordillera true glaciers do not now exist even at much more considerable heights. Farther south on both

sides of the continent, from lat. 41° to the southernmost extremity, we have the clearest evidence of

former glacial action, in numerous immense boulders transported far from their parent source.

From these several facts, namely from the glacial action having extended all round the northern and

southernhemispheres—from theperiodhaving been in a geological sense recent in bothhemispheres—from its having lasted in both during a great length of time, as may be inferred from the

amount of work effected—and lastly from glaciers having recently descended to a low level along the

whole line of the Cordillera, it at one time appeared to me that we could not avoid the conclusion that the

temperature of the whole world had been simultaneously lowered during the Glacial period. But nowMr.

Croll, in a series of admirable memoirs, has attempted to show that a glacial condition of climate is the

result of various physical causes, brought into operation by an increase in the eccentricity of the earth's

orbit. All these causes tend towards the same end; but the most powerful appears to be the indirect

influence of the eccentricity of the orbit upon oceanic currents. According to Mr. Croll, cold periods

regularly recur every ten or fifteen thousand years; and these at long intervals are extremely severe, owing

to certain contingencies, of which the most important, as Sir C. Lyell has shown, is the relative position of

the land and water. Mr. Croll believes that the last great Glacial period occurred about 240,000 yearsago, and endured with slight alterations of climate for about 160,000 years. With respect to more ancient

Glacial periods, several geologists are convinced from direct evidence that such occurred during the

Miocene and Eocene formations, not tomention stillmore ancient formations. But the most important

result for us, arrived at by Mr. Croll, is that whenever the northern hemisphere passes through a cold

period the temperature of the southern hemisphere is actually raised, with the winters rendered much

milder, chiefly through changes in the direction of the ocean-currents. So conversely it will be with the

northernhemisphere, whilst the southern passes through a Glacial period. This conclusion throws so

much light on geographical distribution that I am strongly inclined to trust in it; but I will first give the facts,

which demand an explanation.

In South America, Dr. Hooker has shown that besides many closely allied species, between forty and

fifty of the flowering plants of Tierra del Fuego, forming no inconsiderable part of its scanty flora, are

common to North America and Europe, enormously remote as these areas in opposite hemispheres are

from each other. On the lofty mountains of equatorial America a host of peculiar species belonging to

European genera occur. On the Organ mountains of Brazil, some few temperate European, some

Antarctic, and some Andean genera were found by Gardner, which do not exist in the low intervening hot

countries. On the Silla of Caraccas, the illustrious Humboldt long ago found species belonging to genera

characteristic of the Cordillera.

In Africa, several forms characteristic of Europe and some few representatives of the flora of the Cape

of Good Hope occur on the mountains of Abyssinia. At the Cape of Good Hope a very few European

species, believed not to have been introduced by man, and on the mountains several representative

European forms are found, which have not been discovered in the intertropical parts of Africa. Dr.





Hooker has also lately shown that several of the plants living on the upper parts of the lofty island of

Fernando Po and on the neighbouring Cameroon mountains, in the Gulf of Guinea, are closely related to

those on the mountains of Abyssinia, and likewise to those of temperate Europe. It now also appears, as

I hear from Dr. Hooker, that some of these same temperate plants have been discovered by the Rev. R.

T. Lowe on the mountains of the Cape Verde islands. This extension of the same temperate forms,

almost under the equator, across the whole continent of Africa and to the mountains of the Cape Verde

archipelago, is one of the most astonishing facts ever recorded in the distribution of plants.

On the Himalaya, and on the isolated mountain ranges of the peninsula of India, on the heights of Ceylon,

and on the volcanic cones of Java, many plants occur, either identically the same or representing each

other, and at the same time representing plants of Europe, not found in the intervening hot lowlands. A list

of the genera of plants collected on the loftier peaks of Java, raises a picture of a collection made on a

hillock in Europe! Still more striking is the fact that peculiar Australian forms are represented by certain

plants growing on the summits of the mountains of Borneo. Some of these Australian forms, as I hear

from Dr. Hooker, extend along the heights of the peninsula of Malacca, and are thinly scattered on the

one hand over India, and on the other hand as far north as Japan.

On the southern mountains of Australia, Dr. F. Müller has discovered several European species; other

species, not introduced by man, occur on the lowlands; and a long list can be given, as I am informed by

Dr. Hooker, of European genera, found in Australia, but not in the intermediate torrid regions. In the

admirable 'Introduction to the Flora of New Zealand,' by Dr. Hooker, analogous and striking facts are

given in regard to the plants of that large island. Hence we see that certain plants growing on the more

lofty mountains of the tropics in all parts of the world, and on the temperate plains of the north and south,

are either the same species or varieties of the same species. It should, however, be observed that these

plants are not strictly arctic forms; for, as Mr. H. C. Watson has remarked, "in receding from polar

towards equatorial latitudes, the Alpine or mountain floras really become less and less Arctic." Besides

these identical and closely allied forms, many species inhabiting the same widely sundered areas, belong

to genera not now found in the intermediate tropical lowlands.

These brief remarks apply to plants alone; but some few analogous facts could be given in regard to

terrestrial animals. In marine productions, similar cases likewise occur; as an example, I may quote a

statement by the highest authority, Prof. Dana, that "it is certainly a wonderful fact that New Zealand

should have a closer resemblance in its crustacea to Great Britain, its antipode, than to any other part of

the world." Sir J. Richardson, also, speaks of the reappearance on the shores of New Zealand,

Tasmania, &c., of northern forms of fish. Dr. Hooker informs me that twenty-five species of Algæ are

common to New Zealand and to Europe, but have not been found in the intermediate tropical seas.

From the foregoing facts, namely, the presence of temperate forms on the highlands across the whole of equatorial Africa, and along the Peninsula of India, to Ceylon and the Malay Archipelago, and in a less

well-marked manner across the wide expanse of tropical South America, it appears almost certain that at

some former period, no doubt during the most severe part of a Glacial period, the lowlands of these great

continents were everywhere tenanted under the equator by a considerable number of temperate forms.

At this period the equatorial climate at the level of the sea was probably about the same with that now

experienced at the height of from five to six thousand feet under the same latitude, or perhaps even rather

cooler. During this, the coldest period, the lowlands under the equator must have been clothed with a

mingled tropical and temperate vegetation, like that described byHooker as growing luxuriantly at the

height of from four to five thousand feet on the lower slopes of the Himalaya, but with perhaps a still

greater preponderance of temperate forms. So again in the mountainous island of Fernando Po, in the

Gulf of Guinea, Mr. Mann found temperate European forms beginning to appear at the height of about

five thousand feet. On the mountains of Panama, at the height of only two thousand feet, Dr. Seemann

found the vegetation like that of Mexico, "with forms of the torrid zone harmoniously blended with those







centuries from La Plata and during the last forty or fifty years from Australia. The Neilgherrie mountains in

India, however, offer a partial exception; for here, as I hear from Dr. Hooker, Australian forms are

rapidly sowing themselves and becoming naturalised. Before the last great Glacial period, no doubt the

intertropical mountainswere stockedwith endemic Alpine forms; but these have almost everywhere

yielded to the more dominant forms generated in the larger areas and more efficient workshops of the

north. In many islands the native productions are nearly equalled, or even outnumbered, by those which

have become naturalised; and this is the first stage towards their extinction. Mountains are islands on theland, and their inhabitants have yielded to those produced within the larger areas of the north, just in the

same way as the inhabitants of real islands have everywhere yielded and are still yielding to continental

forms naturalised through man's agency.

The same principles apply to the distribution of terrestrial animals and of marine productions, in the

northern and southern temperate zones, and on the intertropical mountains. When, during the height of the

Glacial period, the ocean-currents were widely different to what they now are, some of the inhabitants of

the temperate seas might have reached the equator; of these a few would perhaps at once be able to

migrate southward, by keeping to the cooler currents, whilst others might remain and survive in the colder

depths until the southern hemisphere was in its turn subjected to a glacial climate and permitted their further progress; in nearly the same manner as, according to Forbes, isolated spaces inhabited by Arctic

productions exist to the present day in the deeper parts of the northern temperate seas.

I am far from supposing that all the difficulties in regard to the distribution and affinities of the identical

and allied species, which now live so widely separated in the north and south, and sometimes on the

intermediate mountain-ranges, are removed on the viewsabove given. The exact lines ofmigration cannot

be indicated. We cannot say why certain species and not others have migrated; why certain species have

been modified and have given rise to new forms, whilst others have remained unaltered. We cannot hope

to explain such facts, until we can say why one species and not another becomes naturalised by man's

agency in a foreign land; why one species ranges twice or thrice as far, and is twice or thrice as common,

as another species within their own homes.

Various special difficulties also remain to be solved; for instance, the occurrence, as shown by Dr.

Hooker, of the same plants at points so enormously remote as Kerguelen Land, New Zealand, and

Fuegia; but icebergs, as suggested by Lyell, may have been concerned in their dispersal. The existence at

these and other distant points of the southern hemisphere, of species, which, though distinct, belong to

genera exclusively confined to the south, is a more remarkable case. Some of these species are so

distinct, that we cannot suppose that there has been time since the commencement of the last Glacial

period for their migration and subsequent modification to the necessary degree. The facts seem to

indicate that distinct species belonging to the same genera have migrated in radiating lines from a common

centre; and I am inclined to look in the southern, as in the northern hemisphere, to a former and warmer period, before the commencement of the last Glacial period, when the Antarctic lands, now covered with

ice, supported a highly peculiar and isolated flora. It may be suspected that before this flora was

exterminated during the last Glacial epoch, a few forms had been already widely dispersed to various

points of the southern hemisphere by occasional means of transport, and by the aid as halting-places, of

now sunken islands. Thus the southern shores of America, Australia, and New Zealand may have

become slightly tinted by the same peculiar forms of life.

Sir C. Lyell in a striking passage has speculated, in language almost identical with mine, on the effects of

great alterations of climate throughout the world on geographical distribution. And we have now seen that

Mr.Croll's conclusion that successiveGlacial periods in the one hemisphere coincidewith warmer

periods in the opposite hemisphere, together with the admission of the slow modification of species,

explains a multitude of facts in the distribution of the same and of the allied forms of life in all parts of the

globe. The living waters have flowed during one period from the north and during another from the south,





and in both cases have reached the equator; but the stream of life has flowed with greater force from the

north than in the opposite direction, and has consequently more freely inundated the south. As the tide

leaves its drift in horizontal lines, rising higher on the shores where the tide rises highest, so have the living

waters left their living drift on our mountain summits, in a line gently rising from the Arctic lowlands to a

great altitude under the equator. The various beings thus left stranded may be compared with savage

races of man, driven up and surviving in the mountain fastnesses of almost every land, which serve as a

record, full of interest to us, of the former inhabitants of the surrounding lowlands.

|Go to Contents |

Chapter XIII Geographical Distribution— continued .

Distribution of fresh-water productions—On the inhabitants of oceanic islands—Absenceof Batrachians

and of terrestrial Mammals—On the relation of the inhabitants of islands to those of the nearest

mainland—Oncolonisation from the nearest source with subsequentmodification—Summaryof the last

and present chapter.

Fresh-water Productions.

As lakes and river-systems are separated from each other by barriers of land, it might have been thought

that fresh-water productions would not have ranged widely within the same country, and as the sea is

apparently a still more formidable barrier, that they would never have extended to distant countries. But

the case is exactly the reverse. Not only have many freshwater species, belonging to different classes, an

enormous range, but allied species prevail in a remarkable manner throughout the world. When first

collecting in the fresh waters of Brazil, I well remember feeling much surprise at the similarity of thefresh-water insects, shells,&c., and at the dissimilarityof the surrounding terrestrial beings, compared

with those ofBritain.

But the wide ranging power of fresh-water productions can, I think, in most cases be explained by their

having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to

pond, or from stream to stream, within their own countries; and liability to wide dispersal would follow

from this capacity as an almost necessary consequence. We can here consider only a few cases; of these,

some of the most difficult to explain are presented by fish. It was formerly believed that the same

fresh-water species never existed on two continents distant from each other. But Dr. Günther has lately

shown that the Galaxias attenuatus inhabits Tasmania, New Zealand, the Falkland Islands, and themainland of South America. This is a wonderful case, and probably indicates dispersal from an Antarctic

centre during a former warm period. This case, however, is rendered in some degree less surprising by





the species of this genus having the power of crossing by some unknown means considerable spaces of

open ocean: thus there is one species common to New Zealand and to the Auckland Islands, though

separated by a distance of about 230 miles. On the same continent fresh-water fish often range widely,

and as if capriciously; for in two adjoining river-systems some of the species may be the same, and some

wholly different.

It is probable that they are occasionally transported by what may be called accidental means. Thus fishesstill alive are not very rarely dropped at distant points by whirlwinds; and it is known that the ova retain

their vitality for a considerable time after removal from the water. Their dispersal may, however, be

mainly attributed to changes in the level of the land within the recent period, causing rivers to flow into

each other. Instances, also, could be given of this having occurred during floods, without any change of

level. The wide difference of the fish on the opposite sides of most mountain-ranges, which are

continuous, and which consequentlymust from an early period have completelyprevented the

inosculation of the river-systems on the two sides, leads to the same conclusion. Some freshwater fish

belong to very ancient forms, and in such cases there will have been ample time for great geographical

changes, and consequently time and means for much migration. Moreover, Dr. Günther has recently been

led by several considerations to infer that with fishes the same forms have a long endurance. Saltwater fish can with care be slowly accustomed to live in fresh water; and, according to Valenciennes, there is

hardly a single group of which all the members are confined to fresh water, so that a marine species

belonging to a fresh-water group might travel far along the shores of the sea, and could, it is probable,

become adapted without much difficulty to the fresh waters of a distant land.

Some species of fresh-water shells have very wide ranges, and allied species which, on our theory, are

descended from a common parent, and must have proceeded from a single source, prevail throughout the

world. Their distribution at first perplexed me much, as their ova are not likely to be transported by birds;

and the ova, as well as the adults, are immediately killed by sea-water. I could not even understand how

some naturalised species have spread rapidly throughout the same country. But two facts, which I have

observed—and many others no doubt will be discovered—throw some light on this subject. When duckssuddenly emerge from a pond covered with duck-weed, I have twice seen these little plants adhering to

their backs; and it has happened to me, in removing a little duck-weed from one aquarium to another,

that I have unintentionally stocked the one with fresh-water shells from the other. But another agency is

perhaps more effectual: I suspended the feet of a duck in an aquarium, where many ova of fresh-water

shells were hatching; and I found that numbers of the extremely minute and just-hatched shells crawled on

the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though

at a somewhat more advanced age they would voluntarily drop off. These just-hatched molluscs, though

aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this

length of time a duck or heron might fly at least six or seven hundred miles, and if blown across the sea to

an oceanic island, or to any other distant point, would be sure to alight on a pool or rivulet. Sir CharlesLyell informs me that a Dytiscus has been caught with an Ancylus (a fresh-water shell like a limpet) firmly

adhering to it; and a water-beetle of the same family, a Colymbetes, once flew on board the 'Beagle,'

when forty-five miles distant from the nearest land: how much farther it might have been blown by a

favouring gale no one can tell.

With respect to plants, it has long been known what enormous ranges many fresh-water, and even

marsh species, have, both over continents and to the most remote oceanic islands. This is strikingly

illustrated, according to Alph. de Candolle, in those large groups of terrestrial plants, which have very

few aquatic members; for the latter seem immediately to acquire, as if in consequence, a wide range. I

think favourable means of dispersal explain this fact. I have before mentioned that earth occasionally

adheres in some quantity to the feet and beaks of birds. Wading birds, which frequent the muddy edges

of ponds, if suddenly flushed, would be the most likely to have muddy feet. Birds of this order wander

more than those of any other; and they are occasionally found on the most remote and barren islands of





the open ocean; they would not be likely to alight on the surface of the sea, so that any dirt on their feet

would not be washed off; and when gaining the land, they would be sure to fly to their natural freshwater

haunts. I do not believe that botanists are aware how charged the mud of ponds is with seeds; I have

tried several little experiments, but will here give only the most striking case: I took in February three

tablespoonfuls of mud from three different points, beneath water, on the edge of a little pond: this mud

when dried weighed only 6 3/4 ounces; I kept it covered up in my study for six months, pulling up and

counting each plant as it grew; the plants were of many kinds, and were altogether 537 in number; andyet the viscid mud was all contained in a breakfast cup! Considering these facts, I think it would be an

inexplicable circumstance if water-birds did not transport the seeds of fresh-water plants to unstocked

ponds and streams, situated at very distant points. The same agency may have come into play with the

eggs of some of the smaller fresh-water animals.

Other and unknown agencies probably have also played a part. I have stated that fresh-water fish eat

some kinds of seeds, though they reject many other kinds after having swallowed them; even small fish

swallow seeds of moderate size, as of the yellow water-lily and Potamogeton. Herons and other birds,

century after century, have gone on daily devouring fish; they then take flight and go to other waters, or

are blown across the sea; and we have seen that seeds retain their power of germination, when rejectedmany hours afterwards in pellets or in the excrement. When I saw the great size of the seeds of that fine

water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks on thedistribution of this plant,

I thought that the means of its dispersal must remain inexplicable; but Audubon states that he found the

seeds of the great southern water-lily (probably, according to Dr. Hooker, the Nelumbium luteum) in a

heron's stomach. Now this bird must often have flown with its stomach thus well stocked to distant

ponds, and then getting a hearty meal of fish, analogy makes me believe that it would have rejected the

seeds in a pellet in a fit state for germination.

In considering these several means of distribution, it should be remembered that when a pond or stream

is first formed, for instance, on a rising islet, it will be unoccupied; and a single seed or egg will have a

good chance of succeeding. Although there will always be a struggle for life between the inhabitants of the same pond, however few in kind, yet as the number even in a well-stocked pond is small in

comparison with the number of species inhabiting an equal area of land, the competition between them

will probably be less severe than between terrestrial species; consequently an intruder from the waters of

a foreign country would have a better chance of seizing on a new place, than in the case of terrestrial

colonists. We should also remember that many fresh-water productions are low in the scale of nature,

and we have reason to believe that such beings become modified more slowly than the high; and this will

give time for the migration of aquatic species. We should not forget the probability of many freshwater

forms having formerly ranged continuously over immense areas, and then having become extinct at

intermediate points. But thewide distribution of freshwater plants and of the lower animals,whether

retaining the same identical form or in some degree modified, apparently depends in main part on thewide dispersal of their seeds and eggs by animals, more especially by fresh-water birds, which have great

powers of flight, and naturally travel from one piece of water to another.

On the Inhabitants of Oceanic Islands.

We now come to the last of the three classes of facts, which I have selected as presenting the greatest

amount of difficulty with respect to distribution, on the view that not only all the individuals of the same

species have migrated from some one area, but that allied species, although now inhabiting the most

distant points, have proceeded from a single area,—the birthplace of their early progenitors. I have

alreadygiven my reasons for disbelieving in continental extensionswithin the period of existing species,

on so enormous a scale that all the many islands of the several oceans were thus stocked with their







number of peculiar land-shells, whereas not one species of sea-shell is peculiar to its shores: now, though

we do not know how sea-shells are dispersed, yet we can see that their eggs or larvæ, perhaps attached

to seaweed or floating timber, or to the feet of wading-birds, might be transported across three or four

hundred miles of open sea far more easily than land-shells. The different orders of insects inhabiting

Madeira present nearly parallel cases.

Oceanic islands are sometimes deficient in animals of certain whole classes, and their places areoccupied by other classes; thus in the Galapagos Islands reptiles, and in New Zealand gigantic wingless

birds, take, or recently took, the place of mammals. Although New Zealand is here spoken of as an

oceanic island, it is in some degree doubtful whether it should be so ranked; it is of large size, and is not

separated from Australia by a profoundly deep sea; from its geological character and the direction of its

mountain-ranges, the Rev. W. B. Clarke has lately maintained that this island, as well as New Caledonia,

should be considered as appurtenances of Australia. Turning to plants, Dr. Hooker has shown that in the

Galapagos Islands the proportional numbers of the different orders are very different from what they are

elsewhere. All such differences in number, and the absence of certain whole groups of animals and plants,

are generally accounted for by supposed differences in the physical conditions of the islands; but this

explanation is not a little doubtful. Facility of immigration seems to have been fully as important as thenature of the conditions.

Many remarkable little facts could be given with respect to the inhabitants of oceanic islands. For

instance, in certain islands not tenanted bya single mammal, someof the endemic plants have beautifully

hooked seeds; yet few relations are more manifest than that hooks serve for the transportal of seeds in

the wool or fur of quadrupeds. But a hooked seed might be carried to an island by other means; and the

plant then becomingmodifiedwould form an endemic species, still retaining itshooks,whichwould form

a useless appendage like the shrivelled wings under the soldered wing-covers of many insular beetles.

Again, islands often possess trees or bushes belonging to orders which elsewhere include only

herbaceous species; now trees, as Alph. de Candolle has shown, generally have, whatever the cause may

be, confined ranges. Hence trees would be little likely to reach distant oceanic islands; and an herbaceous plant, which had no chance of successfully competing with the many fully developed trees growing on a

continent, might, when established on an island, gain an advantage over other herbaceous plants by

growing taller and taller and overtopping them. In this case, natural selection would tend to add to the

stature of the plant, to whatever order it belonged, and thus first convert it into a bush and then into a

tree.

Absence of Batrachians and Terrestrial Mammals on Oceanic Islands.

With respect to the absence of whole orders of animals on oceanic islands, Bory St. Vincent long ago

remarked that Batrachians (frogs, toads, newts) are never found on any of the many islands with which

the great oceans are studded. I have taken pains to verify this assertion, and have found it true, with the

exception of New Zealand, New Caledonia, the Andaman Islands, and perhaps the Salomon Islands and

the Seychelles. But I have already remarked that it is doubtful whether New Zealand and New Caledonia

ought to be classed as oceanic islands; and this is still more doubtful with respect to the Andaman and

Salomon groups and the Seychelles. This general absence of frogs, toads, and newts on so many true

oceanic islands cannot be accounted for by their physical conditions: indeed it seems that islands are

peculiarly fitted for these animals; for frogs have been introduced into Madeira, the Azores, and

Mauritius, and have multiplied so as to become a nuisance. But as these animals and their spawn are

immediately killed (with the exception, as far as known, of one Indian species) by seawater, there would

be great difficulty in their transportal across the sea, and therefore we can see why they do not exist on

strictly oceanic islands. But why, on the theory of creation, they should not have been created there, it





would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the oldest voyages, and have not

found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by

the natives) inhabiting an island situated above 300 miles from a continent or great continental island; and

many islands situated at a much less distance are equally barren. The Falkland Islands, which are

inhabited by a wolf-like fox, come nearest to an exception; but this group cannot be considered asoceanic, as it lies on a bank in connection with the mainland at the distance of about 280 miles;

moreover, icebergs formerly brought boulders to its western shores, and they may have formerly

transported foxes, as now frequently happens in the arctic regions. Yet it cannot be said that small islands

will not support at least small mammals, for they occur in many parts of the world on very small islands,

when lying close to a continent; and hardly an island can be named on which our smaller quadrupeds

have not become naturalised and greatly multiplied. It cannot be said, on the ordinary view of creation,

that there has not been time for the creation of mammals; many volcanic islands are sufficiently ancient, as

shown by the stupendous degradation which they have suffered, and by their tertiary strata: there has also

been time for the production of endemic species belonging to other classes; and on continents it is known

that new species of mammals appear and disappear at a quicker rate than other and lower animals.Although terrestrial mammals do not occur on oceanic islands, aerial mammals do occur on almost every

island. New Zealand possesses two bats found nowhere else in the world: Norfolk Island, the Viti

Archipelago, theBonin Islands, the Caroline andMarianne Archipelagoes, andMauritius, all possess

their peculiar bats. Why, it may be asked, has the supposed creative force produced bats and no other

mammals on remote islands? On my view this question can easily be answered; for no terrestrial mammal

can be transported across a wide space of sea, but bats can fly across. Bats have been seen wandering

by day far over the Atlantic Ocean; and two North American species either regularly or occasionally visit

Bermuda, at the distance of 600 miles from the mainland. I hear from Mr. Tomes, who has specially

studied this family, that many species have enormous ranges, and are found on continents and on far

distant islands. Hence we have only to suppose that such wandering species have been modified in their

new homes in relation to their new position, and we can understand the presence of endemic bats onoceanic islands, with the absence of all other terrestrial mammals.

Another interesting relation exists, namely between the depth of the sea separating islands from each

other or from the nearest continent, and the degree of affinity of their mammalian inhabitants. Mr.

Windsor Earl has made some striking observations on this head, since greatly extended by Mr. Wallace's

admirable researches, in regard to the great Malay Archipelago, which is traversed near Celebes by a

space of deep ocean, and this separates two widely distinct mammalian faunas. On either side the islands

stand on a moderately shallow submarine bank, and these islands are inhabited by the same or by closely

allied quadrupeds. I have not as yet had time to follow up this subject in all quarters of the world; but as

far as I have gone, the relation holds good. For instance, Britain is separated by a shallow channel fromEurope, and the mammals are the same on both sides; and so it is with all the islands near the shores of

Australia. The West Indian Islands, on the other hand, stand on a deeply submerged bank, nearly 1000

fathoms in depth, and here we find American forms, but the species and even the genera are quite

distinct. As the amount of modification which animals of all kinds undergo partly depends on the lapse of

time, and as the islands which are separated from each other or from the mainland by shallow channels,

are more likely to have been continuously united within a recent period than the islands separated by

deeper channels, we can understand how it is that a relation exists between the depth of the sea

separating two mammalian faunas, and the degree of their affinity,—a relation which is quite inexplicable

on the theory of independent acts of creation.

The foregoing statements in regard to the inhabitants of oceanic islands,—namely, the fewness of the

species, with a large proportion consisting of endemic forms—the members of certain groups, but not

those of other groups in the same class, having been modified—the absence of certain whole orders, as





of batrachians and of terrestrial mammals, notwithstanding the presence of aerial bats,—the singular

proportions of certain orders of plants,—herbaceous forms having been developed into trees,

&c.,—seem to me to accord better with the belief in the efficiency of occasional means of transport,

carried on during a long course of time, than with the belief in the former connection of all oceanic islands

with the nearest continent; for on this latter view it is probable that the various classes would have

immigrated more uniformly, and from the species havingentered in a body theirmutual relationswould

not have been much disturbed, and consequently they would either have not been modified, or all thespecies in a more equable manner.

I do not deny that there are many and serious difficulties in understanding how many of the inhabitants of

the more remote islands, whether still retaining the same specific form or subsequentlymodified, have

reached their present homes. But the probability of other islands having once existed as halting-places, of

which not a wreck now remains, must not be overlooked. I will specify one difficult case. Almost all

oceanic islands, even themost isolated and smallest, are inhabited by land-shells, generally by endemic

species, but sometimes by species found elsewhere,—striking instances of which have been given by Dr.

A. A. Gould in relation to the Pacific. Now it is notorious that land-shells are easily killed by sea-water;

their eggs, at least such as I have tried, sink in it and are killed. Yet there must be some unknown, butoccasionally efficient means for their transportal. Would the just-hatched young sometimes adhere to the

feet of birds roosting on the ground, and thus get transported? It occurred to me that land-shells, when

hybernating and having a membranousdiaphragm over the mouth of the shell, might be floated in chinks

of drifted timber across moderately wide arms of the sea. And I find that several species in this state

withstand uninjured an immersion in sea-water during seven days: one shell, the Helixpomatia, after

having been thus treated and again hybernating was put into sea-water for twenty days, and perfectly

recovered. During this length of time the shell might have been carried by a marine current of average

swiftness, to a distance of 660 geographical miles. As this Helix has a thick calcareous operculum, I

removed it, and when it had formed a new membranous one, I again immersed it for fourteen days in

sea-water, and again it recovered and crawled away. Baron Aucapitaine has since tried similar

experiments: he placed 100 land-shells, belonging to ten species, in a box pierced with holes, andimmersed it for a fortnight in the sea. Out of the hundred shells, twenty-seven recovered. The presence of

an operculum seems to have been of importance, as out of twelve specimens of Cyclostoma elegans,

which is thus furnished, eleven revived. It is remarkable, seeing how well the Helix pomatia resisted with

me the salt-water, that not one of fifty-four specimens belonging to four other species of Helix tried by

Aucapitaine, recovered. It is, however, not at all probable that land-shells have often been thus

transported; the feet of birds offer a more probable method.

On the Relations of the Inhabitants of Islands to those of the nearest Mainland .

The most striking and important fact for us is the affinity of the species which inhabit islands to those of

the nearestmainland, without being actually the same.Numerous instances could be given. The

Galapagos Archipelago, situated under the equator, lies at the distance of between 500 and 600 miles

from the shores of South America. Here almost every product of the land and of the water bears the

unmistakable stamp of theAmerican continent. There are twenty-six land-birds; of these, twenty-one, or

perhaps twenty-three are ranked as distinct species, and would commonly be assumed to have been here

created; yet the close affinity of most of these birds to American species is manifest in every character, in

their habits, gestures, and tones of voice. So it is with the other animals, and with a large proportion of

the plants, as shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, looking at

the inhabitants of these volcanic islands in the Pacific, distant several hundredmiles from the continent,

feels that he is standing on American land. Why should this be so? why should the species which are

supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plainly the







different enemies. If then it varied, natural selection would probably favour different varieties in the

different islands. Some species, however, might spread and yet retain the same character throughout the

group, just as we see some species spreading widely throughout a continent and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago, and in a lesser degree in some

analogous cases, is that each new species after being formed in any one island, did not spread quickly to

the other islands. But the islands, though in sight of each other, are separated by deep arms of the sea, inmost cases wider than the British Channel, and there is no reason to suppose that they have at any former

period been continuously united. The currents of the sea are rapid and sweep between the islands, and

gales of wind are extraordinarily rare; so that the islands are far more effectually separated from each

other than they appear on a map. Nevertheless some of the species, both of those found in other parts of

the world and of those confined to the archipelago, are common to the several islands; and we may infer

from their present manner of distribution, that they have spread from one island to the others. But we

often take, I think, an erroneous view of the probability of closely-allied species invading each other's

territory,when put into free intercommunication. Undoubtedly, if one species has anyadvantage over

another, it will in a very brief time wholly or in part supplant it; but if both are equally well fitted for their

own places, both will probably hold their separate places for almost any length of time. Being familiar with the fact that many species, naturalised through man's agency,have spread with astonishing rapidity

over wide areas, we are apt to infer that most species would thus spread; but we should remember that

the species which become naturalised in new countries are not generally closely allied to the aboriginal

inhabitants, but are very distinct forms, belonging in a large proportion of cases, as shown by Alph. de

Candolle, to distinct genera. In the Galapagos Archipelago, many even of the birds, though so well

adapted for flying from island to island, differ on the different islands; thus thereare threeclosely-allied

species of mocking-thrush, each confined to its own island. Now let us suppose the mocking-thrush of

Chatham Island to be blown to Charles Island, which has its own mocking-thrush; why should it succeed

in establishing itself there? We may safely infer that Charles Island is well stocked with its own species,

for annually more eggs are laid and young birds hatched, than can possibly be reared; and we may infer

that the mocking-thrush peculiar to Charles's Island is at least as well fitted for its home as is the species peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston have communicated to me a remarkable fact

bearing on this subject; namely, that Madeira and the adjoining islet of Porto Santo possess many distinct

but representative species of land-shells, some of which live in crevices of stone; and although large

quantities of stone are annually transported from Porto Santo to Madeira, yet this latter island has not

become colonised by the Porto Santo species; nevertheless both islands have been colonised by

European land-shells, which no doubt had some advantage over the indigenous species. From these

considerations I think we need not greatly marvel at the endemic species which inhabit the several islands

of the Galapagos Archipelago, not having all spread from island to island. On the same continent, also,

preoccupation has probably playedan importantpart in checking the commingling of the species which

inhabit different districtswith nearly the same physical conditions. Thus, the south-east and south-westcorners of Australia have nearly the same physical conditions, and are united by continuous land, yet they

are inhabited by a vast number of distinct mammals, birds, and plants; so it is, according to Mr. Bates,

with the butterflies and other animals inhabiting the great, open, and continuous valley of the Amazons.

The same principle which governs the general character of the inhabitants of oceanic islands, namely, the

relation to the source whence colonists could have been most easily derived, together with their

subsequent modification, is of the widest application throughout nature. We see this on every

mountain-summit, in every lake and marsh. For Alpine species, excepting in as far as the same species

have become widely spread during the Glacial epoch, are related to those of the surrounding lowlands;

thuswehave inSouth America,Alpine humming-birds,Alpine rodents,Alpine plants, &c., all strictly

belonging to American forms; and it is obvious that a mountain, as it became slowly upheaved, would be

colonised from the surrounding lowlands. So it is with the inhabitants of lakes and marshes, excepting in

so far as great facility of transport has allowed the same forms to prevail throughout large portions of the





world. We see this same principle in the character of most of the blind animals inhabiting the caves of

America and of Europe. Other analogous facts could be given. It will, I believe, be found universally true,

that wherever in two regions, let them be ever so distant, many closely allied or representative species

occur, there will likewise be found some identical species; andwherevermany closely-allied species

occur, there will be found many forms which some naturalists rank as distinct species, and others as mere

varieties; these doubtful formsshowing us the steps in the progress of modification.

The relation between the power and extent of migration in certain species, either at the present or at

some former period, and the existence at remote points of the world of closely-allied species, is shown in

another and more general way. Mr. Gould remarked to me long ago, that in those genera of birds which

range over the world, many of the species have very wide ranges. I can hardly doubt that this rule is

generally true, thoughdifficult of proof.Amongst mammals,we see it strikingly displayed inBats, and in a

lesser degree in the Felidæ and Canidæ. We see the same rule in the distribution of butterflies and

beetles. So it is with most of the inhabitants of fresh water, for many of the genera in the most distinct

classes range over the world, and many of the species have enormous ranges. It is not meant that all, but

that some of the species have very wide ranges in the genera which range very widely. Nor is it meant

that the species in such genera have on an average a very wide range; for this will largely depend on howfar the process of modification has gone; for instance, two varieties of the same species inhabit America

and Europe, and thus the species has an immense range; but, if variation were to be carried a little further,

the two varieties would be ranked as distinct species, and their range would be greatly reduced. Still less

is it meant, that species which have the capacity of crossing barriers and ranging widely, as in the case of

certain powerfully-winged birds,will necessarily rangewidely; for we should never forget that to range

widely implies not only the power of crossing barriers, but the more important power of being victorious

in distant lands in the struggle for life with foreign associates. But according to the view that all the species

of a genus, though distributed to the most remote points of the world, are descended from a single

progenitor, we ought to find, and I believe as a general rule we do find, that some at least of the species

range verywidely.

We should bear in mind that many genera in all classes are of ancient origin, and the species in this case

will have had ample time for dispersal and subsequent modification. There is also reason to believe from

geological evidence, that within each great class the lower organisms change at a slower rate than the

higher; consequently they will have had a better chance of ranging widely and of still retaining the same

specific character. This fact, together with that of the seeds and eggs of most lowly organised forms being

very minute and better fitted for distant transportal, probably accounts for a law which has long been

observed, and which has lately been discussed by Alph. de Candolle in regard to plants, namely, that the

lower any group of organisms stands the more widely it ranges.

The relations just discussed,—namely, lowerorganisms rangingmore widely than the higher,—some of the species ofwidely-ranging genera themselves rangingwidely,—such facts, as alpine, lacustrine, and

marsh productions being generally related to those which live on the surrounding low lands and dry

lands,—thestriking relationship between the inhabitants of islands and those of the nearestmainland—the

still closer relationship of the distinct inhabitants of the islands in the same archipelago—are inexplicable

on the ordinary view of the independent creation of each species, but are explicable if we admit

colonisation from the nearest or readiest source, together with the subsequent adaptation of the colonists

to their new homes.

Summary of the last and present Chapters.

In these chapters I have endeavoured to show, that if we make due allowance for our ignorance of the





full effects of changes of climate and of the level of the land, which have certainly occurred within the

recent period, and of other changes which have probably occurred,—if we remember how ignorant we

are with respect to the many curious means of occasional transport,—if we bear in mind, and this is a

very important consideration, how often a species may have ranged continuously over a wide area, and

then havebecome extinct in the intermediate tracts,—the difficulty is not insuperable in believing that all

the individuals of the same species, wherever found, are descended from common parents. And we are

led to this conclusion, which has been arrived at by many naturalists under the designation of singlecentres of creation, by various general considerations, more especially from the importance of barriers of

all kinds, and from the analogicaldistribution of sub-genera, genera, and families.

With respect to distinct species belonging to the same genus, which on our theory have spread from one

parent-source; if we make the same allowances as before for our ignorance, and remember that some

forms of life have changed very slowly, enormous periods of time having been thus granted for their

migration, the difficulties are far from insuperable; though in this case, as in that of the individuals of the

same species, they are often great.

As exemplifying the effects of climatal changes on distribution, I have attempted to show how importanta part the last Glacial period has played, which affected even the equatorial regions, and which, during

the alternations of the cold in the north and south, allowed the productions of opposite hemispheres to

mingle, and left some of them stranded on the mountain-summits in all parts of the world. As showing

how diversified are the means of occasional transport, I have discussed at some little length the means of

dispersal of freshwater productions.

If the difficulties be not insuperable in admitting that in the long course of time all the individuals of the

same species, and likewise of the several species belonging to the same genus, have proceeded from

some one source; then all the grand leading facts of geographical distribution are explicable on the theory

ofmigration, togetherwith subsequentmodification and the multiplication of new forms.We can thus

understand the high importance of barriers, whether of land or water, in not only separating, but inapparently forming the several zoological andbotanical provinces. We can thus understand the

concentration of related species within the same areas; and how it is that under different latitudes, for

instance in South America, the inhabitants of the plains and mountains, of the forests, marshes, and

deserts, are linked together in so mysterious a manner, and are likewise linked to the extinct beings which

formerly inhabited the samecontinent. Bearing inmind that the mutual relation of organism to organism is

of the highest importance, we can see why two areas having nearly the same physical conditions should

often be inhabited by very different forms of life; for according to the length of time which has elapsed

since the colonists entered one of the regions, or both; according to the nature of the communication

which allowed certain forms and not others to enter, either in greater or lesser numbers; according or not,

as those which entered happened to come into more or less direct competition with each other and withthe aborigines; and according as the immigrants were capable of varying more or less rapidly, there

would ensue in the two ormore regions, independentlyof theirphysical conditions, infinitely diversified

conditions of life,—there would be an almost endless amount of organic action and reaction,—and we

should find somegroups of beings greatly, and someonly slightlymodified,—some developed in great

force, some existing in scanty numbers—and this we do find in the several great geographical provinces

of the world.

On these same principles we can understand, as I have endeavoured to show, why oceanic islands

should have few inhabitants, but that of these, a large proportion should be endemic or peculiar; and why,

in relation to the means of migration, one group of beings should have all its species peculiar, and another

group, even within the same class, should have all its species the same with those in an adjoining quarter

of the world. We can see why whole groups of organisms, as batrachians and terrestrial mammals,

should be absent from oceanic islands, whilst the most isolated islands should possess their own peculiar





species of aerial mammals or bats. We can see why, in islands, there should be some relation between

the presence of mammals, in a more or less modified condition, and the depth of the sea between such

islands and the mainland.Wecanclearly see why all the inhabitants of an archipelago, though specifically

distinct on the several islets, should be closely related to each other; and should likewise be related, but

less closely, to those of the nearest continent, or other source whence immigrants might have been

derived. We can see why, if there exist very closely allied or representative species in two areas,

however distant from each other, some identical species will almost always there be found.

As the late Edward Forbes often insisted, there is a striking parallelism in the laws of life throughout time

and space; the laws governing the succession of forms in past times being nearly the same with those

governing at the present time the differences in different areas. We see this in many facts. The endurance

of each species and group of species is continuous in time; for the apparent exceptions to the rule are so

few, that they may fairly be attributed to our not having as yet discovered in an intermediate deposit

certain forms which are absent in it, but which occur both above and below: so in space, it certainly is the

general rule that the area inhabited by a single species, or by a group of species, is continuous, and the

exceptions, which are not rare, may, as I have attempted to show, be accounted for by former migrations

under different circumstances, or through occasional means of transport, or by the species having become extinct in the intermediate tracts. Both in time and space species and groups of species have their

points ofmaximum development.Groupsof species, livingduring the sameperiodof time, or livingwithin

the same area, are often characterised by trifling features in common, as of sculpture or colour. In looking

to the long succession of past ages, as in looking to distant provinces throughout the world, we find that

species in certain classes differ little from each other, whilst those in another class, or only in a different

section of the same order, differ greatly from each other. In both time and space the lowly organised

members of each class generally change less than the highly organised; but there are in both cases

marked exceptions to the rule. According to our theory, these several relations throughout time and

space are intelligible; for whether we look to the allied forms of life which have changed during successive

ages, or to those which have changed after having migrated into distant quarters, in both cases they are

connected by the same bond of ordinary generation; in both cases the laws of variation have been thesame, and modifications have been accumulated by the same means of natural selection.

|Go to Contents |

Chapter XIV

Mutual Affinities of Organic Beings:

Morphology: Embryology: Rudimentary

Organs.

CLASSIFICATION, groups subordinate to groups—Natural system-Rules anddifficulties in

classification, explained on the theory of descentwith modification—Classification of varieties—Descent





always used in classification—Analogical or adaptive characters—Affinities, general, complex, and

radiating—Extinction separates and defines groups—Morphology, between members of the same class,

between parts of the same individual—Embryology, laws of, explained byvariations not supervening at

an early age, andbeing inherited at a corresponding age—RudimentaryOrgans; their origin

explained—Summary.

Classification.

FROM the most remote period in the history of the world organic beings have been found to resemble

each other in descending degrees, so that they can be classed in groups under groups. This classification

is not arbitrary like the grouping of the stars in constellations. The existence of groups would have been of

simple significance, if one group had been exclusively fitted to inhabit the land and another the water; one

to feed on flesh, another on vegetable matter, and so on; but the case is widely different, for it is

notorious how commonly members of even the same sub-group have different habits. In the second and

fourth chapters, on Variation and on Natural Selection, I have attempted to show that within eachcountry it is the widely ranging, the much diffused and common, that is the dominant species, belonging to

the larger genera in each class, which vary most. The varieties, or incipient species, thus produced,

ultimately become converted into new and distinct species; and these, on the principle of inheritance, tend

to produce other new and dominant species. Consequently the groups which are now large, and which

generally include many dominant species, tend to go on increasing in size. I further attempted to show that

from the varying descendants of each species trying to occupy as many and as different places as

possible in the economy of nature, they constantly tend to diverge in character. This latter conclusion is

supported by observing the great diversity of forms which, in any small area, come into the closest

competition, and by certain facts in naturalisation.

I attempted also to show that there is a steady tendency in the forms which are increasing in number anddiverging in character, to supplant and exterminate the preceding, less divergent and less improved forms.

I request the reader to turn to the diagram illustrating the action, as formerly explained, of these several

principles; and he will see that the inevitable result is, that the modified descendants proceeding from one

progenitor become broken up into groups subordinate to groups. In the diagram each letter on the

uppermost line may represent a genus including several species, and the whole of the genera along this

upper line form together one class, for all are descended from one ancient parent, and, consequently,

have inherited something in common. But the three genera on the left hand have, on this same principle,

much in common, and form a sub-family, distinct from that containing the next two genera on the right

hand, which diverged from a common parent at the fifth stage of descent. These five genera have also

much in common, though less thanwhengrouped in sub-families; and they forma familydistinct from thatcontaining the three genera still farther to the right hand, which diverged at an earlier period. And all these

genera, descended from (A), form an order distinct from the genera descended from (I). So that we here

have many species descended from a single progenitor grouped into genera; and the genera into

sub-families, families, and orders, all under one great class. The grand fact of the natural subordination of

organic beings in groupsunder groups, which, from its familiarity, does not always sufficiently strike us, is

in my judgment thus explained. No doubt organic beings, like all other objects, can be classed in many

ways, either artificially by single characters, or more naturally by a number of characters. We know, for

instance, that minerals and the elemental substances can be thus arranged. In this case there is of course

no relation to genealogical succession, and no cause can at present be assigned for their falling into

groups. But with organic beings the case is different, and the view above given accords with their natural

arrangement in group under group; and no other explanation has ever been attempted.

Naturalists, as we have seen, try to arrange the species, genera, and families in each class, on what is





called the Natural System. But what is meant by this system? Some authors look at it merely as a scheme

for arranging together those living objects which are most alike, and for separating those which are most

unlike; or as an artificial method of enunciating, as briefly as possible, general propositions,—that is, by

one sentence to give the characters common, for instance, to all mammals, by another those common to

all carnivora, by another those common to the dog-genus, and then, by adding a single sentence, a full

description is given of each kind of dog. The ingenuity and utility of this system are indisputable. But many

naturalists think that something more is meant by the Natural System; they believe that it reveals the planof the Creator; but unless it be specified whether order in time or space, or both, or what else is meant

by the plan of the Creator, it seems to me that nothing is thus added to our knowledge. Expressions such

as that famous one by Linnæus, which we often meet with in a more or less concealed form, namely, that

the characters do not make the genus, but that the genus gives the characters, seem to imply that some

deeper bond is included in our classifications than mere resemblance. I believe that this is the case, and

that community of descent—the one known cause of close similarity in organic beings—is the bond,

which though observed by various degrees of modification, is partially revealed to us by our

classifications.

Let us now consider the rules followed in classification, and the difficulties which are encountered on theview that classification either gives someunknown plan of creation, or is simply a scheme for enunciating

general propositions and of placing together the forms most like each other. It might have been thought

(and was in ancient times thought) that those parts of the structure which determined the habits of life, and

the general place of each being in the economy of nature, would be of very high importance in

classification. Nothing can be more false. No one regards the external similarity of a mouse to a shrew, of

a dugong to a whale, of a whale to a fish, as of any importance. These resemblances, though so intimately

connected with the whole life of the being, are ranked as merely "adaptive or analogical characters;" but

to the consideration of these resemblances we shall recur. It may even be given as a general rule, that the

less any part of the organisation is concerned with special habits, the more important it becomes for

classification.As an instance: Owen, in speakingof the dugong, says, "The generative organs, being those

which are most remotely related to the habits and food of an animal, I have always regarded as affordingvery clear indications of its true affinities.We are least likely in the modifications of these organs to

mistake a merely adaptive for an essential character." With plants how remarkable it is that the organs of

vegetation, onwhich their nutrition and life depend, areof little signification;whereas the organs of

reproduction, with their product the seed and embryo, are of paramount importance! So again in

formerly discussing certain morphological characterswhicharenot functionally important, we have seen

that they areoften of the highest service in classification. This depends on their constancy throughout

many alliedgroups; and their constancy chieflydepends on any slight deviations not havingbeen

preserved and accumulated by natural selection, which acts only on serviceable characters.

That the mere physiological importance of an organ does not determine its classificatory value, is almost proved by the fact that in allied groups, in which the same organ, as we have every reason to suppose,

has nearly the samephysiological value, its classificatory value iswidely different. No naturalist can have

worked long at any group without being struck with this fact; and it has been fully acknowledged in the

writings of almost every author. It will suffice to quote the highest authority, Robert Brown, who, in

speaking of certain organs in the Proteaceæ, says their generic importance, "like that of all their parts, not

only in this, but, as I apprehend, in every natural family, is very unequal, and in some cases seems to be

entirely lost." Again, in another work he says, the genera of the Connaraceæ "differ in having one or more

ovaria, in the existence or absence of albumen, in the imbricate or valvular æstivation. Any one of these

characters singly is frequentlyofmore than generic importance, though here even when all taken together

they appear insufficient to separate Cnestis from Connarus." To give an example amongst insects: in one

great division of the Hymenoptera, the antennæ, as Westwood has remarked, are most constant in

structure; in another division they differ much, and the differences areof quite subordinatevalue in

classification; yet no one will say that the antennæ in these two divisions of the same order are of unequal





physiological importance.Any number of instances could be given of the varying importance for

classification of the same important organ within the samegroup of beings.

Again, no one will say that rudimentary or atrophied organs are of high physiological or vital importance;

yet, undoubtedly, organs in this condition are often of much value in classification. No one will dispute

that the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg,

arehighly serviceable in exhibiting the close affinitybetween ruminants and pachyderms. RobertBrownhas strongly insisted on the fact that the position of the rudimentary florets is of the highest importance in

the classificationof the grasses.

Numerous instances could be given of characters derived from parts which must be considered of very

trifling physiological importance, but which are universally admitted as highlyserviceable in the definition

of whole groups. For instance, whether or not there is an open passage from the nostrils to the mouth, the

only character, according toOwen, which absolutely distinguishes fishes and reptiles—the inflection of

the angle of the lower jaw in Marsupials—the manner in which the wings of insects are folded—mere

colour in certain Algæ—mere pubescence on parts of the flower in grasses—the nature of the dermal

covering, as hair or feathers, in the Vertebrata. If the Ornithorhynchus had been covered with feathersinstead of hair, this external and trifling character would have been considered by naturalists as an

important aid in determining the degree of affinity of this strange creature to birds.

The importance, for classification, of trifling characters, mainly depends on their being correlatedwith

many other characters of more or less importance. The value indeed of an aggregate of characters is very

evident in natural history. Hence, as has often been remarked, a species may depart from its allies in

several characters, both of high physiological importance, and of almost universal prevalence, andyet

leave us in no doubt where it should be ranked. Hence, also, it has been found that a classification

founded on any single character, however important that may be, has always failed; for no part of the

organisation is invariably constant. The importance of an aggregate of characters, even when none are

important, alone explains the aphorismenunciated byLinnæus, namely, that the characters do not givethe genus, but the genus gives the characters; for this seems founded on the appreciation of many trifling

points of resemblance, too slight to be defined. Certain plants, belonging to the Malpighiaceæ, bear

perfect and degraded flowers; in the latter, as A. de Jussieu has remarked, "the greater number of the

characters proper to the species, to the genus, to the family, to the class, disappear, and thus laugh at our

classification."When Aspicarpaproduced in France, during several years, only these degraded flowers,

departing so wonderfully in a number of the most important points of structure from the proper type of

the order, yet M. Richard sagaciously saw, as Jussieu observes, that this genus should still be retained

amongst the Malpighiaceæ.This case well illustrates the spirit of our classifications.

Practically, when naturalists are at work, they do not trouble themselves about the physiological value of the characters which they use in defining a group or in allocating any particular species. If they find a

character nearly uniform, and common to a great number of forms, and not common to others, they use it

as one of high value; if common to some lesser number, they use it as of subordinate value. This principle

has been broadly confessed by some naturalists to be the true one; and by none more clearly than by that

excellent botanist, Aug. St. Hilaire. If several trifling characters arealways found in combination, though

no apparent bond of connection can be discovered between them, especial value is set on them. As in

most groups of animals, important organs, such as those for propelling the blood, or for aerating it, or

those for propagating the race, are found nearly uniform, they are considered as highly serviceable in

classification; but in some groups all these, the most important vital organs, are found to offer characters

of quite subordinate value. Thus, as Fritz Müller has lately remarked, in the same group of crustaceans,

Cypridina is furnished with a heart, whilst in two closely allied genera, namely Cypris and Cytherea, there

is no such organ; one species of Cypridina has well-developed branchiæ, whilst another species is

destitute of them.







I), a species has transmited modified descendants to the present day, represented by the fifteen genera (a

14to z 14 ) on the uppermost horizontal line. Now all these modified descendants from a single species,

are related in blood or descent in the same degree; they may metaphorically be called cousins to the

same millionth degree; yet they differ widely and in different degrees from each other. The forms

descended from A, now broken up into two or three families, constitute a distinct order from those

decended from I, also broken up into two families. Nor can the existing species, descended from A, be

ranked n the same genus with the parent A; or those from I, with the parent I. But the existing genusF14may be supposed to have been but slightly modified; and it will then rank with the parent-genus F;

just as some few still living organisms belong to Silurian genera. So that the comparative value of the

differences between these organic beings, which are all related to each other in the same degree in blood,

hascome to be widely different. Nevertheless their genealogicalarrangement remains strictly true, not

only at the present time, but at each successive period of descent. All the modified descendants from A

will have inherited something in common from their common parent, as will all the descendants from I; so

will it be with each subordinate branch of descendants, at each successive stage. If, however, we

suppose any descendant of A, or of I, to have become so much modified as to have lost all traces of its

parentage, in this case, its place in the natural system will be lost, as seems to have occurred with some

few existing organisms. All the descendants of the genus F, along its whole line of descent, are supposedto have been but little modified, and they form a single genus. But this genus, though much isolated, will

still occupy its proper intermediate position. The representation of the groups, as here given in the

diagram on a flat surface, is much too simple. The branches ought to have diverged in all directions. If the

names of the groups had been simply written down in a linear series, the representation would have been

still less natural; and it is notoriously not possible to represent in a series, on a flat surface, the affinities

which we discover in nature amongst the beings of the same group. Thus, the natural system is

genealogical in its arrangement, like a pedigree: but the amount ofmodificationwhich the different groups

have undergone has to be expressed by ranking them under different so-called genera, sub-families,

families, sections, orders, and classes.

It may be worth while to illustrate this view of classification, by taking the case of languages. If we possessed a perfect pedigree of mankind, a genealogical arrangement of the races of man would afford

the best classification of the various languages now spoken throughout the world; and if all extinct

languages, and all intermediate andslowly changing dialects, were to be included, such an arrangement

would be the only possible one. Yet it might be that some ancient languages had altered very little and

had given rise to few new languages, whilst others had altered much owing to the spreading, isolation,

and state of civilisation of the several co-descended races, and had thus given rise to many new dialects

and languages. The various degrees of difference between the languages of the same stock, would have

to be expressed by groups subordinate to groups; but the proper or even the only possible arrangement

would still be genealogical; and this would be strictly natural, as it would connect together all languages,

extinct and recent, by the closest affinities, and would give the filiation and origin of each tongue.

In confirmation of this view, let us glance at the classification of varieties, which are known or believed to

be descended from a single species. These are grouped under the species, with the sub-varieties under

the varieties; and in some cases, as with the domestic pigeon, with several other grades of difference.

Nearly the same rules are followed as in classifying species. Authors have insisted on the necessity of

arranging varieties on a natural instead of an artificial system; we are cautioned, for instance, not to class

two varieties of the pine-apple together, merely because their fruit, though the most important part,

happens to be nearly identical; no one puts the Swedish and common turnip together, though the esculent

and thickened stems are so similar. Whatever part is found to be most constant, is used in classing

varieties: thus the great agriculturist Marshall says the horns are very useful for this purpose with cattle,

because they are less variable than the shape or colour of the body, &c.; whereas with sheep the horns

are much less serviceable, because less constant. In classing varieties, I apprehend that if we had a real

pedigree, a genealogical classificationwould be universally preferred; and it has been attempted in some





cases. For we might feel sure, whether there had been more or less modification, that the principle of

inheritance would keep the forms together which were allied in the greatest number of points. In tumbler

pigeons, though some of the sub-varieties differ in the important character of the length of the beak, yet

all are kept together from having the common habit of tumbling; but the short-faced breed has nearly or

quite lost his habit: nevertheless, without any thought on the subject, these tumblers are kept in the same

group, because allied in blood and alike in some other respects.

With species in a state of nature, every naturalist has in fact brought descent into his classification; for he

includes in his lowest grade, that of species, the two sexes; and how enormously these sometimes differ in

the most important characters, is known to every naturalist: scarcely a single fact can be predicated in

common of the adult males and hermaphrodites of certain cirripedes, and yet no one dreams of

separating them. As soon as the three Orchidean forms, Monachanthus, Myanthus, and Catasetum,

which had previously been ranked as three distinct genera, were known to be sometimes produced on

the same plant, they were immediately considered as varieties; and now I have been able to show that

they are the male, female, and hermaphrodite forms of the same species. The naturalist includes as one

species the various larval stages of the same individual, however much they may differ from each other

and from the adult, as well as the so-called alternate generations of Steenstrup, which can only in atechnical sense be considered as the same individual. He includes monsters and varieties, not from their

partial resemblance to the parent-form, but because they are descended from it.

As descent has universally been used in classing together the individuals of the same species, though the

males and females and larvæ are sometimes extremely different; and as it has been used in classing

varieties whichhave undergone a certain, and sometimes a considerable amount ofmodification, maynot

this same element of descent have been unconsciously used in grouping species under genera, and genera

under higher groups, all under the so-called natural system? I believe it has been unconsciously used; and

thus only can I understand the several rules and guides which have been followed by our best

systematists. As we have no written pedigrees, we are forced to trace community of descent by

resemblances of any kind. Therefore we chose those characters which are the least likely to have beenmodified, in relation to the conditions of life to which each species has been recently exposed.

Rudimentary structures on this view are as good as, or even sometimes better than, other parts of the

organisation. We care not how trifling a character may be—let it be the mere inflection of the angle of the

jaw, the manner in which an insect's wing is folded, whether the skin be covered by hair or feathers—if it

prevail throughoutmanyand different species, especially those having very different habits of life, it

assumes high value; for we can account for its presence in so many forms with such different habits, only

by inheritance from a common parent. We may err in this respect in regard to single points of structure,

but when several characters, let them be ever so trifling, concur throughout a large group of beings having

different habits, we may feel almost sure, on the theory of descent, that these characters have been

inherited from a common ancestor; and we know that such aggregated characters have especial value inclassification.

We can understand why a species or a group of species may depart from its allies, in several of its most

important characteristics, and yet be safely classed with them. This may be safely done, and is often

done, as long as a sufficient number of characters, let them be ever so unimportant, betrays the hidden

bond of community of descent. Let two forms have not a single character in common, yet, if these

extreme forms are connected together by a chain of intermediate groups, we may at once infer their

community of descent, and we put them all into the same class. As we find organs of high physiological

importance—those which serve to preserve life under themost diverse conditions of existence—are

generally the most constant, we attach especial value to them; but if these same organs, in another group

or section of a group, are found to differ much, we at once value them less in our classification.We shall

presently seewhy embryological characters are of such high classificatory importance.Geographical

distribution may sometimes be brought usefully into play in classing large genera,because all the species





of the same genus, inhabiting any distinct and isolated region, are in all probability descended from the

same parents.

Analogical Resemblances.— We can understand, on the above views, the very important distinction

between real affinities and analogical or adaptive resemblances. Lamarck first called attention to this

subject, and he has been ably followed by Macleay and others. The resemblance in the shape of the

body and in the fin-like anterior limbs between dugongs and whales, and between these two orders of mammals and fishes, are analogical. So is the resemblance between a mouse and a shrew-mouse

(Sorex), which belong to different orders; and the still closer resemblance, insisted on by Mr. Mivart,

between the mouse anda smallmarsupial animal (Antechinus) ofAustralia.These latter resemblances

may be accounted for, as it seems to me, by adaptation for similarly active movements through thickets

andherbage, together with concealment from enemies.

Amongst insects there are innumerable similar instances; thusLinnæus,misled by external appearances,

actually classed an homopterous insect as a moth.We see something of the same kind even with our

domestic varieties, as in the strikingly similar shape of the body in the improved breeds of the Chinese

and common pig, which are descended from distinct species; and in the similarly thickened stems of thecommon and specifically distinct Swedish turnip. The resemblance between the greyhound and the

racehorse is hardly more fanciful than the analogies which have been drawn by some authors between

widely different animals.

On the view of characters being of real importance for classification, only in so far as they reveal

descent, we can clearly understand why analogical or adaptive characters, although of the utmost

importance to the welfare of the being, are almost valueless to the systematist. For animals, belonging to

two most distinct lines of descent, may have become adapted to similar conditions, and thus have

assumed a close external resemblance; but such resemblances will not reveal—will rather tend to conceal

their blood-relationship. We can thus also understand the apparent paradox, that the very same

characters are analogical when one group is compared with another, but give true affinities when themembers of the same group are compared together: thus, the shape of the body and fin-like limbs are

only analogicalwhenwhales are compared with fishes, being adaptations in both classes for swimming

through the water; but between the several members of the whale family, the shape of the body and the

fin-like limbsoffer characters exhibiting true affinity; for as these parts are so nearly similar throughout the

whole family, we cannot doubt that they have been inherited from a common ancestor. So it is with

fishes.

Numerous cases could be given of striking resemblances in quite distinct beings between single parts or

organs, which have been adapted for the same functions. A good instance is afforded by the close

resemblance of the jaws of the dog and Tasmanian wolf or Thylacinus,—animals which are widelysundered in the natural system. But this resemblance is confined to general appearance, as in the

prominence of the canines, and in the cutting shape of the molar teeth. For the teeth really differ much:

thus the dog has on each side of the upper jaw four pre-molars and only two molars; whilst the

Thylacinus has three pre-molars and four molars. The molars also differ much in the two animals in

relative size and structure. The adult dentition is preceded by a widely different milk dentition. Any one

may of course deny that the teeth in either case have been adapted for tearing flesh, through the natural

selection of successive variations; but if this be admitted in the one case, it is unintelligible to me that it

should be denied in the other. I am glad to find that so high an authority as Professor Flower has come to

this same conclusion.

The extraordinary cases given in a former chapter, of widely different fishes possessing electric

organs,—of widelydifferent insects possessing luminous organs,—and of orchids and asclepiads having

pollen-masses with viscid discs, come under this same head of analogical resemblances. But these cases





are so wonderful that they were introduced as difficulties or objections to our theory. In all such cases

some fundamental difference in the growth or development of the parts, and generally in their matured

structure, can be detected. The end gained is the same, but the means, though appearing superficially to

be the same, are essentially different. The principle formerly alluded to under the term of analogical

variation has probably in these cases often come into play; that is, the members of the same class,

although only distantly allied, have inherited so much in common in their constitution, that they are apt to

vary under similar exciting causes in a similar manner;and thiswould obviously aid in the acquirementthroughnatural selection of parts or organs, strikingly like each other, independentlyof their direct

inheritance from a commonprogenitor.

As species belonging to distinct classes have often been adapted by successive slight modifications to

live undernearly similar circumstances,—to inhabit, for instance, the three elementsof land, air, and

water,—we can perhaps understand how it is that a numerical parallelism has sometimes been observed

between the subgroups of distinct classes. A naturalist, struck with a parallelism of this nature, by

arbitrarily raising or sinking the value of the groups in several classes (and all our experience shows that

their valuation is as yet arbitrary), could easily extend the parallelism over a wide range; and thus the

septenary, quinary, quarternary and ternary classifications have probably arisen.

There is another and curious class of cases in which close external resemblance does not depend on

adaptation to similar habits of life, but has been gained for the sake of protection. I allude to the

wonderful manner in which certain butterflies imitate, as first described by Mr. Bates, other and quite

distinct species. This excellent observer has shown that in some districts of S. America, where, for

instance, an Ithomia abounds in gaudy swarms, another butterfly, namely, a Leptalis, is often found

mingled in the same flock; and the latter so closely resembles the Ithomia in every shade and stripe of

colour and even in the shape of its wings, that Mr. Bates, with his eyes sharpened by collecting during

eleven years, was, though always on his guard, continually deceived. When the mockers and the mocked

are caught and compared, they are found to be very different in essential structure, and to belong not only

to distinct genera, but often to distinct families. Had this mimicry occurred in only one or two instances, itmight have been passed over as a strange coincidence. But, if we proceed from a district where one

Leptalis imitates an Ithomia, another mocking and mocked species belonging to the same two genera,

equally close in their resemblance, may be found. Altogether no less than ten genera are enumerated,

which include species that imitate other butterflies. The mockers and mocked always inhabit the same

region; we never find an imitator living remote from the form which it imitates. The mockers are almost

invariably rare insects; the mocked in almost every case abound in swarms. In the same district in which a

species of Leptalis closely imitates an Ithomia, there are sometimes otherLepidopteramimicking the

same Ithomia: so that in the same place, species of three genera of butterflies and even a moth are found

all closely resembling a butterfly belonging to a fourth genus. It deserves especial notice that many of the

mimicking forms of the Leptalis, as well as of the mimicked forms, can be shown by a graduated series to be merely varieties of the same species; whilst others are undoubtedly distinct species. But why, it may

be asked, are certain forms treated as the mimicked and others as the mimickers? Mr. Bates

satisfactorily answers this question, by showing that the form which is imitated keeps the usual dress of

the group to which it belongs, whilst the counterfeiters have changed their dress and do not resemble their

nearest allies.

We are next led to inquire what reason can be assigned for certain butterflies and moths so often

assuming the dress of another and quite distinct form; why, to the perplexity of naturalists, has nature

condescended to the tricks of the stage? Mr. Bates has, no doubt, hit on the true explanation. The

mocked forms, which always abound in numbers, must habitually escape destruction to a large extent,

otherwise they could not exist in such swarms; and a large amount of evidence has now been collected,

showing that they are distasteful to birds and other insect-devouring animals. The mocking forms, on the

other hand, that inhabit the same district, are comparatively rare, and belong to rare groups; hence they





must suffer habitually from somedanger, for otherwise, from the number of eggs laid by all butterflies,

they would in three or four generations swarm over the whole country. Now if a member of one of these

persecuted and rare groups were to assume a dress so like that of a well-protected species that it

continually deceived the practised eyes of an entomologist, it would often deceive predaceous birds and

insects, and thus often escape destruction. Mr. Bates may almost be said to have actually witnessed the

process by which the mimickers have come so closely to resemble the mimicked; for he found that some

of the forms of Leptalis which mimic so many other butterflies, varied in an extreme degree. In onedistrict several varieties occurred, and of these one alone resembled to a certain extent, the common

Ithomia of the same district. In another district there were two or three varieties, one of which was much

commoner than the others, and this closely mocked another form of Ithomia. From facts of this nature,

Mr. Bates concludes that the Leptalis first varies; and when a variety happens to resemble in some

degree any commonbutterfly inhabiting the samedistrict, this variety, from its resemblance to a flourishing

and little-persecuted kind, has a better chance of escaping destruction from predaceous birds and

insects, and is consequently oftener preserved;—"the less perfect degrees of resemblance being

generation after generation eliminated, and only the others left to propagate their kind." So that here we

have an excellent illustration of natural selection.

Messrs. Wallace and Trimen have likewise described several equally striking cases of imitation in the

Lepidoptera of the Malay Archipelago and Africa, and with some other insects. Mr. Wallace has also

detected one such case with birds, but we have none with the larger quadrupeds. The much greater

frequency of imitation with insects thanwith other animals, is probably the consequence of their small

size; insects cannot defend themselves, excepting indeed the kinds furnished with a string, and I have

never heard of an instance of such kinds mocking other insects, though they are mocked; insects cannot

easilyescape by flight from the larger animalswhichprey on them; therefore, speaking metaphorically,

they are reduced, like most weak creatures, to trickery and dissimulation.

It should be observed that the process of imitation probably never commenced between forms widely

dissimilar in colour. But startingwith species already somewhat like each other, the closest resemblance,if beneficial, could readily be gained by the above means; and if the imitated form was subsequently and

gradually modified through any agency, the imitating form would be led along the same track, and thus be

altered to almost any extent, so that it might ultimately assume an appearance or colouring wholly unlike

that of the other members of the family to which it belonged. There is, however, some difficulty on this

head, for it is necessary to suppose in some cases that ancient members belonging to several distinct

groups, before they had diverged to their present extent, accidentally resembled a member of another

and protected group in a sufficient degree to afford some slight protection; this having given the basis for

the subsequent acquisition of themost perfect resemblance.

On the Nature of the Affinities connecting Organic Beings.— As the modified descendants of dominant species, belonging to the larger genera, tend to inherit the advantages which made the groups to

which they belong large and their parents dominant, they are almost sure to spread widely, and to seize

on more and more places in the economy of nature. The larger and more dominant groups within each

class thus tend to go on increasing in size; and they consequently supplant many smaller and feebler

groups. Thus we can account for the fact that all organisms, recent and extinct, are included under a few

great orders, and under still fewer classes. As showing how few the higher groups are in number, and

how widely they are spread throughout the world, the fact is striking that the discovery of Australia has

not added an insect belonging to a new class; and that in the vegetable kingdom, as I learn from Dr.

Hooker, it has added only two or three families of small size.

In the chapter on Geological Succession I attempted to show, on the principle of each group having

generally divergedmuch in character during the long-continued process ofmodification, how it is that the

more ancient forms of life often present characters in some degree intermediate between existinggroups.





As some few of the old and intermediate forms have transmitted to the present day descendants but little

modified, these constitute our so-called osculant or aberrant species. The more aberrant any form is, the

greater must be the number of connecting forms which have been exterminated and utterly lost. And we

have some evidence of aberrant groups having suffered severely from extinction, for they are almost

always represented by extremely few species; and such species as do occur are generally very distinct

from each other, whichagain implies extinction. The genera Ornithorhynchus and Lepidosiren, for

example, would not have been less aberrant had each been represented by a dozen species, instead of asat present by a single one, or by two or three. We can, I think, account for this fact only by looking at

aberrant groups as forms which have been conquered by more successful competitors, with a few

members still preserved under unusually favourable conditions.

Mr. Waterhouse has remarked that, when a member belonging to one group of animals exhibits an

affinity to a quite distinct group, this affinity in most cases is general and not special; thus, according to

Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to Marsupials; but in the points in

which it approaches this order, its relations are general, that is, not to anyone marsupial species more

than to another. As these points of affinity are believed to be real and not merely adaptive, they must be

due in accordance with our view to inheritance from a common progenitor. Therefore we must supposeeither that allRodents, including the bizcacha, branched off fromsome ancientMarsupial, whichwill

naturally have been more or less intermediate in character with respect to all existing Marsupials; or that

both Rodents and Marsupials branched off from a common progenitor, and that both groups have since

undergone much modification in divergentdirections. On either view wemust suppose that the bizcacha

has retained, by inheritance, more of the characters of its ancient progenitor than have other Rodents; and

therefore it will not be specially related to any one existing Marsupial, but indirectly to all or nearly all

Marsupials, from having partially retained the character of their common progenitor, or of some early

member of the group. On the other hand, of all Marsupials, as Mr. Waterhouse has remarked, the

Phascolomys resembles most nearly, not any one species, but the general order of Rodents. In this case,

however, it may be strongly suspected that the resemblance is only analogical, owing to the Phascolomys

having become adapted to habits like those of a Rodent. The elder De Candolle has made nearly similar observations on the general nature of the affinities of distinct familiesof plants.

On the principle of the multiplication and gradual divergence in character of the species descended from

a common progenitor, together with their retention by inheritance of some characters in common, we can

understand the excessively complex and radiating affinities bywhich all the membersof the same family

or higher group are connected together. For the common progenitor of a whole family, now broken up

by extinction into distinct groups andsub-groups, will have transmitted some of its characters,modified in

various ways and degrees, to all the species; and they will consequently be related to each other by

circuitous lines of affinity of various lengths (as may be seen in the diagram so often referred to), mounting

up through many predecessors. As it is difficult to show the blood-relationship between the numerouskindred of any ancient and noble family even by the aid of a genealogical tree, and almost impossible to

do sowithout this aid, we canunderstand theextraordinary difficultywhichnaturalists have experienced

in describing, without the aid of a diagram, the various affinities which they perceive between the many

living and extinct members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an important part in defining and widening

the intervals between the several groups in each class. We may thus account for the distinctness of whole

classes from each other—for instance, of birds from all other vertebrate animals—by the belief that many

ancient forms of life have been utterly lost, through which the early progenitors of birds were formerly

connected with the early progenitors of the other and at that time less differentiated vertebrate classes.

There has been much less extinction of the forms of life which once connected fishes with batrachians.

There has been still less within some whole classes, for instance the Crustacea, for here the most

wonderfully diverse forms are still linked together bya longand only partially broken chain of affinities.





Extinction has only defined the groups: it has by no means made them; for if every form which has ever

lived on this earth were suddenly to reappear, though it would be quite impossible to give definitions by

whicheach group couldbe distinguished, still a natural classification, or at least a natural arrangement,

would be possible. We shall see this by turning to the diagram; the letters, A to L, may represent eleven

Silurian genera, some of which have produced large groups of modified descendants, with every link in

each branch and sub-branch still alive; and the links not greater than those between existing varieties. In

this case it would be quite impossible to give definitions by which the several members of the severalgroups couldbe distinguished from their more immediate parents anddescendants.Yet the arrangement

in the diagram would still hold good and would be natural; for, on the principle of inheritance, all the

forms descended, for instance, from A, would have something in common. In a tree we can distinguish

this or that branch, though at the actual fork the two unite and blend together. We could not, as I have

said, define the several groups; but we could pick out types, or forms, representing most of the

characters of each group, whether large or small, and thus give a general idea of the value of the

differences between them. This is what we should be driven to, if we were ever to succeed in collecting

all the forms in any one class which have lived throughout all time and space. Assuredly we shall never

succeed in making so perfect a collection: nevertheless, in certain classes, we are tending towards this

end; and Milne Edwards has lately insisted, in an able paper, on the high importance of looking to types,whether or not we can separate and define the groups to which such types belong.

Finally, we have seen that natural selection, which follows from the struggle for existence, and which

almost inevitably leads to extinction and divergence of character in thedescendants from anyone

parent-species, explains that great and universal feature in the affinities of all organic beings, namely, their

subordination in group under group. We use the element of descent in classing the individuals of both

sexes and of all ages under one species, although they may have but few characters in common; we use

descent in classing acknowledged varieties, however different they may be from their parents; and I

believe that this element of descent is the hidden bond of connection which naturalists have sought under

the term of the Natural System. On this idea of the natural system being, in so far as it has been

perfected, genealogical in its arrangement, with the grades of difference expressed by the terms genera,families, orders, &c., we can understand the rules which we are compelled to follow in our classification.

We can understand why we value certain resemblances far more than others; why we use rudimentary

and useless organs, or others of trifling physiological importance;why, in finding the relations between

one group and another, we summarily reject analogical or adaptive characters, and yet use these same

characters within the limits of the same group. We can clearly see how it is that all living and extinct forms

can be grouped together within a few great classes; and how the several members of each class are

connected together by the most complex and radiating lines of affinities. We shall never, probably,

disentangle the inextricable web of the affinities between the members of any one class; but when we

have a distinct object in view, and do not look to some unknown plan of creation, we may hope to make

sure but slow progress.

Professor Häckel in his 'Generelle Morphologie' and in other works, has recently brought his great

knowledge and abilities to bear on what he calls phylogeny, or the lines of descent of all organic beings.

In drawing up the several series he trusts chiefly to embryological characters, but receives aid from

homologous and rudimentary organs, as well as from the successive periods at which the various forms of

life are believed to have first appeared in our geological formations. He has thus boldly made a great

beginning, and shows us how classification will in the future be treated.

Morphology.

We have seen that the members of the same class, independently of their habits of life, resemble each







account for the infinite diversity in the structure and functions of the mouths of insects. Nevertheless, it is

conceivable that the general pattern of an organ might become so much obscured as to be finally lost, by

the reduction and ultimately by the complete abortion of certain parts, by the fusion of other parts, and by

the doublingormultiplication of others,—variations whichweknowto bewithin the limits of possibility.

In the paddles of the gigantic extinct sea-lizards, and in the mouths of certain suctorial crustaceans, the

general pattern seems thus to have become partially obscured.

There is another and equally curious branch of our subject; namely, serial homologies, or the comparison

of the different parts or organs in the same individual, and not of the same parts or organs in different

members of the same class. Most physiologists believe that the bones of the skull are homologous—that

is, correspond in number and in relative connexion—with the elemental parts of a certain number of

vertebræ. The anterior and posterior limbs in all the higher vertebrate classes are plainly homologous. So

it is with the wonderfully complex jaws and legs of crustaceans. It is familiar to almost every one, that in a

flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure,

are intelligible on the view that they consist of metamorphosed leaves, arranged in a spire. In monstrous

plants, we often get direct evidence of the possibility of one organ being transformed into another; and

we can actually see, during the early or embryonic stages of development in flowers, as well as incrustaceans and many other animals, that organs, which when mature become extremely different are at

first exactlyalike.

How inexplicable are the cases of serial homologies on the ordinary view of creation! Why should the

brain be enclosed in a box composed of such numerous and such extraordinarily shaped pieces of bone,

apparently representing vertebræ? As Owen has remarked, the benefit derived from the yielding of the

separate pieces in the act of parturition by mammals, will by no means explain the same construction in

the skulls of birds and reptiles.Why should similar bones have been created to form the wing and the leg

of a bat, used as they are for such totally different purposes, namely flying and walking? Why should one

crustacean,which has an extremely complexmouth formed of many parts, consequently always have

fewer legs; or conversely, those with many legs have simpler mouths? Why should the sepals, petals,stamens, and pistils, in each flower, though fitted for such distinct purposes, be all constructed on the

same pattern?

On the theory of natural selection, we can, to a certain extent, answer these questions. We need not here

consider how the bodies of some animals first became divided into a series of segments, or how they

became divided into right and left sides, with corresponding organs, for such questions are almost beyond

investigation. It is, however, probable that some serial structuresare the result of cellsmultiplying by

division, entailing the multiplication of the partsdeveloped from such cells. Itmust suffice for our purpose

to bear in mind that an indefinite repetition of the same part or organ is the common characteristic, as

Owen has remarked, of all low or little specialised forms; therefore the unknown progenitor of theVertebrata probably possessedmany vertebræ; the unknown progenitor of theArticulata, many

segments; and the unknown progenitor of flowering plants, many leaves arranged in one or more spires.

We have also formerly seen that parts many times repeated are eminently liable to vary, not only in

number, but in form. Consequently such parts, being already present in considerable numbers, and being

highly variable,would naturally afford the materials for adaptation to the most different purposes; yet they

would generally retain, through the force of inheritance,plain traces of their original or fundamental

resemblance. They would retain this resemblance all the more, as the variations, which afforded the basis

for their subsequentmodification through natural selection,would tend from the first to be similar; the

parts being at an early stage of growth alike, and being subjected to nearly the same conditions. Such

parts, whether more or less modified, unless their common origin became wholly obscured, would be

serially homologous.

In the great class of molluscs, though the parts in distinct species can be shown to be homologous, only a





few serial homologies, such as the valves of Chitons, can be indicated; that is, we are seldom enabled to

say that one part is homologous with another part in the same individual. And we can understand this

fact; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite

repetition of any one part as we find in the other great classes of the animal and vegetable kingdoms.

But morphology is a much more complex subject than it at first appears, as has lately been well shown in

a remarkable paper by Mr. E. Ray Lankester, who has drawn an important distinction between certainclasses of cases which have all been equally ranked by naturalists as homologous. He proposes to call

the structures which resemble each other in distinct animals owing to their descent from a common

progenitor with subsequent modification,homogenous; and the resemblances which cannot thus be

accounted for, he proposes to callhomoplastic . For instance, he believes that the hearts of birds and

mammals are as a whole homogenous,—that is, have been derived from a common progenitor; but that

the four cavities of the heart in the two classes are homoplastic,—that is, have been independently

developed. Mr. Lankester also adduces the close resemblance of the parts on the right and left sides of

the body, and in the successive segments of the same individual animal; and here we have parts

commonly called homologous, which bear no relation to the descent of distinct species from a common

progenitor. Homoplastic structures are the same with those which I have classed, though in a veryimperfectmanner, as analogous modifications or resemblances. Their formation maybe attributed in part

to distinct organisms, or to distinct parts of the same organism, having varied in an analogous manner; and

in part to similar modifications, having been preserved for the same general purpose or function,—of

which many instanceshavebeen given.

Naturalists frequently speak of the skull as formed of metamorphosed vertebræ; the jaws of crabs as

metamorphosed legs; the stamens and pistils in flowers as metamorposed leaves; but it would in most

cases be more correct, as Professor Huxley has remarked, to speak of both skull and vertebræ, jaws

and legs, &c., as having been metamorphosed, not one from the other, as they now exist, but from some

common and simpler element. Most naturalists, however, use such language only in a metaphorical sense;

they are far from meaning that during a long course of descent, primordial organs of any kind—vertebræin the one case and legs in the other—have actually been converted into skulls or jaws. Yet so strong is

the appearance of this having occurred, that naturalists canhardly avoid employing language having this

plain signification.According to the views here maintained, such languagemay be used literally; and the

wonderful fact of the jaws, for instance, of a crab retaining numerous characters which they probably

wouldhave retained through inheritance, if they had really beenmetamorphosed from true through

extremelysimple legs, is in part explained.

Development and Embryology.

This is one of the most important subjects in the whole round of history. The metamorphoses of insects,

with which every one is familiar, are generally effected abruptly by a few stages; but the transformations

are in reality numerous andgradual, though concealed. A certain ephemerous insect (Chlöeon) during its

development, moults, as shown by Sir J. Lubbock, above twenty times, and each time undergoes a

certain amount of change; and in this case we see the act of metamorphosis performed in a primary and

gradual manner. Many insects, andespecially certain crustaceans, showuswhat wonderful changes of

structure can be effected during development. Such changes, however, reach their acme in the so-called

alternate generations of some of the lower animals. It is, for instance, an astonishing fact that a delicate

branching coralline, studded with polypi and attached to a submarine rock, should produce, first by

budding and then by transverse division, a host of huge floating jelly-fishes; and that these should produce

eggs, from whichare hatched swimming animalcules, which attach themselves to rocks and become

developed into branching corallines; and so on in an endless cycle. The belief in the essential identity of





the process of alternate generation and of ordinary metamorphosis has been greatly strengthened by

Wagner's discovery of the larvæ or maggot of a fly, namely the Cecidomyia, producing asexually other

larvæ, and these others, which finally are developed into mature males and females, propagating their

kind in the ordinary manner by eggs.

It may be worth notice that when Wagner's remarkable discovery was first announced, I was asked how

was it possible to account for the larvæ of this fly having acquired the power of asexual reproduction. Aslong as the case remained unique no answer could be given. But already Grimm has shown that another

fly, a Chironomus, reproduces itself in nearly the same manner, and he believes that this occurs frequently

in the Order. It is the pupa, and not the larva, of the Chironomus which has this power; and Grimm

further shows that this case, to a certain extent, "unites that of the Cecidomyia with the parthenogenesis of

the Coccidæ;"—the term parthenogenesis implying that the mature females of the Coccidæ are capable

of producing fertile eggswithout theconcourseof themales. Certain animals belonging to several classes

are now known to have the power of ordinary reproduction at an unusually early age; and we have only

to accelerate parthenogenetic production by gradual steps to an earlier and earlier age,—Chironomus

showing us an almost exactly intermediate stage, viz., that of the pupa—and we can perhaps account for

themarvellous case of the Cecidomyia.

It has already been stated that various parts in the same individual which are exactly alike during an early

embryonic period, become widely different and serve for widely different purposes in the adult state. So

again it has been shown that generally the embryos of the most distinct species belonging to the same

class are closely similar, but become, when fully developed, widely dissimilar. A better proof of this latter

fact cannot be given than the statement by Von Baer that "the embryos of mammalia, of birds, lizards,

and snakes, probably also of chelonia are in their earliest states exceedingly like one another, both as a

whole and in the mode of development of their parts; so much so, in fact, that we can often distinguish the

embryos only by their size. In my possession are two little embryos in spirit, whose names I have omitted

to attach, and at present I am quite unable to say to what class they belong. They may be lizards or small

birds, or very young mammalia, so complete is the similarity in the mode of formation of the head andtrunk in these animals. The extremities, however, are still absent in these embryos. But even if they had

existed in the earliest stage of their development we should learn nothing, for the feet of lizards and

mammals, the wings and feet of birds, no less than the hands and feet of man, all arise from the same

fundamental form." The larvæof most crustaceans, at corresponding stages of development, closely

resemble each other, however different the adults may become; and so it is with very many other animals.

A trace of the law of embryonic resemblance occasionally lasts till a rather late age: thus birds of the

same genus, and of allied genera, often resemble each other in their immature plumage; as we see in the

spotted feathers in the young of the thrush group. In the cat tribe, most of the species when adult are

striped or spotted in lines; and stripes or spots can be plainly distinguished in the whelp of the lion and the

puma. We occasionally though rarely see something of the same kind in plants; thus the first leaves of theulex or furze, and the first leaves of the phyllodineous acacias, are pinnate or divided like the ordinary

leaves of the leguminosæ.

The points of structure, in which the embryos of widely different animals within the same class resemble

each other, often have no direct relation to their conditions of existence. We cannot, for instance,

suppose that in the embryos of the vertebrata the peculiar loop-like courses of the arteries near the

branchial slitsare related to similarconditions,—in the young mammalwhich is nourished in the wombof

its mother, in the egg of the bird which is hatched in a nest, and in the spawn of a frog under water. We

have no more reason to believe in such a relation, than we have to believe that the similar bones in the

hand of a man, wing of a bat, and fin of a porpoise, are related to similar conditions of life. No one

supposes that the stripes on the whelp of a lion, or the spots on the young blackbird, are of any use to

these animals.





The case, however, is different when an animal during any part of its embryonic career is active, and has

to provide for itself. The period of activity may come on earlier or later in life; but whenever it comes on,

the adaptation of the larvæ to its conditions of life is just as perfect and as beautiful as in the adult animal.

In how important a manner this has acted, has recently been well shown by Sir J. Lubbock in his remarks

on the close similarity of the larvæ of some insects belonging to very different orders, and on the

dissimilarity of the larvæ of other insects within the same order, according to their habits of life. Owing to

such adaptations, the similarityof the larvæof allied animals is sometimes greatly obscured; especiallywhen there is a division of labour during the different stages of development, as when the same larvæ has

during one stage to search for food, and during another stage has to search for a place of attachment.

Cases can even be given of the larvæ of allied species, or groups of species, differing more from each

other than do the adults. In most cases, however, the larvæ, though active, still obey, more or less

closely, the law of common embryonic resemblance. Cirripedes afford a good instance of this; even the

illustrious Cuvier did not perceive that a barnacle was a crustacean: but a glance at the larvæ shows this

in an unmistakable manner. So again the two main divisions of cirripedes, the pedunculated and sessile,

thoughdifferingwidely in external appearance,have larvæ in all their stages barely distinguishable.

The embryo in the course of development generally rises in organisation; I use this expression, though Iam aware that it is hardly possible to define clearly what is meant by the organisation being higher or

lower. But no one probably will dispute that the butterfly is higher than the caterpillar. In some cases,

however, the mature animal must be considered as lower in the scale than the larva, as with certain

parasitic crustaceans. To refer once again to cirripedes: the larvæ in the first stage have three pairs of

locomotive organs, a simple single eye, and a probosciformed mouth, with which they feed largely, for

they increase much in size. In the second stage, answering to the chrysalis stage of butterflies, they have

six pairsof beautifully constructed natatory legs, a pair ofmagnificent compound eyes, andextremely

complex antennæ; but they have a closed and imperfect mouth, and cannot feed: their function at this

stage is, to search out by their well-developed organs of sense, and to reach by their active powers of

swimming, a proper place on which to become attached and to undergo their final metamorphosis. When

this is completed they are fixed for life: their legs are now converted into prehensile organs; they againobtain a well-constructed mouth; but they have no antennæ, and their two eyes are now reconverted into

a minute, single, simple eye-spot. In this last and complete state, cirripedes may be considered as either

more highly or more lowly organised than they were in the larval condition. But in some genera the larvæ

become developed into hermaphrodites having the ordinary structure, and into what I have called

complemental males; and in the latter the development has assuredly been retrograde, for the male is a

mere sack, which lives for a short time and is destitute of mouth, stomach, and every other organ of

importance, excepting those for reproduction.

We are so much accustomed to see a difference in structure between the embryo and the adult, that we

are tempted to look at this difference as in some necessary manner contingent on growth. But there is noreason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out

with all their parts in proper proportion, as soon as any part became visible. In some whole groups of

animals and in certain members of other groups this is the case, and the embryo does not at any period

differ widely from the adult: thus Owen has remarked in regard to cuttle-fish, "there is no metamorphosis;

the cephalopodic character is manifested long before the parts of the embryo are completed."

Land-shells and fresh-water crustaceans are born having their proper forms, whilst themarinemembers

of the same two great classes pass through considerable and often great changes during their

development. Spiders, again, barely undergo anymetamorphosis. The larvæ ofmost insects pass through

a worm-like stage, whether they are active and adapted to diversified habits, or are inactive from being

placed in the midst of proper nutriment or from being fed by their parents; but in some few cases, as in

that of Aphis, if we look to the admirable drawings of the development of this insect, by Professor

Huxley, we see hardly any trace of the vermiform stage.





Sometimes it is only the earlier developmental stages which fail. Thus Fritz Müller has made the

remarkable discovery that certain shrimp-like crustaceans (allied to Penæus) first appear under the simple

nauplius-form, and after passing through twoor more zoea-stages, and then through themysis-stage,

finally acquire their mature structure: now in the whole great malacostracan order, to which these

crustaceans belong, no other member is as yet known to be first developed under the nauplius-form,

though many appear as zoeas; nevertheless Müler assigns reasons for his belief, that if there had been no

suppressionof development, all these crustaceanswould have appeared as nauplii.

How, then, can we explain these several facts in embryology,—namely, the very general, though not

universal, difference in structure between the embryo and the adult;—the various parts in the same

individual embryo, whichultimately become very unlike and serve for diversepurposes, being at an early

period of growth alike;—the common, but not invariable, resemblance between the embryos or larvæ of

the most distinct species in the same class;—the embryo often retaining whilst within the egg or womb,

structures which are of no service to it, either at that or at a later period of life; on the other hand larvæ,

which have to provide for their own wants, being perfectly adapted to the surrounding conditions;—and

lastly the fact of certain larvæ standing higher in the scale of organisation than the mature animal into

which they are developed? I believe that all these facts can be explained, as follows.

It is commonly assumed, perhaps from monstrosities affecting the embryo at a very early period, that

slight variations or individual differences necessarily appear at an equally early period.Wehave little

evidence on this head, but what we have certainly points the other way; for it is notorious that breeders of

cattle, horses, and various fancy animals, cannot positively tell, until some time after birth, what will be the

merits or demerits of their young animals. We see this plainly in our own children; we cannot tell whether

a child will be tall or short, or what its precise features will be. The question is not, at what period of life

each variation may have been caused, but at what period the effects are displayed. The cause may have

acted, and I believe often has acted, on one or both parents before the act of generation. It deserves

notice that it is of no importance to a very young animal, as long as it remains in its mother's womb or in

the egg, or as long as it is nourished and protected by its parent, whether most of its characters areacquired a little earlier or later in life. It would not signify, for instance, to a bird which obtained its food

by having a much-curved beak whether or not whilst young it possessed a beak of this shape, as long as

it was fed by its parents.

I have stated in the first chapter, that at whatever age a variation first appears in the parent, it tends to

re-appear at a corresponding age in the offspring. Certain variations can only appear at corresponding

ages; for instance, peculiarities in the caterpillar, cocoon, or imago states of the silk-moth: or, again, in the

full-grown horns of cattle. But variations, which, for all that we can see might have first appeared either

earlier or later in life, likewise tend to re-appear at a corresponding age in the offspring and parent. I am

far from meaning that this is invariably the case and I could give several exceptional cases of variations(taking the word in the largest sense) which have supervened at an earlier age in the child than in the

parent.

These two principles, namely, that slight variations generally appear at a not very early period of life, and

are inherited at a corresponding not early period, explain, as I believe, all the above specified leading

facts in embryology. But first let us look to a few analogous cases in our domestic varieties. Some

authors who have written on Dogs, maintain that the greyhound and bulldog, though so different, are

really closely allied varieties, descended from the same wild stock; hence I was curious to see how far

their puppies differed from each other: I was told by breeders that they differed just as much as their

parents, and this, judging by the eye, seemed almost to be the case; but on actually measuring the old

dogs and their six-days-old puppies, I found that the puppies had not acquired nearly their full amount of

proportional difference. So, again, I was told that the foals of cart and race-horses—breeds which have

been almost wholly formed by selection under domestication—differedasmuch as the full-grown





animals; but having had careful measurements made of the dams and of three-days-old colts of race and

heavy cart-horses, I find that this is by no means the case.

As we have conclusive evidence that the breeds of the Pigeon are descended from a single wild species,

I compared the young within twelve hours after being hatched; I carefully measured the proportions (but

will not here give the details) of the beak, width of mouth, length of nostril and of eyelid, size of feet and

length of leg, in the wild parent-species, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now some of these birds, when mature, differ in so extraordinary a manner in the length and form of

beak, and in other characters, that they would certainly have been ranked as distinct genera if found in a

state of nature. But when the nestling birds of these several breeds were placed in a row, though most of

them could just be distinguished, the proportional differences in the above specified points were

incomparably less than in the full-grown birds. Some characteristic points of difference—for instance, that

of the width of mouth—could hardly be detected in the young. But there was one remarkable exception

to this rule, for the young of the short-faced tumbler differed from the young of the wild rock-pigeon and

of the other breeds, in almost exactly the same proportions as in the adult state.

These facts are explained by the above two principles. Fanciers select their dogs, horses, pigeons, &c.,for breeding, when nearly grown up: they are indifferent whether the desired qualities are acquired earlier

or later in life, if the full-grown animal possesses them. And the cases just given, more especially that of

the pigeons, show that the characteristic differenceswhich have been accumulated byman's selection,

and which give value to his breeds, do not generally appear at a very early period of life, and are

inherited at a corresponding not early period. But the case of the short-faced tumbler, which when twelve

hours old possessed its proper characters, proves that this is not the universal rule; for here the

characteristic differences must either have appeared at an earlier period than usual, or, if not so, the

differences must have been inherited, not at a corresponding, but at an earlier age.

Now let us apply these two principles to species in a state of nature. Let us take a group of birds,

descended from some ancient form and modified through natural selection for different habits. Then, fromthe many slight successive variations having supervened in the several species at a not early age, and

having been inherited at a corresponding age, the young will have been but little modified, and they will

still resemble each other much more closely than do the adults,—just as we have seen with the breeds of

the pigeon. We may extend this view to widely distinct structures and to whole classes. The fore-limbs,

for instance, which once served as legs to a remote progenitor, may have become, through a long course

of modification, adapted in one descendant to act as hands, in another as paddles, in another as wings;

but on the above two principles the fore-limbs will not have been much modified in the embryos of these

several forms; although in each form the forelimbwill differ greatly in the adult state. Whatever influence

long-continued use or disuse may have had in modifying the limbs or other parts of any species, this will

chiefly or solely have affected it when nearly mature, when it was compelled to use its full powers to gainits own living; and the effects thus produced will have been transmitted to the offspring at a corresponding

nearly mature age. Thus the young will not be modified, or will be modified only in a slight degree,

through the effects of the increased use or disuse of parts.

With some animals the successive variations may have supervened at a very early period of life, or the

steps may have been inherited at an earlier age than that at which they first occurred. In either of these

cases, the young or embryo will closely resemble the mature parent-form, as we have seen with the

short-faced tumbler. And this is the rule of development in certain whole groups, or in certain sub-groups

alone, as with cuttle-fish, land-shells, fresh-water crustaceans, spiders, and some members of the great

class of insects. With respect to the final cause of the young in such groups not passing through any

metamorphosis, we cansee that thiswould follow from the followingcontingencies; namely, from the

young having to provide at a very early age for their own wants, and from their following the same habits

of life with their parents; for in this case, it would be indispensable for their existence that they should be





modified in the same manner as their parents. Again, with respect to the singular fact that many terrestrial

and fresh-water animals do not undergo any metamorphosis, whilst marine members of the same groups

pass throughvarious transformations, Fritz Müller has suggested that the process of slowlymodifying and

adapting an animal to live on the land or in fresh water, instead of in the sea, would be greatly simplified

by its not passing through any larval stage; for it is not probable that places well adapted for both the

larval and mature stages, under such new and greatly changed habits of life, would commonly be found

unoccupied or ill-occupied by other organisms. In this case the gradual acquirement at an earlier andearlier age of the adult structure would be favoured by natural selection; and all traces of former

metamorphoseswould finallybe lost.

If, on the other hand, it profited the young of an animal to follow habits of life slightly different from those

of the parent-form, and consequently to be constructed on a slightly different plan, or if it profited a larvæ

already different from its parent to change still further, then, on the principle of inheritance at

corresponding ages, the young or the larvæ might be rendered by natural selection more and more

different from their parents to any conceivable extent. Differences in the larvæmight, also, become

correlated with successive stages of its development; so that the larva, in the first stage, might come to

differ greatly from the larvæ in the second stage, as is the case with many animals. The adult might also become fitted for sites or habits, in which organs of locomotion or of the senses, &c., would be useless;

and in this case the metamorphosis would be retrograde.

From the remarks just made we can see how by changes of structure in the young, in conformity with

changed habits of life, together with inheritance at corresponding ages, animalsmight come to pass

through stages of development, perfectly distinct from the primordial condition of their adult progenitors.

Most of our best authorities are now convinced that the various larval and pupal stages of insects have

thus been acquired through adaptation, and not through inheritance from some ancient form.The curious

case of Sitaris—a beetle whichpasses through certain unusual stagesof development—will illustrate how

this might occur. The first larval form is described by M. Fabre, as an active, minute insect, furnished with

six legs, two long antennæ, and four eyes. These larvæ are hatched in the nests of bees; and when themale-bees emerge from their burrows, in the spring, which they do before the females, the larvæ spring

on them, and afterwards crawl on to the females whilst paired with the males. As soon as the female bee

deposits her eggs on the surface of the honey stored in the cells, the larvæ of the Sitaris leap on the eggs

and devour them. Afterwards they undergo a complete change; their eyes disappear; their legs and

antennae become rudimentary, and they feed on honey; so that they now more closely resemble the

ordinary larvæof insects; ultimately theyundergo a further transformation, and finallyemerge as the

perfect beetle. Now, if an insect, undergoing transformations like those of the Sitaris, were to become the

progenitor of a whole new class of insects, the course of development of the new class would be widely

different from that of our existing insects; and the first larval stage certainly would not represent the

former condition of any adult and ancient form.

On the other hand it is highly probable that with many animals the embryonic or larval stages show us,

more or less completely, the condition of the progenitor of the whole group in its adult state. In the great

class of the Crustacea, formswonderfully distinct from each other, namely, suctorial parasites, cirripedes,

entomostraca, and even the malacostraca, appear at first as larvæ under the nauplius-form; and as these

larvæ live and feed in the open sea, and are not adapted for any peculiar habits of life, and from other

reasons assigned by Fritz Müller, it is probable that at some very remote period an independent adult

animal, resembling theNauplius, existed, andsubsequentlyproduced, along several divergent lines of

descent, the above-named great Crustacean groups. So again it is probable, from what we know of the

embryos of mammals, birds, fishes, and reptiles, that these animals are the modified descendants of some

ancient progenitor, whichwas furnished in its adult statewith branchiæ, a swim-bladder, four fin-like

limbs, and a long tail, all fitted for an aquatic life.





As all the organic beings, extinct and recent, which have ever lived, can be arranged within a few great

classes; and as all within each class have, according to our theory, been connected together by fine

gradations, the best, and, if our collections were nearly perfect, the only possible arrangement, would be

genealogical; descent being thehiddenbondof connexion whichnaturalists have been seeking under the

term of the Natural System. On this view we can understand how it is that, in the eyes of most naturalists,

the structure of the embryo is even more important for classification than that of the adult. In two or more

groups of animals, however much they may differ from each other in structure and habits in their adultcondition, if they pass through closely similar embryonic stages, we may feel assured that they all are

descended from one parent-form, andare therefore closely related. Thus, community in embryonic

structure reveals communityof descent; but dissimilarity in embryonic development does not prove

discommunity of descent, for in one of two groups the developmental stages may have been suppressed,

or may have been so greatly modified through adaptation to new habits of life, as to be no longer

recognisable. Even in groups, in which the adults have been modified to an extreme degree, community of

origin is often revealed by the structure of the larvæ; we have seen, for instance, that cirripedes, though

externally so like shellfish, are at once known by their larvæ to belong to the great class of crustaceans.

As the embryo often shows us more or less plainly the structure of the less modified and ancient

progenitor of the group, we can see why ancient and extinct forms so often resemble in their adult statethe embryos of existing species of the same class. Agassiz believes this to be a universal law of nature;

and we may hope hereafter to see the law proved true. It can, however, be proved true only in those

cases in which the ancient state of the progenitor of the group has not been wholly obliterated, either by

successive variations having supervened at a very early period of growth, or by such variations having

been inherited at an earlier age than that at which they first appeared. It should also be borne in mind, that

the law may be true, but yet, owing to the geological record not extending far enough back in time, may

remain for a long period, or for ever, incapable of demonstration. The law will not strictly hold good in

those cases in which an ancient form became adapted in its larval state to some special line of life, and

transmitted the same larval state to a whole group of descendants; for such larvæ will not resemble any

still more ancient form in its adult state.

Thus, as it seems to me, the leading facts in embryology, which are second to none in importance, are

explained on the principle of variations in the many descendants from some one ancient progenitor,

having appeared at a not very early period of life, and having been inherited at a corresponding period.

Embryology rises greatly in interest, when we look at the embryo as a picture, more or less obscured, of

the progenitor, either in its adult or larval state, of all the members of the same great class.

Rudimentary, Atrophied, and Aborted Organs.

Organs or parts in this strange condition, bearing the plain stamp of inutility, are extremely common, or

even general, throughout nature. It would be impossible to name one of the higher animals in which some

part or other is not in a rudimentary condition. In the mammalia, for instance, the males possess

rudimentarymammæ; in snakes one lobeof the lungs is rudimentary; in birds the "bastard-wing" may

safely be considered as a rudimentary digit, and in some species the whole wing is so far rudimentary that

it cannot be used for flight. What can be more curious than the presence of teeth in fætal whales, which

when grown up have not a tooth in their heads; or the teeth, which never cut through the gums, in the

upper jaws of unborn calves?

Rudimentary organs plainly declare their origin and meaning in various ways. Therearebeetlesbelonging

to closely allied species, or even to the same identical species, which have either full-sized and perfect

wings, ormere rudiments of membrane, whichnot rarely lieunder wing-covers firmly soldered together;

and in these cases it is impossible to doubt, that the rudiments represent wings. Rudimentary organs





sometimes retain their potentiality: this occasionally occurswith the mammæ ofmale mammals, which

have been known to become well developed and to secrete milk. So again in the udders in the genus

Bos, there are normally four developed and two rudimentary teats; but the latter in our domestic cows

sometimes become well developed and yield milk. In regard to plants the petals are sometimes

rudimentary, and sometimes well-developed in the individuals of the same species. In certain plants

having separated sexes Köreuter found that by crossing a species, in which the male flowers included a

rudiment of a pistil,with an hermaphrodite species, having of coursea well-developed pistil, the rudimentin the hybrid offspring was much increased in size; and this clearly shows that the rudimentary and perfect

pistils are essentially alike in nature. An animal may possess various parts in a perfect state, and yet they

may in one sense be rudimentary, for they are useless: thus the tadpole of the common Salamander or

Water-newt, as Mr. G. H. Lewes remarks, "has gills, and passes its existence in the water; but the

Salamandra atra, which lives highupamong the mountains, brings forth its young full-formed.This animal

never lives in the water. Yet if we open a gravid female, we find tadpoles inside her with exquisitely

feathered gills; and when placed in water they swim about like the tadpoles of the water-newt. Obviously

this aquatic organisation has no reference to the future life of the animal, nor has it any adaptation to its

embryonic condition; it has solely reference to ancestral adaptations, it repeats a phase in the

development of its progenitors."

An organ, serving for two purposes, may become rudimentary or utterly aborted for one, even the more

important purpose, and remain perfectly efficient for the other. Thus in plants, the office of the pistil is to

allow the pollen-tubes to reach the ovules within the ovarium. The pistil consists of a stigma supported on

a style; but in some Compositæ, the male florets, which of course cannot be fecundated, have a

rudimentary pistil, for it is not crowned with a stigma; but the style remains well developed and is clothed

in the usual manner with hairs, which serve to brush the pollen out of the surrounding and conjoined

anthers. Again, an organ may become rudimentary for its proper purpose, and be used for a distinct one:

in certain fishes the swim-bladder seems to be rudimentary for its proper function of giving buoyancy, but

has become converted into a nascent breathing organ or lung. Many similar instances could be given.

Useful organs, however little they may be developed, unless we have reason to suppose that they were

formerly more highly developed, ought not to be considered as rudimentary. They may be in a nascent

condition, and in progress towards further development. Rudimentary organs, on the other hand, are

either quite useless, such as teeth which never cut through the gums, or almost useless, such as the wings

of an ostrich, which serve merely as sails. As organs in this condition would formerly, when still less

developed, have been of even less use than at present, they cannot formerly have been produced through

variation and natural selection, whichacts solely by the preservationof useful modifications. Theyhave

been partially retained by the power of inheritance, and relate to a former state of things. It is, however,

oftendifficult to distinguish between rudimentary and nascent organs; for we can judge only by analogy

whether a part is capable of further development, in which case alone it deserves to be called nascent.Organs in this condition will always be somewhat rare; for beings thus provided will commonly have been

supplanted by their successors with the same organ in a more perfect state, and consequently will have

become long ago extinct. The wing of the penguin is of high service, acting as a fin; it may, therefore,

represent the nascent state of the wing: not that I believe this to be the case; it is more probably a

reduced organ, modified for a new function: the wing of the Apteryx, on the other hand, is quite useless,

and is truly rudimentary. Owen considers the simple filamentary limbs of the Lepidosirenas the

"beginnings of organswhich attain full functional development in higher vertebrates;" but, according to the

view lately advocated by Dr. Günther, they are probably remnants, consisting of the persistent axis of a

fin, with the lateral rays or branches aborted. The mammary glands of the Ornithorhynchus may be

considered, in comparison with the udders of a cow, as in a nascent condition. The ovigerous frena of

certain cirripedes, which have ceased to give attachment to the ova and are feebly developed, are

nascent branchiæ.





Rudimentary organs in the individuals of the same species are very liable to vary in the degree of their

development and in other respects. In closely allied species, also, the extent to which the same organ has

been reduced occasionally differs much. This latter fact is well exemplified in the state of the wings of

femalemoths belonging to the same family.Rudimentary organsmay be utterly aborted; and this implies,

that in certain animals or plants, parts are entirely absent which analogy would lead us to expect to find in

them, and whichareoccasionally found inmonstrous individuals. Thus inmost of the Scrophulariaceæ the

fifth stamen is utterly aborted; yet we may conclude that a fifth stamen once existed, for a rudiment of it isfound inmany species of the family, and this rudiment occasionallybecomes perfectly developed, as may

sometimes be seen in the common snap-dragon. In tracing the homologies of any part in different

members of the same class, nothing is more common, or, in order fully to understand the relations of the

parts, more useful than the discovery of rudiments. This is well shown in the drawings given by Owen of

the leg-bones of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants,

can often be detected in the embryo, but afterwards wholly disappear. It is also, I believe, a universal

rule, that a rudimentary part is of greater size in the embryo relatively to the adjoining parts, than in the

adult; so that the organ at this early age is less rudimentary, or even cannot be said to be in any degreerudimentary. Hence rudimentary organs in the adult are often said to have retained their embryonic

condition.

I have now given the leading facts with respect to rudimentary organs. In reflecting on them, every one

must be struck with astonishment; for the same reasoning power which tells us that most parts and organs

are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or

atrophied organs are imperfect and useless. In works on natural history, rudimentary organs are generally

said to have been created "for the sake of symmetry," or in order "to complete the scheme of nature." But

this is not an explanation, merely a re-statement of the fact. Nor is it consistent with itself; thus the

boa-constrictor has rudiments of hind-limbs and of a pelvis, and if it be said that these bones have been

retained "to complete the scheme of nature," why, as Professor Weismann asks, have they not beenretained by other snakes, which do not possess even a vestige of these same bones? What would be the

thought of an astronomer who maintained that the satellites revolve in elliptic courses round their planets

"for the sake of symmetry," because the planets thus revolve round the sun? An eminent physiologist

accounts for the presence of rudimentary organs, by supposing that they serve to excrete matter in

excess, or matter injurious to the system; but can we suppose that the minute papilla, which often

represents the pistil in male flowers, and which is formed of mere cellular tissue, can thus act? Can we

suppose that rudimentary teeth, which are subsequently absorbed, are beneficial to the rapidly growing

embryonic calf by removing matter so precious as phosphate of lime?When a man's fingers have been

amputated, imperfect nails have been known to appear on the stumps, and I could as soon believe that

these vestiges of nails are developed in order to excrete horny matter, as that the rudimentary nails on thefin of the manatee have been developed for this same purpose.

On the view of descent with modification, the origin of rudimentary organs is comparatively simple; and

we can understand to a large extent the laws governing their imperfect development. We have plenty of

cases of rudimentary organs in our domestic productions,—as the stump of a tail in tailless breeds,—the

vestige of an ear in earless breeds of sheep,—the reappearance of minute dangling horns in hornless

breeds of cattle, more especially, according to Youatt, in young animals,—and the state of the whole

flower in the cauliflower. We often see rudiments of various parts in monsters; but I doubt whether any of

these cases throw light on the origin of rudimentary organs in a state of nature, further than by showing

that rudiments can be produced; for the balance of evidence clearly indicates that species under nature

do not undergo great and abrupt changes. But we learn from the study of our domestic productions that

the disuse of parts leads to their reduced size; and that the result is inherited.





It appears probable that disuse has been the main agent in rendering organs rudimentary. It would at first

lead by slow steps to the more and more complete reduction of a part, until at last it became

rudimentary,—as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds

inhabiting oceanic islands, which have seldom been forced by beasts of prey to take flight, and have

ultimately lost the power of flying.Again, an organ, useful under certain conditions,might become

injurious under others, as with the wings of beetles living on small and exposed islands; and in this case

natural selectionwill have aided in reducing the organ, until it was renderedharmless and rudimentary.

Any change in structure and function, which can be effected by small stages, is within the power of

natural selection; so that an organ rendered, through changed habits of life, useless or injurious for one

purpose, might be modified and used for another purpose. An organ might, also, be retained for one

alone of its former functions. Organs, originally formed by the aid of natural selection, when rendered

useless may well be variable, for their variations can no longer be checked by natural selection. All this

agrees well with what we see under nature. Moreover, at whatever period of life either disuse or

selection reduces an organ, and this will generally be when the being has come to maturity and has to

exert its full powers of action, the principle of inheritance at corresponding ages will tend to reproduce

the organ in its reduced state at the same mature age, but will seldom effect it in the embryo. Thus we canunderstand the greater size of rudimentary organs in the embryo relatively to the adjoining parts, and their

lesser relative size in the adult. If, for instance, the digit of an adult animal was used less and less during

many generations, owing to some change of habits, or it an organ or gland was less and less functionally

exercised, we may infer that it would become reduced in size in the adult descendants of this animal, but

would retainnearly its original standard of development in the embryo.

There remains, however, this difficulty. After an organ has ceased being used, and has become in

consequence much reduced, how can it be still further reduced in size until the merest vestige is left; and

how can it be finally quite obliterated? It is scarcely possible that disuse can go on producing any further

effect after the organ has once been rendered functionless. Some additional explanation is here requisite

which I cannot give. If, for instance, it could be proved that every part of the organisation tends to vary ina greater degree towards diminution than towards augmentation of size, then we should be able to

understand how an organ which has become useless would be rendered, independently of the effects of

disuse, rudimentary andwould at last bewholly suppressed; for thevariations towards diminished size

would no longer be checked by natural selection. The principle of the economy of growth, explained in a

former chapter, by which the materials forming any part, if not useful to the possessor, are saved as far as

possible,will perhaps come into play in rendering a useless part rudimentary. But this principle will almost

necessarily be confined to the earlier stages of the process of reduction; for we cannot suppose that a

minute papilla, for instance, representing in a male flower the pistil of the female flower, and formed

merely of cellular tissue, could be further reduced or absorbed for the sake of economising nutriment.

Finally, as rudimentary organs, by whatever steps they may have been degraded into their present

useless condition, are the record of a former state of things, and have been retained solely through the

power of inheritance,—we can understand, on the genealogical view of classification, how it is that

systematists, in placing organisms in their proper places in the natural system, have often found

rudimentary parts as useful as, or even sometimesmore useful than, parts of high physiological

importance. Rudimentary organs may be compared with the letters in a word, still retained in the spelling,

but become useless in the pronunciation, but which serve as a clue for its derivation. On the view of

descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and

useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the

old doctrine of creation, might even have been anticipated in accordance with the views here explained.

Summary.





In this chapter I have attempted to show, that the arrangement of all organic beings throughout all time in

groups under groups—that the nature of the relationships bywhichall living and extinct organisms are

united by complex, radiating, and circuitous lines of affinities into a fewgrand classes,—the rules followed

and the difficulties encountered bynaturalists in their classifications,—the value set upon characters, if

constant and prevalent, whether of high or of the most trifling importance, or, as with rudimentary organs,of no importance,—the wide opposition in value between analogical or adaptive characters, and

characters of true affinity; and other such rules;—all naturally follow if we admit the common parentage of

allied forms, togetherwith theirmodification through variation and natural selection, with the contingencies

of extinction and divergence of character. In considering this view of classification, it should be borne in

mind that the element of descent has been universally used in ranking together the sexes, ages, dimorphic

forms, and acknowledged varieties of the same species, however much they may differ from each other

in structure. If we extend the use of this element of descent,—the one certainly known cause of similarity

in organic beings,—we shall understand what is meant by the Natural System: it is genealogical in its

attempted arrangement, with the grades of acquired difference marked by the terms, varieties, species,

genera, families, orders, and classes.

On this same view of descent with modification, most of the great facts in Morphology become

intelligible,—whether we look to the same pattern displayed by the different species of the same class in

their homologous organs, to whatever purpose applied; or to the serial and lateral homologies in each

individualanimalandplant.

On the principle of successive slight variations, not necessarily or generally supervening at a very early

period of life, and being inherited at a corresponding period, we can understand the leading facts in

Embryology; namely, the close resemblance in the individual embryo of the partswhicharehomologous,

and which when matured become widely different in structure and function; and the resemblance of the

homologous parts or organs in allied though distinct species, though fitted in the adult state for habits asdifferent as is possible. Larvæ are active embryos, which have been specially modified in a greater or less

degree in relation to their habits of life, with their modifications inherited at a corresponding early age. On

these same principles,—and bearing in mind that when organs are reduced in size, either from disuse or

through natural selection, it will generally be at that period of life when the being has to provide for its

own wants, and bearing in mind how strong is the force of inheritance—the occurrence of rudimentary

organs might even have been anticipated. The importance of embryological characters and of rudimentary

organs in classification is intelligible, on the view that a natural arrangementmust begenealogical.

Finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim

so plainly, that the innumerable species, genera and families, with which this world is peopled, are alldescended, each within its own class or group, from common parents, and have all been modified in the

course of descent, that I should without hesitation adopt this view, even if it were unsupported by other

facts or arguments.

|Go to Contents |





Chapter XV

Recapitulation and Conclusion.

Recapitulation of the objections to the theory of Natural Selection—Recapitulation of the general and

special circumstances in its favour—Causes of the general belief in the immutabilityof species—Howfar

the theory of Natural Selection may be extended—Effects of its adoption on the study of Natural

History—Concluding Remarks.

AS this whole volume is one long argument, it may be convenient to the reader to have the leading facts

and inferences briefly recapitulated.

That many and serious objections may be advanced against the theory of descent with modification

through variation and natural selection, I do not deny. I have endeavoured to give to them their full force.

Nothing at first can appear more difficult to believe than that the more complex organs and instincts have

been perfected, not by means superior to, though analogous with, human reason, but by the accumulation

of innumerable slight variations, each good for the individual possessor.Nevertheless, this difficulty,

though appearing to our imagination insuperably great, cannot be considered real if we admit the

following propositions, namely, that all partsof the organisation and instinctsoffer, at least, individual

differences—that there is a struggle for existence leading to the preservation of profitable deviations of

Structure or instinct—and, lastly, that gradations in the state of perfection of each organ may have

existed, each good of its kind. The truth of these propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what gradations many structures have been

perfected,more especially amongst broken and failing groups of organic beings,whichhave suffered

much extinction, but we see so many strange gradations in nature, that we ought to be extremely cautious

in saying that any organ or instinct, or any whole structure, could not have arrived at its present state by

many graduated steps. There are, it must be admitted, cases of special difficulty opposed to the theory of

natural selection; and one of the most curious of these is the existence in the same community of two or

three defined castes of workers or sterile female ants; but I have attempted to show how these difficulties

can be mastered.

With respect to the almost universal sterility of species when first crossed, which forms so remarkable acontrast with the almost universal fertility of varieties when crossed, I must refer the reader to the

recapitulation of the facts given at the end of the ninth chapter, which seem to me conclusively to show

that this sterility is no more a special endowment than is the incapacity of two distinct kinds of trees to be

grafted together; but that it is incidental on differences confined to the reproductive systems of the

intercrossed species. We see the truth of this conclusion in the vast difference in the results of crossing the

same two species reciprocally,—that is, when one species is first used as the father and then as the

mother.Analogy from theconsideration of dimorphic and trimorphic plants clearly leads to the same

conclusion, for when the forms are illegitimately united, they yield few or no seed, and their offspring are

more or less sterile; and these forms belong to the same undoubted species, and differ from each other in

no respect except in their reproductive organs and functions.

Although the fertility of varieties when intercrossed and of their mongrel offspring has been asserted by

so many authors to be universal, this cannot be considered as quite correct after the facts given on the





high authority of Gärtner and Kölreuter. Most of the varieties which have been experimented on have

been produced under domestication; and as domestication (I do not mean mere confinement) almost

certainly tends to eliminate that sterilitywhich, judging fromanalogy, would have affected the

parent-species if intercrossed, we ought not to expect that domesticationwould likewise induce sterility in

theirmodified descendants when crossed. This elimination of sterility apparently follows from the same

causewhichallows our domestic animals to breed freelyunder diversified circumstances; and this again

apparently follows from their having been graduallyaccustomed to frequent changes in their conditions of life.

A double and parallel series of facts seems to throw much light on the sterility of species, when first

crossed, and of their hybrid offspring. On the one side, there is good reason to believe that slight changes

in the conditions of life give vigour and fertility to all organic beings. We know also that a cross between

the distinct individuals of the same variety, and betweendistinct varieties, increases the number of their

offspring, and certainly gives to them increased size and vigour. This is chieflyowing to the forms which

are crossed having been exposed to somewhat different conditions of life; for I have ascertained by a

laborious series of experiments that if all the individuals of the same variety be subjected during several

generations to the same conditions, the good derived from crossing is oftenmuch diminished orwhollydisappears. This is one side of the case. On the other side, we know that species which have long been

exposed to nearly uniform conditions, when they are subjected under confinement to new and greatly

changed conditions, either perish, or if they survive, are rendered sterile, though retainingperfect health.

This does not occur, or only in a very slight degree, with our domesticated productions, which have long

been exposed to fluctuating conditions. Hence when we find that hybrids produced by a cross between

two distinct species are few in number, owing to their perishing soon after conception or at a very early

age, or if surviving that they are rendered more or less sterile, it seems highly probable that this result is

due to their having been in fact subjected to a great change in their conditions of life, from being

compounded of two distinct organisations. He who will explain in a definite manner why, for instance, an

elephant or a fox will not breed under confinement in its native country, whilst the domestic pig or dog

will breed freely under the most diversified conditions, will at the same time be able to give a definiteanswer to the question why two distinct species, when crossed, as well as their hybrid offspring, are

generally rendered more or less sterile, whilst two domesticated varieties when crossed and their mongrel

offspring are perfectly fertile.

Turning to geographicaldistribution, the difficulties encounteredon the theory of descent with

modification are serious enough. All the individuals of the same species, and all the species of the same

genus, or even higher group, are descended from common parents; and therefore, in however distant and

isolated parts of the world they may now be found, they must in the course of successive generations

have travelled from some one point to all the others.We are often wholly unable even to conjecture how

this could have been effected. Yet, as we have reason to believe that some species have retained thesame specific form for very long periods of time, immensely long as measured by years, too much stress

ought not to be laid on the occasional wide diffusion of the same species; for during very long periods

there will always have been a good chance for wide migration by many means. A broken or interrupted

range may often be accounted for by the extinction of the species in the intermediate regions. It cannot be

denied that we are as yet very ignorant as to the full extent of the various climatal and geographical

changes whichhave affected theearth during modern periods; andsuch changes will oftenhave facilitated

migration. As an example, I have attempted to show how potent has been the influence of the Glacial

period on the distribution of the same and of allied species throughout the world. We are as yet

profoundly ignorant of the many occasional means of transport. With respect to distinct species of the

same genus inhabiting distant and isolated regions, as the process ofmodification has necessarilybeen

slow, all the means of migration will have been possible during a very long period; and consequently the

difficulty of the wide diffusion of the species of the same genus is in some degree lessened.





As according to the theory of natural selection an interminable number of intermediate forms must have

existed, linking together all the species in each group by gradations as fine as are our existing varieties, it

may be asked, Why do we not see these linking forms all around us? Why are not all organic beings

blended together in an inextricable chaos? With respect to existing forms, we should remember that we

have no right to expect (excepting in rare cases) to discover directly connecting links between them,but

only between each and some extinct and supplanted form. Even on a wide area, which has during a long

period remained continuous, and ofwhich the climatic and other conditionsof life change insensibly in proceeding from a district occupied by one species into another district occupied by a closely allied

species, we have no just right to expect often to find intermediate varieties in the intermediate zones. For

we have reason to believe that only a few species of a genus ever undergo change; the other species

becoming utterly extinct and leaving no modified progeny. Of the species which do change, only a few

within the same country change at the same time; and all modifications are slowly effected. I have also

shown that the intermediate varieties which probably at first existed in the intermediate zones, would be

liable to be supplanted by the allied forms on either hand; for the latter, from existing in greater numbers,

would generally be modified and improved at a quicker rate than the intermediate varieties, which existed

in lesser numbers; so that the intermediate varieties would, in the long run, be supplanted and

exterminated.

On this doctrine of the extermination of an infinitude of connecting links, between the living and extinct

inhabitants of the world, and at each successive period between the extinct and still older species, why is

not everygeological formation chargedwith such links? Why does not every collection of fossil remains

afford plain evidence of the gradation and mutation of the forms of life? Although geological research has

undoubtedly revealed the former existence ofmany links, bringingnumerous forms of life much closer

together, it does not yield the infinitely many fine gradations between past and present species required

on the theory; and this is the most obvious of the many objections which may be urged against it. Why,

again, do whole groups of allied species appear, though this appearance is often false, to have come in

suddenly on the successive geological stages? Although we now know that organic beings appeared on

this globe, at a period incalculably remote, long before the lowest bed of the Cambrian system wasdeposited, why do we not find beneath this system great piles of strata stored with the remains of the

progenitors of the Cambrian fossils? For on the theory, such strata must somewhere have been deposited

at these ancient and utterly unknown epochs of the world's history.

I can answer these questions and objections only on the supposition that the geological record is far

more imperfect thanmost geologists believe. The number of specimens in all our museums is absolutely

as nothing compared with the countless generations of countless species which have certainly existed.

The parent-form of any two or more species would not be in all its characters directly intermediate

between its modified offspring, any more than the rock-pigeon is directly intermediate in crop and tail

between its descendants, the pouter and fantail pigeons. We should not be able to recognise a species asthe parent of another and modified species, if we were to examine the two ever so closely, unless we

possessed most of the intermediate links; and owing to the imperfection of the geological record, we have

no just right to expect to find so many links. If two or three, or even more linking forms were discovered,

they would simply be ranked by many naturalists as so many new species, more especially if found in

different geological sub-stages, let their differencesbe ever so slight. Numerous existing doubtful forms

could be named which are probably varieties; but who will pretend that in future ages so many fossil links

will be discovered, that naturalists will be able to decide whether or not these doubtful forms ought to be

called varieties? Only a small portion of the world has been geologically explored. Only organic beings of

certain classes can be preserved in a fossil condition, at least in any great number. Many species when

once formed neverundergoanyfurther change but become extinct without leaving modified descendants;

and the periods, during which species have undergone modification, though long asmeasuredby years,

have probably been short in comparison with the periods during which they retain the same form. It is the

dominant and widely ranging species which vary most frequently and vary most, and varieties are often at





first local—both causes rendering the discoveryof intermediate links in any one formation less likely.

Local varieties will not spread into other and distant regions until they are considerably modified and

improved; and when they have spread, and are discovered in a geological formation, they appear as if

suddenly created there, and will be simply classed as new species. Most formations have been

intermittent in their accumulation; and their duration has probably been shorter than theaverage duration

of specific forms. Successive formations are in most cases separated from each other by blank intervals

of time of great length; for fossiliferous formations thick enough to resist future degradation can as ageneral rule be accumulated only where much sediment is deposited on the subsiding bed of the sea.

During the alternate periods of elevation and of stationary level the record will generally be blank. During

these latter periods there will probably be more variability in the forms of life; during periods of

subsidence,more extinction.

With respect to the absence of strata rich in fossils beneath the Cambrian formation, I can recur only to

the hypothesis given in the tenth chapter; namely, that though our continents and oceans have endured for

an enormous period in nearly their present relative positions, we have no reason to assume that this has

always been the case; consequently formations much older than any now known may lie buried beneath

the great oceans. With respect to the lapse of time not having been sufficient since our planet wasconsolidated for the assumed amount of organic change, and this objection, as urged by Sir William

Thompson, is probably one of the gravest as yet advanced, I can only say, firstly, that we do not know at

what rate species change as measured by years, and secondly, that many philosophers are not as yet

willing to admit that we know enough of the constitution of the universe and of the interior of our globe to

speculate with safety on its past duration.

That the geological record is imperfect all will admit; but that it is imperfect to the degree required by our

theory, few will be inclined to admit. If we look to long enough intervals of time, geology plainly declares

that species have all changed; and they have changed in the manner required by the theory, for they have

changed slowly and in a graduated manner. We clearly see this in the fossil remains from consecutive

formations invariably being much more closely related to each other, than are the fossils from widelyseparated formations.

Such is the sum of the several chief objections and difficulties which may be justly urged against the

theory; and I have now briefly recapitulated the answers and explanations which, as far as I can see, may

be given. I have felt these difficulties far too heavily during many years to doubt their weight. But it

deserves especial notice that the more important objections relate to questions on which we are

confessedly ignorant; nor do we know how ignorant we are. We do not know all the possible transitional

gradations between the simplest and the most perfect organs; it cannot be pretended that we know all the

varied means of Distribution during the long lapse of years, or that we know how imperfect is the

Geological Record. Serious as these several objections are, in my judgment they are by no meanssufficient to overthrow the theory of descentwith subsequentmodification.

Now let us turn to the other side of the argument Under domestication we see much variability, caused,

or at least excited, by changed conditions of life; but often in so obscure a manner, that we are tempted

to consider the variations as spontaneous.Variability is governed bymany complex laws,—by correlated

growth, compensation, the increased use and disuse of parts, and the definite action of the surrounding

conditions. There ismuchdifficulty in ascertaining how largelyour domestic productions havebeen

modified; but we may safely infer that the amount has been large, and that modifications can be inherited

for long periods. As long as the conditions of life remain the same, we have reason to believe that a

modificationwhich has already been inherited for many generations, may continue to be inherited for an

almost infinite number of generations. On the other hand, we have evidence that variability when it has

once come into play, does not cease under domestication for a very long period; nor do we know hat it





ever ceases, for new varieties are still occasionally produced by our oldest domesticated productions.

Variability is not actuallycaused byman; he only unintentionally exposes organic beings to new

conditions of life, and then nature acts on the organisation and causes it to vary. But man can and does

select the variations given to him by nature, and thus accumulates hem in any desired manner. He thus

adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it

unconsciously bypreserving the individuals most usefulor pleasing to him without any intention of alteringthe breed. It is certain that he can largely influence the character of a breed by selecting, in each

successive generation, individual differences so slight as to be inappreciable except by an educated eye.

This unconscious process of selection has been the great agency in the formation of the most distinct and

useful domestic breeds. That many breeds produced by man have to a large extent the character of

natural species, is shown by the inextricable doubts whether many of them are varieties or originally

distinct species.

There is no reason why the principles which have acted so efficiently under domestication should not

have acted undernature. In the survival of favoured individuals and races, during the constantly-recurrent

Struggle for Existence, we see a powerful and ever-acting form of Selection. The struggle for existenceinevitably follows from the highgeometrical ratio of increase which is common to all organic beings. This

high rate of increase is proved by calculation,—by the rapid increase of many animals and plants during a

succession of peculiar seasons, and when naturalised in new countries. More individuals are born than

can possibly survive.A grain in the balance may determine which individuals shall live and which shall

die,—which variety or species shall increase in number, and which shall decrease, or finally become

extinct. As the individuals of the same species come in all respects into the closest competition with each

other, the struggle will generally be most severe between them; it will be almost equally severe between

the varieties of the same species, and next in severity between the species of the same genus. On the

other hand the struggle will often be severe between beings remote in the scale of nature. The slightest

advantage in certain individuals, at any age or during any season, over those with which they come into

competition, or better adaptation in however slight a degree to the surrounding physical conditions, will,in the long run, turn the balance.

With animals having separated sexes, there will be in most cases a struggle between the males for the

possession of the females. The most vigorous males, or those whichhave most successfully struggledwith

their conditions of life, will generally leave most progeny. But success will often depend on the males

having special weapons, or means of defence, or charms; and a light advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physical changes, we might have

expected to find that organic beings have varied under nature, in the same way as they have varied under

domestication. And if there has been any variability under nature, it would be an unaccountable fact if natural selection had not come into play. It has often been asserted, but the assertion is incapable of

proof, that the amount of variation under nature is a strictly limited quantity. Man, though acting on

external characters alone and often capriciously, can produce within a short period a great result by

adding upmere individualdifferences in his domestic productions; andevery one admits that species

present individual differences. But, besides such differences, all naturalists admit that natural varieties

exist, which are considered sufficiently distinct to be worthy of record in systematic works. No one has

drawn any clear distinction between individual differencesand slight varieties; or betweenmore plainly

marked varieties and sub-species, and species. On separate continents, and on different parts of the

same continent when divided by barriers of any kind, and on outlying islands, what a multitude of forms

exist, which some experienced naturalists rank as varieties, others as geographical races or sub-species,

andothers as distinct, though closely allied species!

If then, animals and plants do vary, let it be ever so slightly or slowly, why should not variations or





individual differences, whichare in anyway beneficial, be preserved and accumulated through natural

selection, or the survival of the fittest? If man can by patience select variations useful to him, why, under

changingand complexconditions of life, should not variations useful to nature's living products often arise,

and be preserved or selected? What limit can be put to this power, acting during long ages and rigidly

scrutinising the whole constitution, structure, andhabits of each creature,—favouring the good and

rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting each form to the

most complex relations of life. The theory of natural selection, even if we look no farther than this, seemsto be in the highest degree probable. I have already recapitulated, as fairly as I could, the opposed

difficulties and objections: now let us turn to the special facts and arguments in favour of the theory.

On the view that species are only strongly marked and permanent varieties, and that each species first

existed as a variety, we can see why it is that no line of demarcation can be drawn between species,

commonly supposed to have been produced by special acts of creation, and varieties which are

acknowledged to have been produced by secondary laws. On this same view we can understand how it

is that in a region where many species of a genus have been produced, and where they now flourish,

these same species should present many varieties; for where the manufactory of species has been active,we might expect, as a general rule, to find it still in action; and this is the case if varieties be incipient

species. Moreover, the species of the larger genera, which afford the greater number of varieties or

incipient species, retain to a certain degree the character of varieties; for they differ from each other by a

less amount of difference than do the species of smaller genera. The closely allied species also of the

larger genera apparentlyhave restricted ranges, and in their affinities they are clustered in littlegroups

round other species—in both respects resembling varieties. These are strange relations on the view that

each species was independently created, but are intelligible if each existed first as a variety.

As each species tends by its geometrical rate of reproduction to increase inordinately in number; and as

the modified descendants of each species will be enabled to increase by as much as they become more

diversified in habits and structure, so as to be able to seize on many and widely different places in theeconomy of nature, there will be a constant tendency in natural selection to preserve the most divergent

offspring of any one species.Hence, during a long-continued course ofmodification, the slight differences

characteristic of varieties of the same species, tend to be augmented into the greater differences

characteristic of the species of the same genus. New and improved varieties will inevitably supplant and

exterminate the older, less improved, and intermediate varieties; and thus species are rendered to a large

extent defined anddistinct objects. Dominant species belonging to the larger groupswithin each class

tend to give birth to new and dominant forms; so that each large group tends to become still larger, and at

the same time more divergent in character. But as all groups cannot thus go on increasing in size, for the

world would not hold them, the more dominant groups beat the less dominant. This tendency in the large

groups to goon increasing in size and diverging in character, together with the inevitable contingency of much extinction, explains the arrangement of all the forms of life in groups subordinate to groups, all

within a few great classes, which has prevailed throughout all time. This grand fact of the grouping of all

organic beings under what is called the Natural System, is utterly inexplicable on the theory of creation.

As natural selection acts solely by accumulating slight, successive, favourable variations, it canproduce

no great or sudden modifications; it can act only by short and slow steps. Hence, the canon of "Natura

non facit saltum," which every fresh addition to our knowledge tends to confirm, is on this theory

intelligible.We can see why throughout nature the same general end is gained by an almost infinite

diversity of means, for every peculiaritywhen once acquired is long inherited, and structures already

modified in many different ways have to be adapted for the same general purpose. We can, in short, see

why nature is prodigal in variety, though niggard in innovation. But why this should be a law of nature if

each species has been independently created no man can explain.





Many other facts are, as it seems to me, explicable on this theory. How strange it is that a bird, under the

form of a woodpecker, should prey on insects on the ground; that upland geese which rarely or never

swim, should possess webbed feet; that a thrush-like bird should dive and feed on sub-aquatic insects;

and that a petrel should have the habits and structure fitting it for the life of an auk! and so in endless

other cases. But on the view of each species constantly trying to increase in number, with natural

selection always ready to adapt the slowly varying descendants of each to any unoccupied or ill-occupied

place in nature, these facts cease to be strange, or might even have been anticipated.

We can to a certain extent understand how it is that there is so much beauty throughout nature; for this

may be largely attributed to the agency of selection. That beauty, according to our sense of it, is not

universal, must be admitted by every one who will look at some venomous snakes, at some fishes, and at

certain hideous bats with a distorted resemblance to the human face. Sexual selection has given the most

brilliant colours, elegant patterns, and other ornaments to the males, and sometimes to both sexes of

many birds, butterflies, and other animals. With birds it has often rendered the voice of the male musical

to the female, as well as to our ears. Flowers and fruit have been rendered conspicuous by brilliant

colours in contrast with the green foliage, in order that the flowers may be readily seen, visited and

fertilised by insects, and the seeds disseminated by birds. How it comes that certain colours, sounds, andforms should give pleasure to man and the lower animals,—that is, how the sense of beauty in its simplest

form was first acquired,—we do not know any more than how certain odours and flavours were first

rendered agreeable.

As natural selection acts by competition, it adapts and improves the inhabitants of each country only in

relation to their co-inhabitants; so that we need feel no surprise at the species of any one country,

although on the ordinary view supposed to have been created and specially adapted for that country,

being beaten and supplanted by the naturalised productions from another land. Nor ought we to marvel if

all the contrivances in nature be not, as far as we can judge, absolutely perfect, as in the case even of the

human eye; or if some of them be abhorrent to our ideas of fitness. We need not marvel at the sting of the

bee, when used against an enemy, causing the bee's own death; at drones being produced in such greatnumbers for one single act, and being then slaughtered by their sterile sisters; at the astonishing waste of

pollen by our fir-trees; at the instinctive hatred of the queen-bee for her own fertile daughters; at the

ichneumonidæ feeding within the living bodies of caterpillars; or at other such cases. The wonder indeed

is, on the theory of natural selection, that more cases of the want of absolute perfection have not been

detected.

The complex and little known laws governing the production of varieties are the same, as far as we can

judge, with the laws which have governed the production of distinct species. In both cases physical

conditions seem to have produced some direct and definite effect, but how much we cannot say. Thus,

when varieties enter any new station, they occasionally assume some of the characters proper to thespecies of that station. With both varieties and species, use and disuse seem to have produced a

considerable effect; for it is impossible to resist this conclusion when we look, for instance, at the

logger-headed duck, which has wings incapable of flight, in nearly the same condition as in the domestic

duck; or when we look at the burrowing tucu-tucu, which is occasionally blind, and then at certain moles,

which are habitually blind and have their eyes covered with skin; or when we look at the blind animals

inhabiting the dark caves of America and Europe. With varieties and species, correlated variation seems

to have played an important part, so that when one part has been modified other parts have been

necessarily modified. With both varieties and species, reversions to long-lost characters occasionally

occur. How inexplicable on the theory of creation is the occasional appearance of stripes on the

shoulders and legs of the several species of the horse-genus and of their hybrids! How simply is this fact

explained if we believe that these species are all descended from a striped progenitor, in the same manner

as the several domestic breeds of the pigeon are descended from the blue and barred rock-pigeon!





On the ordinary view of each species having been independently created, why should specific

characters, or those by which the species of the same genus differ from each other, be more variable than

generic characters in which they all agree? Why, for instance, should the colour of a flower be more likely

to vary in any one species of a genus, if the other species possess differently coloured flowers, than if all

possessed the same coloured flowers? If species are only well-marked varieties, of which the characters

have become in a high degree permanent, we can understand this fact; for they have already varied since

they branched off from a common progenitor in certain characters, by which they have come to bespecifically distinct from each other; therefore these same characters would be more likely again to vary

than the generic characters which have been inherited without change for an immense period. It is

inexplicable on the theory of creation why a part developed in a very unusual manner in one species alone

of a genus, and therefore, as we may naturally infer, of great importance to that species, should be

eminently liable to variation; but, on our view, this part has undergone, since the several species branched

off from a commonprogenitor, an unusual amount of variabilityand modification, and therefore wemight

expect the part generally to be still variable. But a part may be developed in the most unusual manner,

like the wing of a bat, and yet not be more variable than any other structure, if the part be common to

many subordinate forms, that is, if it has been inherited for a very long period; for in this case it will have

been rendered constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater difficulty than do corporeal structures

on the theory of the natural selection of successive, slight, but profitable modifications. We can thus

understand why nature moves by graduated steps in endowing different animals of the same class with

their several instincts. I have attempted to show how much light the principle of gradation throws on the

admirable architectural powers of the hive-bee. Habit no doubt often comes into play in modifying

instincts; but it certainly is not indispensable, as we see in the case of neuter insects, which leave no

progeny to inherit the effects of long-continued habit. On the view of all the species of the same genus

having descended from a common parent, and having inherited much in common, we can understand

how it is that allied species, when placed under widely different conditions of life, yet follow nearly the

same instincts; why the thrushes of tropical and temperate South America, for instance, line their nestswithmud like our British species. On the viewof instinctshaving been slowly acquired through natural

selection, we need not marvel at some instincts being not perfect and liable to mistakes, and at many

instincts causing other animals to suffer.

If species be only well-marked and permanent varieties, we can at once see why their crossed offspring

should follow the same complex laws in their degrees and kinds of resemblance to their parents,—in

being absorbed into each other by successive crosses, and in other such points,—as do the crossed

offspring of acknowledged varieties. This similarity would be a strange fact, if species had been

independently created and varieties had been produced through secondary laws.

If we admit that the geological record is imperfect to an extreme degree, then the facts, which the record

does give, strongly support the theory of descent with modification. New species have come on the stage

slowly and at successive intervals; and the amount of change, after equal intervals of time, is widely

different in different groups. The extinction of species and of whole groups of species which has played

so conspicuous a part in the history of the organic world, almost inevitably follows from the principle of

natural selection; for old forms are supplanted by new and improved forms. Neither single species nor

groups of species reappear when the chain of ordinary generation is once broken. The gradual diffusion

of dominant forms, with the slow modification of their descendants, causes the forms of life, after long

intervals of time, to appear as if they had changed simultaneously throughout the world. The fact of the

fossil remains of each formation being in some degree intermediate in character between the fossils in the

formations above and below, is simply explained by their intermediate position in the chain of descent.

The grand fact that all extinct beings can be classed with all recent beings, naturally follows from the living

and the extinct being the offspring of common parents. As species have generally diverged in character





during their long course of descent and modification, we can understand why it is that the more ancient

forms, or early progenitors of each group, so often occupy a position in some degree intermediate

between existing groups. Recent forms are generally looked upon as being, on the whole, higher in the

scale of organisation than ancient forms; and they must be higher, in so far as the later and more

improved forms have conquered the older and less improved forms in the struggle for life; they have also

generally had their organsmore specialised for different functions. This fact is perfectly compatible with

numerousbeings still retaining simple and but little improved structures, fitted for simple conditionsof life;it is likewise compatible with some forms having retrograded in organisation, by having become at each

stage of descent better fitted for new and degraded habits of life. Lastly, the wonderful law of the long

endurance of allied forms on the same continent,—of marsupials in Australia, of edentata in America, and

other such cases,—is intelligible, for within the samecountry the existing and the extinct will be closely

allied by descent.

Looking to geographical distribution, if we admit that there has been during the long course of ages much

migration from one part of the world to another, owing to former climatal and geographical changes and

to the many occasional and unknown means of dispersal, then we can understand, on the theory of

descent with modification, most of the great leading facts in Distribution. We can see why there should beso striking a parallelism in the distribution of organic beings throughout space, and in their geological

succession throughout time; for in both cases the beings have been connected by the bond of ordinary

generation, and the means of modification have been the same. We see the full meaning of the wonderful

fact, which has struck every traveller namely, that on the same continent, under the most diverse

conditions, under heat and cold, on mountain and lowland, on deserts and marshes, most of the

inhabitants within each great class are plainly related; for they are the descendants of the same

progenitors and early colonists. On this same principle of former migration, combined inmost caseswith

modification, we can understand, by the aid of the Glacial period, the identity of some few plants, and the

close alliance of many others, on the most distant mountains, and in the northern and southern temperate

zones; and likewise the close alliance of some of the inhabitants of the sea in the northern and southern

temperate latitudes, though separated by thewhole intertropical ocean.Although twocountriesmay present physical conditions as closely similar as the same species ever require, we need feel no surprise

at their inhabitants being widely different, if they have been for a long period completely sundered from

each other; for as the relation of organism to organism is the most important of all relations, and as the

two countries will have received colonists at various periods and in different proportions, from some

other country or from each other, the course of modification in the two areas will inevitably have been

different.

On this view of migration, with subsequent modification, we see why oceanic islands are inhabited by

only few species, but of these, why many are peculiar or endemic forms. We clearly see why species

belonging to those groups of animals which cannot cross wide spaces of the ocean, as frogs andterrestrial mammals, do not inhabit oceanic islands; and why, on the other hand, new and peculiar species

of bats, animals which can traverse the ocean, are often found on islands far distant from any continent.

Such cases as the presence of peculiar species of bats on oceanic islands and the absence of all other

terrestrial mammals, are facts utterly inexplicable on the theory of independent acts of creation.

The existence of closely allied or representative species in any two areas, implies, on the theory of

descent with modification, that the same parent-forms formerly inhabited both areas; andwe almost

invariably find that wherever many closely allied species inhabit two areas, some identical species are still

common to both. Where-ever many closely allied yet distinct species occur, doubtful forms and varieties

belonging to the same groups likewise occur. It is a rule of high generality that the inhabitants of each area

are related to the inhabitants of the nearest source whence immigrants might have been derived. We see

this in the striking relation of nearly all the plants and animals of the Galapagos archipelago, of Juan

Fernandez, and of the other American islands, to the plants and animals of the neighbouring American









facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so

easy to hide our ignorance under such expressions as the "plan of creation," "unity of design," &c., and to

think that we give an explanation when we only re-state a fact. Any one whose disposition leads him to

attach more weight to unexplained difficulties than to the explanation of a certain number of facts will

certainly reject the theory. A few naturalists, endowed with much flexibility of mind, and who have

already begun to doubt the immutability of species, may be influenced by this volume; but I look with

confidence to the future,—to young and rising naturalists, who will be able to view both sides of thequestion with impartiality. Whoever is led to believe that species are mutable will do good service by

conscientiously expressing his conviction; for thus only can the load of prejudice bywhich this subject is

overwhelmed be removed.

Several eminent naturalists have of late published their belief that a multitude of reputed species in each

genus are not real species; but that other species are real, that is, have been independently created. This

seems to me a strange conclusion to arrive at. They admit that a multitude of forms, which till lately they

themselves thought were special creations, and which are still thus looked at by the majority of naturalists,

andwhich consequently have all the external characteristic features of true species,—they admit that

these have been produced by variation, but they refuse to extend the same view to other and slightlydifferent forms. Nevertheless they do not pretend that they can define, or even conjecture, which are the

created forms of life, and which are those produced by secondary laws. They admit variation as avera

causa in one case, they arbitrarily reject it in another, without assigning any distinction in the two cases.

The day will come when this will be given as a curious illustration of the blindness of preconceived

opinion. These authors seem no more startled at a miraculous act of creation than at an ordinary birth.

But do they really believe that at innumerable periods in the earth's history certain elemental atoms have

been commanded suddenly to flash into living tissues? Do they believe that at each supposed act of

creation one individual ormanywere produced? Were all the infinitely numerouskinds of animals and

plants created as eggs or seed, or as full grown? and in the case of mammals, were they created bearing

the false marks of nourishment from the mother's womb? Undoubtedly some of these same questions

cannot be answered by those who believe in the appearance or creation of only a few forms of life, or of some one form alone. It has been maintained by several authors that it is as easy to believe in the creation

of a million beings as of one; but Maupertuis' philosophical axiom "of least action" leads the mind more

willingly to admit the smaller number; and certainly weought not to believe that innumerable beingswithin

each great class have been created with plain, but deceptive, marks of descent from a single parent.

As a record of a former state of things, I have retained in the foregoing paragraphs, and elsewhere,

several sentences which imply that naturalists believe in the separate creation of each species; and I have

beenmuch censured for having thus expressed myself.But undoubtedly thiswas the general belief when

the first edition of the present work appeared. I formerly spoke to very many naturalists on the subject of

evolution, and never once met with any sympathetic agreement. It is probable that some did then believein evolution, but they were either silent, or expressed themselves so ambiguously that it was not easy to

understand their meaning.Now things arewholly changed, andalmost every naturalist admits the great

principle of evolution. Thereare, however, some who still think that species have suddenly given birth,

through quite unexplained means, to new and totally different forms: but, as I have attempted to show,

weighty evidence can be opposed to the admission of great and abrupt modifications. Under a scientific

point of view, and as leading to further investigation, but little advantage is gained by believing that new

forms are suddenly developed in an inexplicable manner from old and widely different forms, over the old

belief in the creation of species from the dust of the earth.

It may be asked how far I extend the doctrine of the modification of species. The question is difficult to

answer, because the more distinct the forms are which we consider, by so much the arguments in favour

of community of descent become fewer in number and less in force. But some arguments of the greatest

weight extend very far. All the members of whole classes are connected together by a chain of affinities,





and all can be classed on the same principle, in groups subordinate to groups. Fossil remains sometimes

tend to fill up very wide intervals between existing orders.

Organs in a rudimentary condition plainly show that an early progenitor had the organ in a fully

developed condition; and this in some cases implies an enormous amount ofmodification in the

descendants. Throughout whole classes various structures are formed on the same pattern, and at a very

early age the embryos closely resemble each other. Therefore I cannot doubt that the theory of descentwith modification embraces all the members of the same great class or kingdom. I believe that animals are

descended from at most only four or five progenitors, and plants from an equal or lesser number.

Analogy would lead me one step farther, namely, to the belief that all animals and plants are descended

from some one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much

in common, in their chemical composition, theircellular structure, their laws of growth, and their liability to

injurious influences. We see this even in so trifling a fact as that the same poison often similarly affects

plants and animals; or that the poison secreted by the gallfly produces monstrous growths on the wild

rose or oaktree. With all organic beings, excepting perhaps some of the very lowest, sexual production

seems to be essentially similar. With all, as far as is at present known, the germinal vesicle is the same; sothat all organisms start from a common origin. If we look even to the two main divisions—namely, to the

animal andvegetable kingdoms—certain lowformsare so far intermediate in character that naturalists

have disputed to which kingdom they should be referred. As Professor Asa Gray has remarked, "the

spores and other reproductive bodies of many of the lower algae may claim to have first a

characteristically animal, and then an unequivocally vegetable existence."Therefore,on the principle of

natural selection with divergence of character, it does not seem incredible that, from some such low and

intermediate form, both animals and plants may have been developed; and, if we admit this, we must

likewise admit that all the organic beings which have ever lived on this earth may be descended from

someone primordial form. But this inference is chiefly groundedonanalogy, and it is immaterial whether

or not it be accepted. No doubt it is possible, as Mr. G. H. Lewes has urged, that at the first

commencement of life many different forms were evolved; but if so, we may conclude that only a veryfew have left modified descendants. For, as I have recently remarked in regard to the members of each

great kingdom, such as the Vertebrata, Articulata, &c., we have distinct evidence in their embryological,

homologous, and rudimentary structures, that within each kingdom all themembers are descended from a

single progenitor.

When the views advanced by me in this volume, and by Mr. Wallace, or when analogous views on the

origin of species are generally admitted, we can dimly foresee that there will be a considerable revolution

in natural history. Systematists will be able to pursue their labours as at present; but they will not be

incessantly haunted by the shadowy doubt whether this or that form be a true species. This, I feel sure

and I speak after experience, will be no slight relief. The endless disputes whether or not some fiftyspecies of British brambles are good species will cease. Systematists will have only to decide (not that

this will be easy) whether any form be sufficiently constant and distinct from other forms, to be capable of

definition; and if definable, whether the differencesbe sufficiently important to deserve a specific name.

This latter point will become a far more essential consideration than it is at present; for differences,

however slight, between any two forms, if not blended by intermediate gradations; are looked at by most

naturalists as sufficient to raise both forms to the rank of species.

Hereafter we shall be compelled to acknowledge that the only distinction between species and

well-marked varieties is, that the latter are known, or believed, to be connected at the present day by

intermediate gradations,whereas species were formerly thus connected. Hence,without rejecting the

consideration of the present existence of intermediate gradations between any two forms, we shall be led

to weigh more carefully and to value higher the actual amount of difference between them. It is quite

possible that forms now generally acknowledged to be merely varieties may hereafter be thought Worthy





of specific names; and in this case scientific and common language will come into accordance. In short,

we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera

are merely artificial combinations made for convenience. This may not be a cheering prospect; but we

shall at least be free from the vain search for the undiscovered and undiscoverable essence of the term

species.

The other and more general departments of natural history will rise greatly in interest. The terms used bynaturalists, of affinity, relationship, community of type, paternity,morphology, adaptive characters,

rudimentary and aborted organs, &c., will cease to be metaphorical, and will have a plain signification.

When we no longer look at an organic being as a savage looks at a ship, as something wholly beyond his

comprehension; when we regard every production of nature as one which has had a long history; when

we contemplate every complex structure and instinct as the summing up of many contrivances, each

useful to the possessor, in the same way as any great mechanical invention is the summing up of the

labour, the experience, the reason, and even the blunders of numerous workmen; when we thus view

each organic being, how far more interesting—I speak from experience—does the study of natural

history become!

A grand and almost untrodden field of inquiry will be opened, on the causes and laws of variation, on

correlation, on the effects of use and disuse, on the direct action of external conditions, and so forth. The

study of domestic productions will rise immensely in value. A new variety raised by man will be a more

important and interesting subject for study than one more species added to the infinitude of already

recorded species. Our classifications will come to be, as far as they can be so made, genealogies; and

will then truly give what may be called the plan of creation. The rules for classifying will no doubt become

simpler when we have a definite object in view. We possess no pedigrees or armorial bearings; and we

have to discover and trace the many diverging lines of descent in our natural genealogies, by characters of

any kindwhichhave longbeen inherited. Rudimentary organswill speak infallibly with respect to the

nature of long-lost structures. Species and groups of species which are called aberrant, and which may

fancifullybecalled living fossils,will aid us in forming a picture of the ancient forms of life. Embryologywill often reveal to us the structure, in some degree obscured, of the prototypes of each great class.

When we can feel assured that all the individuals of the same species, and all the closely allied species of

most genera, have within a not very remote period descended from one parent, and have migrated from

some one birth-place; and when we better know the many means of migration, then, by the light which

geology now throws, and will continue to throw, on former changes of climate and of the level of the

land, we shall surely be enabled to trace in an admirable manner the former migrations of the inhabitants

of the whole world. Even at present, by comparing the differences between the inhabitants of the sea on

the opposite sides of a continent, and the nature of the various inhabitants on that continent in relation to

their apparentmeansof immigration, some light canbe thrownon ancient geography.

The noble science of Geology loses glory from the extreme imperfection of the record. The crust of the

earth with its imbedded remains must not be looked at as a well-filled museum, but as a poor collection

made at hazard and at rare intervals. The accumulation of each great fossiliferous formation will be

recognised as having depended on an unusual concurrence of favourable circumstances, and the blank

intervals between the successive stages as having been of vast duration. But we shall be able to gauge

with some security the duration of these intervals by a comparison of the preceding and succeeding

organic forms. We must be cautious in attempting to correlate as strictly contemporaneous two

formations, which do not include many identical species, by the general succession of the forms of life. As

species are produced and exterminated by slowly acting and still existing causes, and not by miraculous

acts of creation; and as the most important of all causes of organic change is one which is almost

independent of altered andperhaps suddenly altered physical conditions, namely, the mutual relation of

organism to organism,—the improvement of one organismentailing the improvement or the extermination





of others; it follows, that the amount of organic change in the fossils of consecutive formations probably

serves as a fair measure of the relative though not actual lapse of time. A number of species, however,

keeping in a body might remain for a long period unchanged, whilst within the same period several of

these species bymigrating into new countries and coming into competitionwith foreign associates,might

become modified; so that we must not overrate the accuracy of organic change as a measure of time.

In the future I see open fields for far more important researches. Psychology will be securely based onthe foundation already well laid by Mr. Herbert Spencer, that of the necessary acquirement of each

mental power and capacity by gradation. Much light will be thrown on the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with the view that each species has been

independently created. To my mind it accords better with what we know of the laws impressed on matter

by the Creator, that the production and extinction of the past and present inhabitants of the world should

have been due to secondary causes, like those determining the birth and death of the individual. When I

view all beings not as special creations, but as the lineal descendants of some few beings which lived long

before the first bed of the Cambrian system was deposited, they seem to me to become ennobled.

Judging from the past, we may safely infer that not one living species will transmit its unaltered likeness toa distant futurity. And of the species now living very few will transmit progeny of any kind to a far distant

futurity; for the manner in which all organic beings are grouped, shows that the greater number of species

in each genus, and all the species in many genera, have left no descendants, but have become utterly

extinct. We can so far take a prophetic glance into futurity as to foretell that it will be the common and

widely-spread species, belonging to the larger anddominant groupswithin each class, whichwill

ultimately prevail and procreate new and dominant species. As all the living forms of life are the lineal

descendants of those which lived long before the Cambrian epoch, we may feel certain that the ordinary

succession by generation has never once been broken, and that no cataclysm has desolated the whole

world. Hence we may look with some confidence to a secure future of great length. And as natural

selection works solely by and for the good of each being, all corporeal and mental endowments will tend

to progress towards perfection.

It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing

on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to

reflect that these elaborately constructed forms, so different from each other, and dependent upon each

other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the

largest sense, beingGrowthwith Reproduction; Inheritance which is almost implied by reproduction;

Variability from the indirect and direct action of the conditions of life, and from use and disuse: a Ratio of

Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing

Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from

famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows.There is grandeur in this viewof life,with its several powers, having

been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone

cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful

and most wonderful have been, and are being evolved.

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Chapter II

Variation Under Nature.

Variability—Individual differences—Doubtful species—Wide ranging,much diffused, and common

species, vary most—Species of the larger genera in each country vary more frequently than the species

of the smaller genera—Many of the species of the larger genera resemble varieties in being very closely,

but unequally, related to each other, and in having restricted ranges.

BEFORE applying the principles arrived at in the last chapter to organic beings in a state of nature, we

must briefly discuss whether these latter are subject to any variation. To treat this subject properly, a long

catalogue of dry facts ought to be given; but these I shall reserve for a future work. Nor shall I here

discuss the various definitions which have been given of the term species. No one definition has satisfied

all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally

the term includes the unknown element of a distinct act of creation. The term "variety" is almost equally

difficult to define; but here community of descent is almost universally implied, though it canrarely be

proved. We have also what are called monstrosities; but they graduate into varieties. By a monstrosity I

presume ismeant some considerabledeviationof structure, generally injurious, or not useful to the

species. Some authors use the term "variation" in a technical sense, as implying a modification directly due

to the physical conditions of life; and "variations" in this sense are supposed not to be inherited; but who

can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on

Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited

for at least a few generations? and in this case I presume that the form would be called a variety.

It may be doubted whether sudden and considerable deviations of structure such as we occasionally see

in our domestic productions, more especially with plants, are ever permanently propagated in a state of

nature. Almost every part of every organic being is so beautifully related to its complex conditions of life

that it seems as improbable that any part should have been suddenly produced perfect, as that a complex

machine should have been invented by man in a perfect state. Under domestication monstrosities

sometimes occur which resemble normal structures inwidely different animals. Thuspigs have

occasionally been born with a sort of proboscis, and if any wild species of the same genus had naturally

possessed a proboscis, it might have been argued that this had appeared as a monstrosity; but I have as

yet failed to find, after diligent search, cases ofmonstrosities resembling normal structures in nearly allied

forms, and these alone bear on the question. If monstrous forms of this kind ever do appear in a state of nature and are capable of reproduction (which is not always the case), as they occur rarely and

singularly, their preservationwould dependonunusually favourable circumstances. Theywould, also,

during the first and succeeding generations cross with the ordinary form, and thus their abnormal

character would almost inevitably be lost. But I shall have to return in a future chapter to the preservation

andperpetuation of single or occasional variations.

Individual Differences.

The many slight differences which appear in the offspring from the same parents, or which it may be presumed have thus arisen, from being observed in the individuals of the same species inhabiting the same

confined locality, may be called individual differences. No one supposes that all the individuals of the





same species are cast in the same actual mould. These individual differences are of the highest importance

for us, for they are often inherited, as must be familiar to every one; and they thus afford materials for

natural selection to act on and accumulate, in the same manner as man accumulates in any given direction

individual differences in his domesticated productions.These individual differences generally affect what

naturalists consider unimportant parts; but I could show by a long catalogue of facts, that parts which

must be called important, whether viewed under a physiological or classificatory point of view, sometimes

vary in the individuals of the same species. I am convinced that the most experienced naturalist would besurprised at the number of the cases of variability, even in important parts of structure, which he could

collect on good authority, as I have collected, during a course of years. It should be remembered that

systematists are far from being pleased at finding variability in important characters, and that there are not

manymen who will laboriously examine internal and important organs, and compare them inmany

specimens of the same species. It would never have been expected that the branching of the main nerves

close to the great central ganglion of an insect would have been variable in the same species; it might have

been thought that changes of this nature could have been effected only by slow degrees; yet Sir J.

Lubbock has shown a degree of variability in these main nerves in Coccus, which may almost be

compared to the irregular branching of the stem of a tree. This philosophical naturalist, I may add, has

also shown that the muscles in the larvæ of certain insects are far from uniform. Authors sometimes arguein a circle when they state that important organs never vary; for these same authors practically rank those

parts as important (as some few naturalists have honestly confessed) which do not vary; and, under this

point of view, no instance will ever be found of an important part varying; but under any other point of

viewmany instancesassuredly canbe given.

There is one point connected with individual differences, which is extremely perplexing: I refer to those

genera whichhave been called "protean" or "polymorphic," inwhich the species present an inordinate

amount of variation. With respect to many of these forms, hardly two naturalists agree whether to rank

them as species or as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several

genera of insects and of Brachiopod shells. In most polymorphic genera some of the species have fixed

and definite characters. Genera which are polymorphic in one country seem to be, with a few exceptions, polymorphic in other countries, and likewise, judging from Brachiopod shells, at formerperiods of time.

These facts are very perplexing, for they seem to show that this kind of variability is independent of the

conditions of life. I am inclined to suspect that we see, at least in some of these polymorphic genera,

variations which are of no service or disservice to the species, and which consequently have not been

seized on and rendered definite by natural selection, as hereafter to be explained.

Individuals of the same species often present, as is known to every one, great differences of structure,

independently of variation, as in the two sexes of various animals, in the two or three castes of sterile

females or workers amongst insects, and in the immature and larval states of many of the lower animals.

There are, also, cases of dimorphism and trimorphism, both with animals and plants. Thus, Mr. Wallace,who has lately called attention to the subject, has shown that the females of certain species of butterflies,

in the Malayan archipelago, regularly appear under two or even three conspicuously distinct forms, not

connected by intermediate varieties. Fritz Müller has described analogous butmore extraordinary cases

with the males of certain Brazilian Crustaceans: thus, the male of a Tanais regularly occurs under two

distinct forms; one of these has strong and differently shaped pincers, and the other has antennæ much

more abundantly furnished with smelling-hairs. Although inmost of these cases, the two or three forms,

both with animals and plants, are not now connected by intermediate gradations, it is probable that they

were once thus connected. Mr. Wallace, for instance, describes a certain butterfly which presents in the

same island a great range of varieties connected by intermediate links, and the extreme links of the chain

closely resemble the two forms of an allied dimorphic species inhabiting another part of the Malay

archipelago. Thus also with ants, the several worker-castes are generally quite distinct; but in some cases,

as we shall hereafter see, the castes are connected together by finely graduated varieties. So it is, as I

have myself observed, with some dimorphic plants. It certainly at first appears a highly remarkable fact





that the same female butterfly should have the power of producing at the same time three distinct female

forms and a male; and that an hermaphrodite plant should produce from the same seed-capsule three

distinct hermaphrodite forms, bearing threedifferent kinds of females and three or even six different kinds

of males. Nevertheless these cases are only exaggerations of the common fact that the female produces

offspring of two sexes which sometimes differ from each other in a wonderful manner.

Doubtful Species.

The forms which possess in some considerable degree the character of species, but which are so closely

similar to other forms, or are so closely linked to them by intermediate gradations, that naturalists do not

like to rank them as distinct species, are in several respects the most important for us. We have every

reason to believe that many of these doubtful and closely allied forms have permanently retained their

characters for a long time; for as long, as far as we know, as have good and true species. Practically,

when a naturalist can unite by means of intermediate links any two forms, he treats the one as a variety of

the other; ranking the most common; but sometimes the one first described, as the species, and the other as the variety. But cases of great difficulty, which I will not here enumerate, sometimes arise in deciding

whether or not to rank one form as a variety of another, even when they are closely connected by

intermediate links; nor will the commonly-assumed hybrid nature of the intermediate forms always

remove the difficulty. In very many cases, however, one form is ranked as a variety of another, not

because the intermediate links have actually been found, but because analogy leads the observer to

suppose either that they do now somewhere exist, or may formerly have existed; and here a wide door

for the entry of doubt and conjecture is opened.

Hence, in determining whether a form should be ranked as a species or a variety, the opinion of

naturalists having sound judgment and wide experience seems the only guide to follow.We must,

however, in many cases, decide by a majority of naturalists, for few well-marked and well-knownvarieties can be named which have not been ranked as species by at least some competent judges.

That varieties of this doubtful nature are far from uncommon cannot be disputed. Compare the several

floras of Great Britain, of France, or of the United States, drawn up by different botanists, and see what

a surprising number of forms have been ranked by one botanist as good species, and by another as mere

varieties. Mr. H. C. Watson, to whom I lie under deep obligation for assistance of all kinds, has marked

for me 182 British plants, which are generally considered as varieties, but which have all been ranked by

botanists as species; and inmaking this list hehas omittedmany trifling varieties, but whichnevertheless

have been ranked by some botanists as species, and he has entirely omitted several highly polymorphic

genera. Under genera, including themost polymorphic forms, Mr.Babington gives 251 species,whereasMr. Bentham gives only 112,—a difference of 139 doubtful forms!Amongst animalswhich unite for each

birth, and which are highly locomotive, doubtful forms, ranked by one zoologist as a species and by

another as a variety, can rarely be found within the same country, but are common in separated areas.

How many of the birds and insects in North America and Europe, which differ very slightly from each

other, have been ranked by one eminent naturalist as undoubted species, and by another as varieties, or,

as they are often called geographical races! Mr. Wallace, in several valuable papers on the various

animals, especially on the Lepidoptera, inhabiting the islands of the greatMalayan archipelago, shows

that they may be classed under four heads, namely, as variable forms, as local forms, as geographical

races or sub-species, and as true representative species. The first or variable forms vary much within the

limits of the same island. The local forms are moderately constant and distinct in each separate island; but

when all from the several islands are compared together, the differences are seen to be so slight and

graduated, that it is impossible to define or describe them, though at the same time the extreme forms are

sufficiently distinct. The geographical races or sub-species are local forms completely fixed and isolated;







several interesting lines of argument, from geographical distribution, analogical variation, hybridism, &c.,

have been brought to bear in the attempt to determine their rank; but space does not here permit me to

discuss them. Close investigation, in many cases, will no doubt bring naturalists to agree how to rank

doubtful forms. Yet it must be confessed that it is in the best known countries that we find the greatest

number of them. I have been struck with the fact, that if any animal or plant in a state of nature be highly

useful to man, or from any cause closely attracts his attention, varieties of it will almost universally be

found recorded. These varieties, moreover, will often be ranked by some authors as species. Look at thecommon oak, how closely it has been studied; yet a German author makes more than a dozen species

out of forms, which are almost universally considered by other botanists to be varieties; and in this

country the highest botanical authorities and practical men can be quoted to show that the sessile and

pedunculated oaks are either good and distinct species or mere varieties.

I may here allude to a remarkable memoir lately published by A. de Candolle, on the oaks of the whole

world. No one ever had more ample materials for the discrimination of the species, or could have

worked on them with more zeal and sagacity. He first gives in detail all the many points of structure which

vary in the several species, and estimates numerically the relative frequency of thevariations. He specifies

above a dozen characters which may be found varying even on the same branch, sometimes according toage or development, sometimes without any assignable reason. Such characters are not of course of

specific value, but they are, as Asa Gray has remarked in commenting on this memoir, such as generally

enter into specific definitions. De Candolle then goes on to say that he gives the rank of species to the

forms that differ by characters never varying on the same tree, and never found connected by

intermediate states.After this discussion, the result of somuch labour, he emphatically remarks: "Theyare

mistaken, who repeat that the greater part of our species are clearly limited, and that the doubtful species

are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and its

species were founded upon a few specimens, that is to say, were provisional. Just as we come to know

them better, intermediate forms flow in, and doubts as to specific limits augment." He also adds that it is

the best known species which present the greatest number of spontaneous varieties and sub-varieties.

Thus Quercus robur has twenty-eight varieties, all of which, excepting six, are clustered round threesub-species, namely, Q. pedunculata, sessiliflora, and pubescens. The forms which connect these three

sub-species are comparatively rare; and, as Asa Gray again remarks, if these connecting forms which are

now rare, were to become wholly extinct, the three sub-species would hold exactly the same relation to

each other, as do the four or five provisionally admitted species which closely surround the typical

Quercus robur. Finally, De Candolle admits that out of the 300 species, which will be enumerated in his

Prodromus as belonging to the oak family, at least two-thirds are provisional species, that is, are not

known strictly to fulfil the definition above given of a true species. It should be added that De Candolle

no longer believes that species are immutable creations, but concludes that the derivative theory is the

most natural one, "and the most accordant with the known facts in palæontology, geographical botany

and zoology, of anatomical structure and classification."

When a young naturalist commences the study of a group of organisms quite unknown to him, he is at

first much perplexed in determining what differences to consider as specific, and what as varietal; for he

knows nothing of the amount and kind of variation to which the group is subject; and this shows, at least,

how very generally there is some variation. But if he confine his attention to one class within one country,

he will soon make up his mind how to rank most of the doubtful forms. His general tendency will be to

make many species, for he will become impressed, just like the pigeon or poultry fancier before alluded

to, with the amount of difference in the forms which he is continually studying; and he has little general

knowledge of analogical variation in other groups and in other countries, by which to correct his first

impressions. As he extends the range of his observations, he will meet with more cases of difficulty; for he

will encounter a greater number of closely-allied forms. But if his observations be widely extended, he

will in the end generally be able to make up his own mind: but he will succeed in this at the expense of

admittingmuchvariation,—and the truth of this admission will often be disputed byother naturalists.





When he comes to study allied forms brought from countries not now continuous, in which case he

cannot hope to find intermediate links, he will be compelled to trust almost entirely to analogy, and his

difficultieswill rise toa climax.

Certainly no clear line of demarcation has as yet been drawn between species and sub-species—that is,

the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank

of species: or, again, between sub-species and well-marked varieties, or between lesser varieties andindividualdifferences.These differences blend into each other by an insensible series; anda series

impresses the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to the systematist, as of the highest

importance for us, as being the first steps towards such slight varieties as are barely thought worth

recording in works on natural history. And I look at varieties which are in any degree more distinct and

permanent, as steps towards more strongly-marked and permanent varieties; and at the latter, as leading

to sub-species, and then to species. The passage from one stage of difference to another may, in many

cases, be the simple result of the nature of the organism and of the different physical conditions to which it

has long been exposed; but with respect to the more important and adaptive characters, the passagefrom one stage of difference to another, may be safely attributed to the cumulative action of natural

selection, hereafter to be explained, and to the effects of the increased use or disuse of parts. A

well-marked varietymay therefore be called an incipient species; but whether this belief is justifiable must

be judged by the weight of the various facts and considerations to be given throughout this work.

It need not be supposed that all varieties or incipient species attain the rank of species. They may

become extinct, or they may endure as varieties for very long periods, as has been shown to be the case

by Mr. Wollaston with the varieties of certain fossil land-shells in Madeira, and with plants by Gaston de

Saporta. If a variety were to flourish so as to exceed in numbers the parent species, it would then rank as

the species, and the species as the variety; or it might come to supplant and exterminate the parent

species; or both might co-exist, and both rank as independent species. But we shall hereafter return tothis subject.

From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of

convenience, to a set of individuals closely resembling each other, and that it does not essentially differ

from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in

comparison with mere individual differences, is also applied arbitrarily, for convenience' sake.

Wide-ranging, much diffused, and common Species vary most .

Guided by theoretical considerations, I thought that some interesting results might be obtained in regard

to the nature and relations of the species which vary most, by tabulating all the varieties in several

well-worked floras. At first this seemed a simple task; but Mr. H. C. Watson, to whom I am much

indebted for valuable advice and assistance on this subject, soon convinced me that there were many

difficulties, as did subsequently Dr. Hooker, even in stronger terms. I shall reserve for a future work the

discussion of these difficulties, and the tables of the proportional numbers of the varying species. Dr.

Hooker permits me to add that after having carefully read my manuscript, and examined the tables, he

thinks that the following statements are fairly well established. The whole subject, however, treated as it

necessarily here is with much brevity, is rather perplexing, and allusions cannot be avoided to the

"struggle for existence," "divergence of character," andother questions, hereafter to be discussed.

Alphonse de Candolle and others have shown that plants which have very wide ranges generally present





varieties; and this might have been expected, as they are exposed to diverse physical conditions, and as

they come into competition (which, as we shall hereafter see, is an equally or more important

circumstance)with different sets of organic beings. But my tables further show that, in any limited

country, the species which are the most common, that is abound most in individuals, and the species

whicharemostwidely diffused within their own country (and this is a different consideration from wide

range, and to a certain extent from commonness), oftenest give rise to varieties sufficiently well-marked

to have been recorded in botanical works. Hence it is the most flourishing, or, as they may be called, thedominant species,—those which range widely, are the most diffused in their own country, and are the

most numerous in individuals,—which oftenest produce well-marked varieties, or, as I consider them,

incipient species. And this, perhaps, might have been anticipated; for, as varieties, in order to become in

any degree permanent, necessarily have to struggle with the other inhabitants of the country, the species

which are already dominantwill be the most likely to yield offspring,which, though in some slight degree

modified, still inherit those advantages that enabled their parents to become dominant over their

compatriots. In these remarks on predominance, it should be understood that reference is made only to

the forms which come into competition with each other, and more especially to the members of the same

genus or class having nearly similar habits of life.With respect to the number of individuals or

commonness of species, the comparison of course relates only to the members of the same group. Oneof the higher plants may be said to be dominant if it be more numerous in individuals and more widely

diffused than the other plants of the same country, which live under nearly the same conditions. A plant of

this kind is not the less dominant because some conferva inhabiting the water or some parasitic fungus is

infinitelymore numerous in individuals and morewidelydiffused. But if the confervaorparasitic fungus

exceeds its allies in the above respects, it will then be dominant within its own class.

Species of the Larger Genera in each Country vary more frequently than the Species of the

Smaller Genera.

If the plants inhabiting a country, as described in any Flora, be divided into two equal masses, all those in

the larger genera (i. e., those including many species) being placed on one side, and all those in the

smaller genera on the other side, the former will be found to include a somewhat larger number of the

very common and much diffused or dominant species. This might have been anticipated; for the mere fact

of many species of the same genus inhabiting any country, shows that there is something in the organic or

inorganic conditions of that country favourable to thegenus; and, consequently,we might have expected

to have found in the larger genera or those including many species, a larger proportional number of

dominant species. But so many causes tend to obscure this result, that I am surprised that my tables show

even a small majority on the side of the larger genera. I will here allude to only two causes of obscurity.

Freshwater and salt-loving plants generally have very wide ranges and are much diffused, but this seemsto be connected with the nature of the stations inhabited by them, and has little or no relation to the size

of the genera to which the species belong. Again, plants low in the scale of organisation are generally

much more widely diffused than plants higher in the scale; and here again there is no close relation to the

size of the genera. The cause of lowly-organised plants ranging widely will be discussed in our chapter on

Geographical Distribution.

From looking at species as only strongly-marked and well-defined varieties, I was led to anticipate that

the species of the larger genera in each country would oftener present varieties, than the species of the

smaller genera; for wherever many closely related species (i. e., species of the same genus) have been

formed, many varieties or incipient species ought, as a general rule, to be now forming. Where many

large trees grow, we expect to find saplings. Where many species of a genus have been formed through

variation, circumstances have been favourable for variation; and hence we might expect that the

circumstances would generally be still favourable to variation. On the other hand, if we look at each





species as a special act of creation, there is no apparent reason why more varieties should occur in a

group havingmanyspecies, than in one having few.

To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous

insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and

those of the smaller genera on the other side, and it has invariably proved to be the case that a larger

proportion of the species on the side of the larger genera presented varieties, than on the side of thesmaller genera. Moreover, the species of the large genera which present any varieties, invariably present

a larger average number of varieties than do the species of the small genera. Both these results follow

when another division is made, and when all the least genera, with from only one to four species, are

altogether excluded from the tables. These facts are of plain signification on the view that species are only

strongly-marked and permanent varieties; for wherever many species of the same genus have been

formed, or where, if we may use the expression, the manufactory of species has been active, we ought

generally to find the manufactory still in action, more especially as we have every reason to believe the

process of manufacturing new species to be a slow one. And this certainly holds true, if varieties be

looked at as incipient species; for my tables clearly show as a general rule that, wherever many species of

a genus have been formed, the species of that genus present a number of varieties, that is of incipientspecies, beyond the average. It is not that all large genera are now varying much, and are thus increasing

in the number of their species, or that no small genera are now varying and increasing; for if this had been

so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small genera have in

the lapse of time often increased greatly in size; and that large genera have often come to their maxima,

decline, and disappeared. All that we want to show is, that, where many species of a genus have been

formed, on an average many are still forming; and this certainly holds good.

Many of the Species included within the Larger Genera resemble Varieties in being very closely,

but unequally, related to each other, and in having restricted ranges.

There are other relations between the species of large genera and their recorded varieties which deserve

notice. We have seen that there is no infallible criterion by which to distinguish species and well-marked

varieties; andwhen intermediate links have not been found between doubtful forms, naturalists are

compelled to come to a determination by the amount of difference between them, judging by analogy

whether or not the amount suffices to raise one or both to the rank of species. Hence the amount of

difference is one very important criterion in settling whether two forms should be ranked as species or

varieties. Now Fries has remarked in regard to plants, andWestwood in regard to insects, that in large

genera the amount of difference between the species is often exceedingly small. I have endeavoured to

test this numerically by averages, and, as far as my imperfect results go, they confirm the view. I havealso consulted some sagacious and experienced observers, and, after deliberation, they concur in this

view. In this respect, therefore, the species of the larger genera resemble varieties, more than do the

species of the smaller genera. Or the case may be put in another way, and it may be said, that in the

larger genera, in which a number of varieties or incipient species greater than the average are now

manufacturing,many of the species already manufacturedstill to a certain extent resemble varieties, for

they differ from each other by less than the usual amount of difference.

Moreover, the species of the larger genera are related to each other, in the same manner as the varieties

of any one species are related to each other. No naturalist pretends that all the species of a genus are

equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser

groups. As Fries has well remarked, little groups of species are generally clustered like satellites around

other species. And what are varieties but groups of forms, unequally related to each other, and clustered

round certain forms—that is, round their parent-species.Undoubtedly there is onemost important point







Chapter III

Struggle for Existence.

Its bearing on natural selection—The term used in a wide sense—Geometrical ratio of increase—Rapid

increase of naturalised animals andplants—Nature of the checks to increase—Competition

universal—Effects of Climate—Protection from thenumber of individuals—Complex relations of all

animals and plants throughout nature—Struggle for life most severe between individuals and varieties of

the same species: often severe between species of the same genus—The relation of organism to organism

the most important of all relations.

BEFORE entering on the subject of this chapter, I must make a few preliminary remarks, to show how

the struggle for existence bears on Natural Selection. It has been seen in the last chapter that amongst

organic beings in a state of nature there is some individual variability: indeed I am not aware that this has

ever been disputed. It is immaterial for us whether a multitude of doubtful forms be called species or

sub-species or varieties; what rank, for instance, the two or three hundred doubtful forms of British plants

are entitled to hold, if the existence of any well-marked varieties be admitted. But the mere existence of

individual variability and of some fewwell-marked varieties, thoughnecessary as the foundation for the

work, helps us but little in understanding how species arise in nature. How have all those exquisite

adaptations of one part of the organisation to another part, and to the conditions of life, and of one

organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the

woodpecker and the mistletoe; and only a little less plainly in the humblest parasite which clings to thehairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water; in

the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations

everywhere and in every part of the organic world.

Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately

converted into good and distinct species which in most cases obviously differ from each other far more

than do the varieties of the same species? How do those groups of species, which constitute what are

called distinct genera, and which differ from each other more than do the species of the same genus,

arise? All these results, as we shall more fully see in the next chapter, follow from the struggle for life.

Owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in anydegree profitable to the individualsof a species, in their infinitely complex relations to other organic beings

and to their physical conditions of life,will tend to the preservation of such individuals, and will generally

be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the

many individuals of any species which are periodically born, but a small number can survive. I have called

this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order

to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer

of the Survival of the Fittest is more accurate, and is sometimes equally convenient. We have seen that

man by selection can certainly produce great results, and can adapt organic beings to his own uses,

through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural

Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably

superior to man's feeble efforts, as the works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for existence. In my future work this subject will





be treated, as it well deserves, at greater length. The elder De Candolle and Lyell have largely and

philosophically shown that all organic beings are exposed to severe competition. In regard to plants, no

one has treated this subject with more spirit and ability than W. Herbert, Dean of Manchester, evidently

the result of his great horticultural knowledge. Nothing is easier than to admit in words the truth of the

universal struggle for life, or more difficult—at least I have found it so—than constantly to bear this

conclusion in mind. Yet unless it be thoroughly engrained in the mind, the whole economy of nature, with

every fact on distribution, rarity, abundance, extinction, andvariation,will be dimly seen or quitemisunderstood. We behold the face of nature bright with gladness, we often see superabundance of food;

we do not see or we forget, that the birds which are idly singing round us mostly live on insects or seeds,

and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their

nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that, though food

may be now superabundant, it is not so at all seasons of each recurring year.

The Term, Struggle for Existence, used in a large sense.

I should premise that I use this term in a large and metaphorical sense including dependence of one being

on another, and including (which is more important) not only the life of the individual, but success in

leaving progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each other

which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the

drought, though more properly it should be said to be dependent on the moisture. A plant which annually

produces a thousand seeds, of which only one of an average comes to maturity, may be more truly said

to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is

dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with

these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. But several

seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with

each other. As the mistletoe is disseminated by birds, its existence depends on them; and it maymetaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour and thus

disseminate its seeds. In these several senses, which pass into each other, I use for convenience' sake the

general term of Struggle for Existence.

Geometrical Ratio of Increase.

A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase.

Everybeing,whichduring its natural lifetimeproduces several eggs or seeds,must suffer destructionduring some period of its life, and during some season or occasional year, otherwise, on the principle of

geometrical increase, its numberswouldquickly become so inordinately great that no country could

support the product. Hence, as more individuals are produced than can possibly survive, there must in

every case be a struggle for existence, either one individual with another of the same species, or with the

individuals of distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied

with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial

increase of food, and no prudential restraint from marriage. Although some species may be now

increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.

There is no exception to the rule that every organic being naturally increases at so high a rate, that, if not

destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man

has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not

be standing-room for his progeny. Linnæus has calculated that if an annual plant produced only two







In looking at Nature, it is most necessary to keep the foregoing considerations always in mind—never to

forget that every single organic being may be said to be striving to the utmost to increase in numbers; that

each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young

or old, during each generation or at recurrent intervals. Lighten any check, mitigate the destruction ever

so little, and the number of the species will almost instantaneously increase to any amount.

Nature of the Checks to Increase.

The causes which check the natural tendency of each species to increase are most obscure. Look at the

most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still

further. We know not exactly what the checks are even in a single instance. Nor will this surprise any one

who reflects how ignorant we are on this head, even in regard to mankind, although so incomparably

better known than any other animal. This subject of the checks to increase has been ably treated by

several authors, and I hope in a future work to discuss it at considerable length, more especially in regard

to the feral animals of South America. Here I will make only a few remarks, just to recall to the reader'smind some of the chief points. Eggs or very young animals seem generally to suffer most, but this is not

invariably the case. With plants there is a vast destruction of seeds, but, from some observations which I

have made it appears that the seedlings suffer most from germinating in ground already thickly stocked

with other plants. Seedlings, also, are destroyed in vast numbers by various enemies; for instance, on a

piece of ground three feet long and two wide, dug and cleared, and where there could be no choking

from other plants, I marked all the seedlings of our native weeds as they came up, and out of 357 no less

than 295 were destroyed, chiefly by slugs and insects. If turf which has long been mown, and the case

would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants

graduallykill the less vigorous, though fullygrown plants; thus out of twenty species growing on a little

plot of mown turf (three feet by four) nine species perished, from the other species being allowed to grow

up freely.

The amount of food for each species of course gives the extreme limit to which each can increase; but

very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the

average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse,

and hares on any large estate depends chiefly on the destruction of vermin. If not one head of game were

shot during the next twenty years in England, and, at the same time, if no vermin were destroyed, there

would, in all probability, be less game than at present, although hundreds of thousands of game animals

are now annually shot. On the other hand, in some cases, as with the elephant, none are destroyed by

beasts of prey; for even the tiger in India most rarely dares to attack a young elephant protected by its

dam.

Climate plays an important part in determining the average number of a species, and periodical seasons

of extreme cold or drought seem to be the most effective of all checks. I estimated (chiefly from the

greatly reduced numbers of nests in the spring) that the winter of 1854—5 destroyed four-fifths of the

birds in my own grounds; and this is a tremendous destruction, when we remember that ten per cent. is

an extraordinarily severe mortality from epidemics with man. The action of climate seems at first sight to

be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it

brings on the most severe struggle between the individuals, whether of the same or of distinct species,

which subsist on the same kind of food. Even when climate, for instance, extreme cold, acts directly, it

will be the least vigorous individuals, or those which have got least food through the advancing winter,

which will suffer most.When we travel from south to north, or from a damp region to a dry, we

invariably see some species gradually getting rarer and rarer, and finally disappearing; and the change of

climate being conspicuous, we are tempted to attribute the whole effect to its direct action. But this is a





false view; we forget that each species, even where it most abounds, is constantly suffering enormous

destruction at some period of its life, from enemies or from competitors for the same place and food; and

if these enemies or competitors be in the least degree favoured by any slight change of climate, they will

increase in numbers; and as each area is already fully stocked with inhabitants, the other species must

decrease. When we travel southward and see a species decreasing in numbers, we may feel sure that the

cause lies quite as much in other species being favoured, as in this one being hurt. So it is when we travel

northward, but in a somewhat lesser degree, for the number of species of all kinds, and therefore of competitors, decreases northwards; hence in going northwards, or in ascending a mountain, we far

oftener meet with stunted forms, due to thedirectly injurious action of climate, than we do in proceeding

southwards or in descending a mountain. When we reach the Arctic regions, or snow-capped summits,

or absolute deserts, the struggle for life is almost exclusivelywith the elements.

That climate acts in main part indirectly by favouring other species, we clearly see in the prodigious

number of plants which in our gardens can perfectly well endure our climate, but which never become

naturalised, for they cannot compete with our native plants nor resist destruction by our native animals.

When a species, owing to highly favourable circumstances, increases inordinately in numbers in a smalltract, epidemics—at least, this seems generally to occur with our game animals—often ensue; and here

we have a limiting check independent of the struggle for life. But even some of these so-called epidemics

appear to be due to parasitic worms, which have from some cause, possibly in part through facility of

diffusion amongst the crowded animals, been disproportionally favoured: and here comes in a sort of

struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the same species, relatively to the

numbers of its enemies, is absolutely necessary for its preservation. Thus we can easily raise plenty of

corn and rape-seed, &c., in our fields, because the seeds are in great excess compared with the number

of birds which feed on them; nor can the birds, though having a superabundance of food at this one

season, increase in number proportionally to the supply of seed, as their numbers are checked during thewinter; but any one who has tried, knows how troublesome it is to get seed from a few wheat or other

such plants in a garden: I have in this case lost every single seed. This view of the necessity of a large

stock of the same species for its preservation, explains, I believe, some singular facts in nature such as

that of very rare plants being sometimes extremely abundant, in the few spots where they do exist; and

that of some social plants being social, that is abounding in individuals, even on the extreme verge of their

range. For in such cases, we may believe, that a plant could exist only where the conditions of its life

were so favourable that many could exist together, and thus save the species from utter destruction. I

should add that the good effects of intercrossing, and the ill effects of close interbreeding, no doubt come

into play in many of these cases; but I will not here enlarge on this subject.

Complex Relations of all Animals and Plants to each other in the Struggle for Existence.

Many cases are on record showing how complex and unexpected are the checks and relations between

organic beings, which have to struggle together in the same country. I will give only a single instance,

which, though a simple one, interested me. In Staffordshire, on the estate of a relation, where I had ample

means of investigation, there was a large and extremely barren heath, which had never been touched by

the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five

years previously and planted with Scotch fir. The change in the native vegetation of the planted part of the

heath was most remarkable, more than is generally seen in passing from one quite different soil to

another: not only the proportional numbers of the heath-plants were wholly changed, but twelve species

of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the





heath. The effect on the insects must have been still greater, for six insectivorous birds were very

common in the plantations, which were not to be seen on the heath; and the heath was frequented by two

or three distinct insectivorous birds. Here we see how potent has been the effect of the introduction of a

single tree, nothing whatever else having been done, with the exception of the land having been enclosed,

so that cattle could not enter. But how important an element enclosure is, I plainly saw near Farnham, in

Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hilltops: within

the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes,so close together that all cannot live. When I ascertained that these young trees had not been sown or

planted, I was so much surprised at their numbers that I went to several points of view, whence I could

examine hundreds of acres of the unenclosed heath, and literally I could not see a single Scotch fir,

except the old planted clumps. But on looking closely between the stems of the heath, I found a multitude

of seedlings and little trees which had been perpetually browsed down by the cattle. In one square yard,

at a point some hundred yards distant from one of the old clumps, I counted thirty-two little trees; and

one of them, with twenty-six rings of growth, had, during many years tried to raise its head above the

stems of the heath, and had failed. No wonder that, as soon as the land was enclosed, it became thickly

clothed with vigorously growing young firs. Yet the heath was so extremely barren and so extensive that

no one would ever have imagined that cattle would have so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch fir; but in several parts of the

world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this;

for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and

northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number

in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase

of these flies, numerous as they are, must be habitually checked by some means, probably by other

parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects

would probably increase; and this would lessen thenumber of thenavel-frequenting flies—then cattle and

horses would become feral, and this would certainly greatly alter (as indeed I have observed in parts of

South America) the vegetation: this again would largely affect the insects; and this, as we have just seen inStaffordshire, the insectivorous birds, and so onwards in ever-increasing circles of complexity. Not that

under nature the relations will ever be as simple as this. Battle within battle must be continually recurring

with varying success; and yet in the long-run the forces are so nicely balanced, that the face of nature

remains for long periods of time uniform, though assuredly the merest trifle would give the victory to one

organic being over another. Nevertheless, so profound is our ignorance, and so high our presumption,

that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we

invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!

I am tempted to give one more instance showing how plants and animals remote in the scale of nature,

are bound together by a web of complex relations. I shall hereafter have occasion to show that the exoticLobelia fulgens is never visited inmygarden by insects, and consequently, from its peculiar structure,

never sets a seed. Nearly all our orchidaceous plants absolutely require the visits of insects to remove

their pollen-masses and thus to fertilise them. I find from experiments that humble-bees are almost

indispensable to the fertilisation of the heartsease (Violo tricolor), for other bees do not visit this flower. I

have also found that the visits of bees are necessary for the fertilisation of some kinds of clover; for

instance, 20 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but 20 other heads protected

from bees produced not one. Again, 100 heads of red clover (T. pratense) produced 2,700 seeds, but

the same number of protected heads produced not a single seed. Humble-bees alone visit red clover, as

other bees cannot reach the nectar. It has been suggested that moths may fertilise the clovers; but I doubt

whether they could do so in the case of the red clover, from their weight not being sufficient to depress

the wing petals. Hence we may infer as highly probable that, if the whole genus of humble-bees became

extinct or very rare in England, the heartsease and red clover would become very rare, or wholly

disappear. The number of humble-bees in any district depends in a great measure upon the number of





field-mice, which destroy their combs and nests; and Col. Newman, who has long attended to the habits

of humble-bees, believes that "more than two-thirds of them are thus destroyed all over England." Now

the number of mice is largely dependent, as every one knows, on the number of cats; and Col. Newman

says, "Near villages and small towns I have found the nests of humble-bees more numerous than

elsewhere, which I attribute to the number of cats that destroy the mice." Hence it is quite credible that

the presence of a feline animal in largenumbers in a district might determine, through the intervention first

of mice and then of bees, the frequency of certain flowers in that district!

In the case of every species, many different checks, acting at different periods of life, and during different

seasons or years, probably come into play; some one check or some few being generally the most

potent; but all will concur in determining the average number or even the existence of the species. In

some cases it can be shown that widely-different checks act on the same species in different districts.

When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their

proportional numbers and kinds to what we call chance. But how false a view is this! Every one has

heard that when an American forest is cut down a very different vegetation springs up; but it has been

observed that ancient Indian ruins in the Southern United States, which must formerly have been cleared

of trees, now display the same beautiful diversity and proportion of kinds as in the surrounding virginforest. What a struggle must have gone on during long centuries between the several kinds of trees each

annually scattering its seeds by the thousand; what war between insect and insect—between insects,

snails, and other animals with birds and beasts of prey—all striving to increase, all feeding on each other,

or on the trees, their seeds and seedlings, or on the other plants which first clothed the ground and thus

checked the growth of the trees! Throw up a handful of feathers, and all fall to the ground according to

definite laws; but how simple is the problem where each shall fall compared to that of the action and

reaction of the innumerable plants and animals which have determined, in the course of centuries, the

proportional numbers and kinds of trees now growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its prey, lies generally between

beings remote in the scale of nature. This is likewise sometimes the case with those which may be strictlysaid to struggle with each other for existence, as in the case of locusts and grass-feeding quadrupeds. But

the struggle will almost invariably be most severe between the individuals of the same species, for they

frequent the same districts, require the same food, and are exposed to the same dangers. In the case of

varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see

the contest soon decided: for instance, if several varieties of wheat be sown together, and the mixed seed

be resown, some of the varieties which best suit the soil or climate, or are naturally the most fertile, will

beat the others and so yield more seed, and will consequently in a few years supplant the other varieties.

To keep up a mixed stock of even such extremely close varieties as the variously-coloured sweet peas,

they must be each year harvested separately, and the seed then mixed in due proportion, otherwise the

weaker kinds will steadily decrease in number and disappear. So again with the varieties of sheep; it has been asserted that certain mountain-varietieswill starve out other mountain-varieties, so that they cannot

be kept together. The same result has followed from keeping together different varieties of the medicinal

leech. It may even be doubted whether the varieties of any of our domestic plants or animals have so

exactly the same strength, habits, and constitution, that the original proportions of a mixed stock (crossing

being prevented) could be kept up for half-a-dozen generations, if they were allowed to struggle

together, in the same manner as beings in a state of nature, and if the seed or young were not annually

preserved in due proportion.

Struggle for Life most severe between Individuals and Varieties of the same Species.

As the species of the same genus usually have, though by no means invariably, much similarity in habits







to keep steadily in mind that each organic being is striving to increase in a geometrical ratio; that each at

some period of its life, during some season of the year, during each generation or at intervals, has to

struggle for life and to suffer great destruction. When we reflect on this struggle, we may console

ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is

generally prompt, and that the vigorous, thehealthy, and the happy survive and multiply.

|Go to Contents |

Chapter IV

Natural Selection; or the SurvivaL of the

Fittest.

Natural Selection—its power comparedwith man's selection—its power on characters of trifling

importance—its power at all ages and on both sexes—Sexual Selection—On the generality of

intercrosses between individuals of the same species—Circumstances favourable andunfavourable to the

results of Natural Selection, namely, intercrossing, isolation, number of individuals—Slow

action—Extinction causedbyNatural Selection—Divergence of Character, related to the diversity of

inhabitants of anysmall area, and to naturalisation—Action ofNatural Selection, through Divergenceof

Character, andExtinction, on thedescendants from a common parent—Explains thegrouping of all

organic beings—Advance in organisation—Low forms preserved—Convergence of

character—Indefinite multiplication of species—Summary.

HOW will the struggle for existence, briefly discussed in the last chapter, act in regard to variation? Can

the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I

think we shall see that it can act most efficiently. Let the endless number of slight variations and individual

differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency. Under domestication, it may be truly

said that the whole organisation becomes in some degree plastic. But the variability, which we almost

universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray

have well remarked, by man; he can neither originate varieties, nor prevent their occurrence; he can

preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and

changingconditions of life, and variability ensues; but similar changes of conditionsmight and do occur

under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations

of all organic beings to each other and to their physical conditions of life; and consequently what infinitely

varied diversities of structure might be of use to each being under changing conditions of life. Can it, then,

be thought improbable, seeing that variations useful to manhave undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the

course of many successive generations? If such do occur, can we doubt (remembering that many more





individuals are born than can possibly survive) that individuals having any advantage, however slight, over

others, would have the best chance of surviving and of procreating their kind? On the other hand, we

may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation

of favourable individual differences and variations, and the destruction of those which are injurious, I have

called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be

affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain

polymorphic species, or would ultimately become fixed, owing to the nature of the organism and thenature of the conditions.

Several writers have misapprehended or objected to the term Natural Selection. Some have even

imagined that natural selection inducesvariability, whereas it implies only the preservationof such

variations as arise and are beneficial to the being under its conditions of life. No one objects to

agriculturists speaking of the potent effects ofman's selection; and in this case the individual differences

given by nature, which man for some object selects, must of necessity first occur. Others have objected

that the term selection implies conscious choice in the animals which become modified; and it has even

been urged that, as plants have no volition, natural selection is not applicable to them! In the literal sense

of the word, no doubt, natural selection is a false term; but who ever objected to chemists speaking of the elective affinities of the various elements?—and yet an acid cannot strictly be said to elect the base

with which it in preference combines. It has been said that I speak of natural selection as an active power

or Deity; but who objects to an author speaking of the attraction of gravity as ruling the movements of the

planets? Every one knows what is meant and is implied by such metaphorical expressions; and they are

almost necessary for brevity. So again it is difficult to avoid personifying the word Nature; but I mean by

Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events

as ascertained byus. With a little familiarity such superficial objections will be forgotten.

We shall best understand the probable course of natural selection by taking the case of a country

undergoing some slight physical change, for instance, of climate.The proportional numbersof its

inhabitantswill almost immediately undergo a change, and some specieswill probably become extinct.We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants

of each country are bound together, that any change in the numerical proportions of the inhabitants,

independently of the change of climate itself, would seriously affect the others. If the country were open

on itsborders, new forms would certainly immigrate, and thiswould likewise seriously disturb the

relations of some of the former inhabitants. Let it be remembered how powerful the influence of a single

introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly

surrounded by barriers, into which new and better adapted forms could not freely enter, we should then

have places in the economy of nature which would assuredly be better filled up, if some of the original

inhabitants were in some manner modified; for, had the area been open to immigration, these same places

would have been seized on by intruders. In such cases, slight modifications, which in any way favouredthe individuals of any species, by better adapting them to their altered conditions, would tend to be

preserved; and natural selection would have free scope for the work of improvement.

We have good reason to believe, as shown in the first chapter, that changes in the conditions of life give

a tendency to increased variability; and in the foregoing cases the conditions have changed, and this

would manifestly be favourable to natural selection, by affording a better chance of the occurrence of

profitable variations. Unless such occur, natural selection cando nothing. Under the term of "variations,"

it must never be forgotten that mere individual differences are included. As man can produce a great

resultwith his domestic animals and plants byaddingup in any given direction individual differences, so

could natural selection, but far more easily from having incomparably longer time for action. Nor do I

believe that any great physical change, as of climate, or any unusual degree of isolation to check

immigration, is necessary in order that new and unoccupied places should be left, for natural selection to

fill up by improving some of the varying inhabitants. For as all the inhabitants of each country are





struggling together with nicely balanced forces, extremelyslightmodifications in the structure or habits of

one species would often give it an advantage over others; and still further modifications of the same kind

would often still further increase the advantage, as long as the species continued under the same

conditions of life and profited by similar means of subsistence and defence. No country can be named in

which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions

under which they live, that none of them could be still better adapted or improved; for in all countries, the

natives have been so far conquered by naturalised productions, that they have allowed some foreigners totake firm possession of the land. And as foreigners have thus in every country beaten some of the natives,

we may safely conclude that the natives might have been modified with advantage, so as to have better

resisted the intruders.

As man can produce, and certainly has produced, a great result by his methodical and unconscious

means of selection, what may not natural selection effect? Man can act only on external and visible

characters: Nature, if I may be allowed to personify the natural preservation or survival of the fittest,

cares nothing for appearances, except in so far as they are useful to any being. She can act on every

internal organ, on every shade of constitutional difference, on the whole machinery of life.Man selects

only for his own good: Nature only for that of the being which she tends. Every selected character is fullyexercised by her, as is implied by the fact of their selection. Man keeps the natives of many climates in

the same country; he seldom exercises each selected character in some peculiar and fitting manner; he

feeds a long and a short beaked pigeon on the same food; he does not exercise a long-backed or

long-legged quadruped in any peculiar manner; he exposes sheep with long and short wool to the same

climate. He does not allow the most vigorous males to struggle for the females. He does not rigidly

destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his

productions. He often begins his selection by some half-monstrous form; or at least by some modification

prominent enough to catch the eye or to be plainly useful to him. Under nature, the slightest differences of

structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be

preserved. How fleeting are the wishes and efforts of man! how short his time! and consequently how

poor will be his results, compared with those accumulated by Nature during whole geological periods!Can we wonder, then, that Nature's productions should be far "truer" in character than man's

productions; that they should be infinitely better adapted to the most complex conditions of life, and

should plainly bear the stamp of farhigher workmanship?

Itmay metaphoricallybe said that natural selection is daily and hourly srutinising, throughout the world,

the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently

and insensibly working,whenever and whereever opportunity offers , at the improvement of each

organic being in relation to its organic and inorganic conditions of life.We see nothing of these slow

changes in progress, until the hand of time has marked the lapse of ages, and then so imperfect is our

view into long-past geological ages, that we see only that the forms of life are now different from whatthey formerly were.

In order that any great amount of modification should be effected in a species, a variety when once

formed must again, perhaps after a long interval of time, vary or present individual differences of the same

favourable nature as before; and these must be again preserved, and so onwards step by step. Seeing

that individual differences of the same kind perpetually recur, this can hardly be considered as an

unwarrantable assumption. But whether it is true, we can judge only by seeing how far the hypothesis

accords with and explains the general phenomena of nature. On the other hand, the ordinary belief that

the amount of possible variation is a strictly limited quantity is likewise a simple assumption.

Although natural selection can act only through and for the good of each being, yet characters and

structures, which we are apt to consider as of very trifling importance, may thus be acted on. When we

see leaf-eating insects green, and bark-feeders mottled-grey; the alpineptarmiganwhite in winter, the







would be very slow, and there would be simultaneously the most rigorous selection of all the young birds

within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably

perish; or, more delicate and more easily broken shells might be selected, the thickness of the shell being

known to vary like every other structure.

It may be well here to remark that with all beings there must be much fortuitous destruction, which can

have little or no influence on the course of natural selection. For instance a vast number of eggs or seedsare annually devoured, and these could be modified through natural selection only if they varied in some

manner which protected them from their enemies. Yet many of these eggs or seeds would perhaps, if not

destroyed, have yielded individuals better adapted to their conditions of life than any of those which

happened to survive. So again a vast number of mature animals and plants, whether or not they be the

best adapted to their conditions, must be annually destroyed by accidental causes, which would not be in

the least degree mitigated by certain changes of structure or constitution which would in other ways be

beneficial to the species. But let the destruction of the adults be ever so heavy, if the number which can

exist in any district be not wholly kept down by such causes,—or again let the destruction of eggs or

seeds be so great that only a hundredth or a thousandth part are developed,—yet of those which do

survive, the best adapted individuals, supposing that there is any variability in a favourable direction, willtend to propagate their kind in larger numbers than the less well adapted. If the numbers be wholly kept

down by the causes just indicated, as will often have been the case, natural selection will be powerless in

certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways;

for we are far from having any reason to suppose that many species ever undergo modification and

improvement at the same time in the same area.

Sexual Selection.

Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attachedto that sex, so no doubt it will be under nature. Thus it is rendered possible for the two sexes to be

modified through natural selection in relation to different habits of life, as is sometimes the case; or for one

sex to be modified in relation to the other sex, as commonly occurs. This leads me to say a few words on

what I have called Sexual Selection. This form of selection depends, not on a struggle for existence in

relation to other organic beings or to external conditions, but on a struggle between the individuals of one

sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful

competitor, but fewor no offspring. Sexual selection is, therefore, less rigorous than natural selection.

Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most

progeny. But in many cases, victory depends not so much on general vigor, as on having special

weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving numerousoffspring. Sexual selection, byalways allowing the victor to breed,might surely give

indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, in nearly the

same manner as does the brutal cockfighter by the careful selection of his best cocks. How low in the

scale of nature the law of battle descends, I know not; male alligators have been described as fighting,

bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male

salmons have been observed fighting all day long; male stag-beetles sometimes bear wounds from the

huge mandibles of other males; the males of certain hymenopterous insects have been frequently seen by

that inimitable observer M. Fabre, fighting for a particular female who sits by, an apparently unconcerned

beholder of the struggle, and then retires with the conqueror. The war is, perhaps, severest between the

males of polygamous animals, and these seem oftenest provided with special weapons. The males of

carnivorous animals are already well armed; though to them and to others, special means of defence may

be given through means of sexual selection, as the mane of the lion, and the hooked jaw to the male

salmon; for the shield may be as important for victory, as the sword or spear.





Amongst birds, the contest is often of a more peaceful character. All those who have attended to the

subject, believe that there is the severest rivalry between the males of many species to attract, by singing,

the females. The rock-thrush of Guiana, birds of paradise, and some others, congregate; and successive

males display with the most elaborate care, and show off in the best manner, their gorgeous plumage;

they likewise perform strange antics before the females, which, standing by as spectators, at last choose

the most attractive partner. Those who have closely attended to birds in confinement well know that theyoften take individual preferences and dislikes: thus Sir R. Heron has described how a pied peacock was

eminently attractive to all his hen birds. I cannot here enter on the necessary details; but if man can in a

short time give beauty and an elegant carriage to his bantams, according to his standard of beauty, I can

see no good reason to doubt that female birds, by selecting, during thousands of generations, the most

melodious or beautiful males, according to their standard of beauty, might produce a marked effect.

Some well-known laws, with respect to the plumage of male and female birds, in comparison with the

plumage of the young, can partly be explained through the action of sexual selection on variations

occurring at different ages, and transmitted to the males alone or to both sexes at corresponding ages; but

I have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animal have the same general habits of

life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual

selection: that is, by individual maleshaving had, in successive generations, some slight advantage over

other males, in their weapons, means of defence, or charms, which they have transmitted to their male

offspring alone. Yet, I would not wish to attribute all sexual differences to this agency: for we see in our

domestic animals peculiarities arising and becoming attached to the male sex,which apparentlyhave not

been augmented through selection by man. The tuft of hair on the breast of the wild turkey-cock cannot

be of any use, and it is doubtful whether it can be ornamental in the eyes of the female bird;—indeed, had

the tuft appeared under domestication, it would have been called a monstrosity.

Illustrations of the Action of Natural Selection, or the Survival of the Fittest .

In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or

two imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some

by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for

instance, had from any change in the country increased in numbers, or that other prey had decreased in

numbers, during that season of the year when the wolf was hardest pressed for food. Under such

circumstances the swiftest and slimmest wolves would have the best chance of surviving and so be

preserved or selected,—provided always that they retained strength to master their prey at this or someother period of the year, when they were compelled to prey on other animals. I can see no more reason

to doubt that this would be the result, than that man should be able to improve the fleetness of his

greyhounds by careful and methodical selection, or by that kind of unconscious selection which follows

from each man trying to keep the best dogs without any thought of modifying the breed. I may add, that,

according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains, in the

United States, one with a light greyhound-like form, which pursues deer, and the other more bulky, with

shorter legs, whichmore frequently attacks the shepherd's flocks.

It should be observed that, in the above illustration, I speak of the slimmest individual wolves, and not of

any single strongly-marked variation having been preserved. In former editions of thiswork I sometimes

spoke as if this latter alternative had frequently occurred. I saw the great importance of individual

differences, and this led me fully to discuss the results of unconscious selection by man, which depends

on the preservation of all the more or less valuable individuals, and on the destruction of the worst. I saw,





also, that the preservation in a state of nature of any occasional deviation of structure, such as a

monstrosity, would be a rare event; and that, if at first preserved, it would generally be lost by subsequent

intercrossingwith ordinary individuals. Nevertheless, until reading an able and valuable article in the

'North British Review' (1867), I did not appreciate how rarely single variations, whether slight or

strongly-marked, could be perpetuated. The author takes the case of a pair of animals, producing during

their lifetime two hundred offspring, of which, from various causes of destruction, only two on an average

survive to pro-create their kind. This is rather an extreme estimate for most of the higher animals, but byno means so for many of the lower organisms. He then shows that if a single individual were born, which

varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the

chances would be strongly against its survival. Supposing it to survive and to breed, and that half its

young inherited the favourable variation; still, as the Reviewer goes on to show, the young would have

only a slightly better chance of surviving and breeding; and this chance would go on decreasing in the

succeeding generations. The justice of these remarks cannot, I think, be disputed. If, for instance, a bird

of some kind could procure its food more easily by having its beak curved, and if one were born with its

beak strongly curved, and which consequently flourished, nevertheless there would be a very poor

chance of this one individual perpetuating its kind to the exclusion of the common form; but there can

hardly be a doubt, judging by what we see taking place under domestication, that this result would followfrom the preservation during manygenerationsof a largenumber of individuals withmore or less strongly

curved beaks, and from the destruction of a still larger number with the straightest beaks.

It should not, however, be overlooked that certain rather strongly marked variations, which no one

would rank asmere individual differences, frequently recur owing to a similar organisationbeing similarly

acted on—of which fact numerous instances could be given with our domestic productions. In such

cases, if the varying individual did not actually transmit to itsoffspring its newly-acquiredcharacter, it

would undoubtedly transmit to them, as long as the existing conditions remained the same, a still stronger

tendency to vary in the same manner. There can also be little doubt that the tendency to vary in the same

manner has often been so strong that all the individuals of the same species have been similarly modified

without the aid of any form of selection. Or only a third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be given. Thus Graba estimates that about

one-fifth of the guillemots in the Faroe Islands consist of a variety so well marked, that it was formerly

ranked as a distinct species under the name of Uria lacrymans. In cases of this kind, if the variation were

of a beneficial nature, the original form would soon be supplanted by the modified form, through the

survival of the fittest.

To the effects of intercrossing in eliminating variations of all kinds, I shall have to recur; but it may be

here remarked that most animals and plants keep to their proper homes, and do not needlessly wander

about; we see this even with migratory birds, which almost always return to the same spot. Consequently

each newly-formed variety would generally be at first local, as seems to be the common rule withvarieties in a state of nature; so that similarly modified individuals would soon exist in a small body

together, and would often breed together. If the new variety were successful in its battle for life, it would

slowly spread from a central district, competingwith and conquering the unchanged individuals on the

margins of an ever-increasing circle.

It may be worth while to give another and more complex illustration of the action of natural selection.

Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap:

this is effected, for instance, by glands at the base of the stipules in some Leguminosæ, and at the backs

of the leaves of the common laurel. This juice, though small in quantity, is greedily sought by insects; but

their visits do not in any way benefit the plant. Now, let us suppose that the juice or nectar was excreted

from the inside of the flowers of a certain number of plants of any species. Insects in seeking the nectar

would get dusted with pollen, and would often transport it from one flower to another. The flowers of

two distinct individuals of the same species would thus get crossed; and the act of crossing, as can be





fullyproved, gives rise to vigorous seedlings whichconsequentlywould have the best chanceof

flourishing and surviving.The plantswhichproduced flowerswith the largest glands or nectaries,

excreting most nectar, would oftenest be visited by insects, and would oftenest be crossed; and so in the

long-run would gain the upper hand and form a local variety. The flowers, also, which had their stamens

and pistils placed, in relation to the size and habits of the particular insects which visited them, so as to

favour in any degree the transportal of the pollen, would likewise be favoured. We might have taken the

case of insects visiting flowers for the sake of collecting pollen instead of nectar; and as pollen is formedfor the sole purpose of fertilisation, its destruction appears to be a simple loss to the plant; yet if a little

pollen were carried, at first occasionally and then habitually, by thepollen-devouring insects from flower

to flower, and a cross thus effected, although nine-tenths of the pollen were destroyed it might still be a

great gain to the plant to be thus robbed; and the individuals which produced more and more pollen, and

had larger anthers, would be selected.

When our plant, by the above process long continued, had been rendered highly attractive to insects,

they would, unintentionally on their part, regularly carry pollen from flower to flower; and that they do this

effectually, I could easily show by many striking facts. I will give only one, as likewise illustrating one step

in the separation of the sexes of plants. Some holly-trees bear only male flowers, which have four stamens producing a rather small quantity of pollen, anda rudimentary pistil; other holly-trees bear only

female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers, inwhichnot a grain

of pollen can be detected. Having found a female tree exactly sixty yards from a male tree, I put the

stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without

exception, there were a few pollen-grains, and on some a profusion. As the wind had set for several days

from the female to the male tree, the pollen could not thus have been carried. The weather had been cold

and boisterous, and therefore not favourable to bees, nevertheless every female flower which I examined

had been effectually fertilised by the bees, which had flown from tree to tree in search of nectar. But to

return to our imaginary case: as soon as the plant had been rendered so highly attractive to insects that

pollen was regularly carried from flower to flower, another processmight commence.No naturalist

doubts the advantage of what has been called the "physiological division of labour;" hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole

plant, and pistils alone in another flower or on another plant. In plants under culture and placed under

new conditions of life, sometimes the male organs and sometimes the female organs become more or less

impotent; now if we suppose this to occur in ever so slight a degree under nature, then, as pollen is

already carried regularly from flower to flower, and as a more complete separation of the sexes of our

plant would be advantageous on the principle of the division of labour, individualswith this tendency

more and more increased, would be continually favoured or selected, until at last a complete separation

of the sexes might be effected. It would take up too much space to show the various steps, through

dimorphism and other means, by which the separation of the sexes in plants of various kinds is apparently

now in progress; but I may add that some of the species of holly in North America, are, according to AsaGray, in an exactly intermediate condition, or, as he expresses it, are more or less diæciously

polygamous.

Let us now turn to the nectar-feeding insects; we may suppose the plant, of which we have been slowly

increasing the nectar by continued selection, to be a common plant; and that certain insects depended in

main part on its nectar for food. I could give many facts showing how anxious bees are to save time: for

instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which, with a

very little more trouble, they can enter by the mouth. Bearing such facts in mind, it may be believed that

under certain circumstances individual differences in the curvature or lengthof the proboscis, &c., too

slight to be appreciated by us, might profit a bee or other insect, so that certain individuals would be able

to obtain their food more quickly than others; and thus the communities to which they belonged would

flourish and throw off many swarms inheriting the same peculiarities. The tubes of the corolla of the

common red and incarnate clovers (Trifolium pratense and incarnatum) do not be a hasty glance appear





to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of

the common red clover, which is visited by humble-bees alone; so that whole fields of red clover offer in

vain an abundant supply of precious nectar to the hive-bee. That this nectar is much liked by the hive-bee

is certain; for I have repeatedly seen, but only in the autumn, many hive-bees sucking the flowers through

holes bitten in the base of the tube by humble-bees. The difference in the length of the corolla in the two

kinds of clover, which determines the visits of the hive-bee, must be very trifling; for I have been assured

that when red clover has been mown, the flowers of the second crop are somewhat smaller, and thatthese are visited by many hive-bees. I do not know whether this statement is accurate; nor whether

another published statement can be trusted, namely, that the Ligurian bee which is generally considered a

mere variety of the common hive-bee, and which freely crosses with it, is able to reach and suck the

nectar of the red clover. Thus, in a country where this kind of clover abounded, it might be a great

advantage to the hive-bee to have a slightly longer or differently constructed proboscis. On the other

hand, as the fertility of this clover absolutely depends on bees visiting the flowers, if humble-bees were to

become rare in any country, it might be a great advantage to the plant to have a shorter or more deeply

divided corolla, so that the hive-bees should be enabled to suck its flowers. Thus I can understand how a

flower and a bee might slowly become, either simultaneously or one after the other, modified and

adapted to each other in the most perfect manner, by the continued preservation of all the individualswhichpresented slight deviations of structure mutually favourable to each other.

I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is

open to the same objections which were first urged against Sir Charles Lyell's noble views on "the

modern changes of the earth, as illustrative of geology;" but we now seldom hear the agencies which we

see still at work, spoken of as trifling or insignificant, when used in explaining the excavation of the

deepest valleys or the formation of long lines of inland cliffs. Natural selection acts only by the

preservation andaccumulation of small inherited modifications, each profitable to the preserved being;

and as modern geology has almost banished such views as the excavation of a great valley by a single

diluvial wave, so will natural selection banish the belief of the continued creation of new organic beings, or

of any great and suddenmodification in their structure.

On the Intercrossing of Individuals.

I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of

course obvious that two individuals must always (with the exception of the curious and not well

understood cases of parthenogenesis) unite for each birth; but in one case or hermaphrodites this is far

from obvious.Nevertheless there is reason to believe that with all hermaphrodites two individuals, either

occasionallyor habitually, concur for the reproduction of their kind. This viewwas long ago doubtfullysuggested by Sprengel, Knight and Kölreuter. We shall presently see its importance; but I must here treat

the subject with extreme brevity, though I have the materials prepared for an ample discussion. All

vertebrate animals, all insects, and some other large groups of animals, pair for each birth. Modern

research has much diminished the number of supposed hermaphrodites, and of real hermaphrodites a

large number pair; that is, two individuals regularly unite for reproduction,which is all that concerns us.

But still there are many hermaphrodite animals which certainly do not habitually pair, and a vast majority

of plants are hermaphrodites. What reason, it may be asked, is there for supposing in these cases that

two individuals ever concur in reproduction? As it is impossible here to enter on details, I must trust to

some general considerations alone.

In the first place, I have collected so large a body of facts, and made so many experiments, showing, in

accordance with the almost universal belief of breeders, that with animals and plants a cross between

different varieties, or between individuals of the same variety but of another strain, gives vigour and





fertility to the offspring; and on the other hand, thatclose interbreedingdiminishes vigour and fertility; that

these facts alone incline me to believe that it is a general law of nature that no organic being fertilises itself

for a perpetuity of generations; but that a cross with another individual is occasionally—perhaps at long

intervals of time—indispensable.

On the belief that this is a law of nature, we can think, understand several large classes of facts, such a

the following, whichon any other vieware inexplicable. Everyhybridizer knows how unfavourableexposure to wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and

stigmas fully exposed to theweather! If an occasional cross be indispensable, notwithstanding that the

plant's own anthers and pistil stand so near each other as almost to insure self-fertilisation, the fullest

freedom for the entrance of pollen from another individual will explain the above state of exposure of the

organs. Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the

great papilionaceous or pea-family; but these almost invariably present beautiful and curious adaptations

in relation to the visits of insects. So necessary are the visits of bees to many papilionaceous flowers, that

their fertility is greatly diminished if these visits be prevented. Now, it is scarcely possible for insects to fly

from flower to flower, and not to carry pollen from one to the other, to the great good of the plant.

Insects act like a camel-hair pencil, and it is sufficient, to ensure fertilisation, just to touch with the same brush the anthers of one flower and then the stigma of another; but it must not be supposed that bees

would thus produce a multitude of hybrids between distinct species; for if a plant's own pollen and that

from another species are placed on the same stigma, the former is so prepotent that it invariably and

completely destroys, as has been shown by Gärtner, the influence of the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other

towards it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for

this end: but the agency of insects is often required to cause the stamens to spring forward, as Kölreuter

has shown to be the case with the barberry; and in this very genus, which seems to have a special

contrivance for self-fertilisation, it is well-known that, if closely-allied formsor varieties are plantednear

each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross. In numerousother cases, far from self-fertilisation being favoured; there are special contrivanceswhicheffectually

prevent the stigma receiving pollen from its own flower, as I could show from the works of Sprengel and

others, as well as from my own observations: for instance, in Lobelia fulgens, there is a really beautiful

and elaborate contrivance bywhichall the infinitely numerous pollen-granules are swept out of the

conjoined anthers of each flower, before the stigma of that individual flower is ready to receive them; and

as this flower is never visited, at least in my garden, by insects, it never sets a seed, though by placing

pollen from one flower on the stigma of another, I raise plenty of seedlings. Another species of Lobelia

which is visited by bees, seeds freely in my garden. In very many other cases, though there is no special

mechanical contrivance to prevent the stigma receiving pollen from the same flower, yet, as Sprengel, and

more recently Hildebrand, and others, have shown, and as I can confirm, either the anthers burst beforethe stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that

these so-named dichogamous plants have in fact separated sexes, and must habitually be crossed. So it is

with the reciprocally dimorphic and trimorphic plants previously alluded to.How strange are these facts!

How strange that the pollen and stigmatic surface of the same flower, though placed so close together, as

if for the very purpose of self-fertilisation, should be in so many cases mutually useless to each other?

How simply are these facts explained on the view of an occasional cross with a distinct individual being

advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each

other, a large majority of the seedlings thus raised turn out, as I have found, mongrels: for instance, I

raised 233 seedling cabbages from some plants of different varieties growing near each other, and of

these only 78 were true to their kind, and some even of these were not perfectly true. Yet the pistil of

each cabbage-flower is surrounded not only by its own six stamens but by those of the many other





flowers on the same plant; and the pollen of each flower readily gets on its own stigma without insect

agency; for I have found that plants carefully protected from insects produce the full number of pods.

How, then, comes it that such a vast number of the seedlings are mongrelized? It must arise from the

pollen of a distinctvariety having a prepotent effect over the flower's own pollen; and that this is part of

the general law of good being derived from the intercrossing of distinct individuals of the same species.

When distinct species are crossed the case is reversed, for a plant's own pollen is almost always

prepotent over foreign pollen; but to this subject we shall return in a future chapter.

In the case of a large tree covered with innumerable flowers, it may be objected that pollen could

seldom be carried from tree to tree, and at most only from flower to flower on the same tree; and flowers

on the same tree can be considered as distinct individuals only in a limited sense. I believe this objection

to be valid, but that nature has largely provided against it by giving to trees a strong tendency to bear

flowers with separated sexes. When the sexes are separated, although the male and female flowers may

be produced on the same tree, pollen must be regularly carried from flower to flower; and this will give a

better chance of pollen being occasionally carried from tree to tree. That trees belonging to all Orders

have their sexes more often separated than other plants, I find to be the case in this country; and at my

request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States,and the result was as I anticipated. On the other hand, Dr. Hooker informs me that the rule does not hold

good in Australia: but if most of the Australian trees are dichogamous, the same result would follow as if

they bore flowers with separated sexes. I have made these few remarks on trees simply to call attention

to the subject.

Turning for a brief space to animals: various terrestrial species are hermaphrodites, such as the

land-mollusca and earth-worms; but these all pair. As yet I have not found a single terrestrial animal

which can fertilise itself. This remarkable fact, which offers so strong a contrast with terrestrial plants, is

intelligible on the viewof an occasional cross being indispensable; for owing to the nature of the fertilising

element there are no means, analogous to the action of insects and of the wind with plants, by which an

occasional cross couldbe effectedwith terrestrial animals without the concurrenceof two individuals. Of aquatic animals, there aremany self-fertilising hermaphrodites; but here the currentsof water offer an

obvious means for an occasional cross. As in the case of flowers, I have as yet failed, after consultation

with one of the highest authorities, namely, Professor Huxley, to discover a single hermaphrodite animal

with the organs of reproduction so perfectly enclosed that access from without, and the occasional

influence of a distinct individual, can be shown to be physically impossible. Cirripedes long appeared to

me to present, under this point of view, a case of great difficulty; but I have been enabled, by a fortunate

chance, to prove that two individuals, though both are self-fertilising hermaphrodites, do sometimes

cross.

It must have struck most naturalists as a strange anomaly that, both with animals and plants, somespecies of the same family and even of the same genus, though agreeing closely with each other in their

whole organisation are hermaphrodites, and some unisexual.But if, in fact, all hermaphrodites do

occasionally intercross, the difference between them and unisexual species is, as far as function is

concerned, very small.

From these several considerations and from the many special facts which I have collected, but which I

am unable here to give, it appears that with animals and plants an occasional intercross between distinct

individuals is a very general, if not universal, law of nature.

Circumstances favourable for the production of new forms through Natural Selection.





This is an extremely intricate subject. A great amount of variability, under which term individual

differences are always included,will evidentlybe favourable. A large number of individuals, bygiving a

better chance within any given period for the appearance of profitable variations, will compensate for a

lesser amount of variability in each individual, and is, I believe, a highly important element of success.

Though Nature grants long periods of time for the work of natural selection, she does not grant an

indefinite period; for as all organic beings are striving to seize on each place in the economy of nature, if

any one species does not become modified and improved in a corresponding degree with its competitors,it will be exterminated. Unless favourable variations be inherited by some at least of the offspring, nothing

can be effected by natural selection. The tendency to reversion may often check or prevent the work; but

as this tendency has not prevented man from forming by selection numerous domestic races, why should

it prevail against natural selection?

In the case of methodical selection, a breeder selects for some definite object, and if the individuals be

allowed freely to intercross, his workwill completely fail.But whenmanymen,without intending to alter

the breed, have a nearly common standard of perfection, and all try to procure and breed from the best

animals, improvement surely but slowly follows from this unconscious process of selection,

notwithstanding that there is no separation of selected individuals. Thus it will be under nature; for within aconfined area,with someplace in the natural polity not perfectly occupied, all the individualsvarying in

the right direction, though in different degrees, will tend to be preserved. But if the area be large, its

several districtswill almost certainly present different conditions of life; and then, if the same species

undergoes modification in different districts, the newly-formed varietieswill intercross on the confines of

each.But we shall see in the sixth chapter that intermediatevarieties, inhabiting intermediatedistricts, will

in the long run generallybe supplanted byone of the adjoining varieties. Intercrossingwill chiefly affect

those animals which unite for each birth and wander much, and which do not breed at a very quick rate.

Hence with animals of this nature, for instance, birds, varieties will generally be confined to separated

countries; and this I find to be the case. With hermaphrodite organisms which cross only occasionally,

and likewise with animals which unite for each birth, but which wander little and can increase at a rapid

rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body and afterwards spread, so that the individuals of the new variety would chiefly cross together.

On this principle, nurserymen always prefer saving seed from a large body of plants, as the chance of

intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not propagate rapidly, we must not assume

that free intercrossing would always eliminate the effects of natural selection; for I can bring forward a

considerable body of facts showing that within the same area, two varieties of the same animal may long

remain distinct, fromhauntingdifferent stations, frombreedingat slightly different seasons, or from the

individualsof each variety preferring to pair together.

Intercrossing plays a very important part in nature by keeping the individuals of the same species, or of

the same variety, true and uniform in character. It will obviously thus act far more efficiently with those

animals which unite for each birth; but, as already stated, we have reason to believe that occasional

intercrosses take place with all animals and plants. Even if these take place only at long intervals of time,

the young thusproducedwill gain somuch in vigour and fertility over the offspring from long-continued

self-fertilisation, that they will have a better chance of surviving and propagating their kind; and thus in the

long run the influence of crosses, even at rare intervals, will be great. With respect to organic beings

extremely low in the scale, which do not propagate sexually, nor conjugate, and which cannot possibly

intercross, uniformity of character can be retained by them under the same conditions of life, only through

the principle of inheritance, and through natural selectionwhich will destroy any individuals departing from

the proper type. If the conditions of life change and the formundergoes modification, uniformityof

character canbe given to the modified offspring, solely by natural selection preserving similar favourable

variations.





Isolation, also, is an important element in the modification of species through natural selection. In a

confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be

almost uniform; so that natural selectionwill tend tomodify all the varying individuals of the same species

in the samemanner. Intercrossing,with the inhabitants of the surrounding districtswill, also, be thus

prevented. Moritz Wagner has lately published an interesting essay on this subject, and has shown that

the service rendered by isolation in preventing crosses between newly-formed varieties is probablygreater even than I supposed. But from reasons already assigned I can by no means agree with this

naturalist, that migration and isolation are necessary elements for the formationof newspecies. The

importance of isolation is likewise great in preventing, after any physical change in the conditions, such as

of climate, elevation of the land, &c., the immigration of better adapted organisms; and thus new places in

the natural economy of the district will be left open to be filled up by the modification of the old

inhabitants. Lastly, isolation will give time for a new variety to be improved at a slow rate; and this may

sometimes be of much importance. If, however, an isolated area be very small, either from being

surrounded by barriers, or from having very peculiar physical conditions, the total number of the

inhabitants will be small; and this will retard the production of new species, through natural selection, by

decreasing the chances of favourable variations arising.

The mere lapse of time by itself does nothing, either for or against natural selection. I state this because it

has been erroneously asserted that the element of time has been assumed by me to play an all-important

part inmodifying species, as if all the forms of life were necessarily undergoingchange through some

innate law. Lapse of time is only so far important, and its importance in this respect is great, that it gives a

better chance of beneficial variations arising and of their being selected, accumulated, and fixed. It

likewise tends to increase the direct action of the physical conditions of life, in relation to the constitution

of each organism.

If we turn to nature to test the truth of these remarks, and look at any small isolated area, such as an

oceanic island, although the number of species inhabiting it is small, as we shall see in our chapter onGeographical Distribution; yet of these species a very large proportion are endemic,—that is, have been

produced there and nowhere else in the world. Hence an oceanic island at first sight seems to have been

highly favourable for the production of new species. But we may thus deceive ourselves, for to ascertain

whether a small isolated area, or a large open area like a continent, has been most favourable for the

production of new organic forms, we ought to make the comparison within equal times; and this we are

incapable of doing.

Although isolation is of great importance in the production of new species, on the whole I am inclined to

believe that largeness of area is still more important, especially for the production of species which shall

prove capable of enduring for a long period, and of spreading widely. Throughout a great and open area,not only will there be a better chance of favourable variations, arising from the large number of individuals

of the same species there supported, but the conditions of life are much more complex from the large

number of already existing species; and if some of these many species become modified and improved,

others will have to be improved in a corresponding degree, or they will be exterminated. Each new form,

also, as soon as it has been much improved, will be able to spread over the open and continuous area,

and will thus come into competition with many other forms. Moreover, great areas, though now

continuous, will often, owing to former oscillations of level, have existed in a broken condition; so that the

good effects of isolation will generally, to a certain extent, have concurred. Finally, I conclude that,

although small isolated areas have been in some respects highly favourable for the production of new

species, yet that the course of modification will generally have been more rapid on large areas; and what

is more important, that the new forms produced on large areas, which already have been victorious over

many competitors, will be those that will spread most widely, and will give rise to the greatest number of

new varieties and species. They will thus play a more important part in the changing history of the organic







beings, one with another and with their physical conditions of life, which may have been effected in the

long course of time through nature's power of selection, that is by the survival of the fittest.

Extinction caused by Natural Selection.

This subject will be more fully discussed in our chapter on Geology; but it must here be alluded to from

being intimately connected with natural selection.Natural selection acts solely through the preservation of

variations in somewayadvantageous, which consequently endure.Owing to the high geometrical rate of

increase of all organic beings, each area is already fully stocked with inhabitants; and it follows from this,

that as the favoured forms increase in number, so, generally, will the less favoured decrease and become

rare. Rarity, as geology tells us, is the precursor to extinction. We can see that any form which is

representedby few individualswill run a good chance of utter extinction, during great fluctuations in the

nature of the seasons, or from a temporary increase in the number of its enemies. But we may go further

than this; for, as new forms are produced, unless we admit that specific forms can go on indefinitely

increasing in number, many old forms must become extinct. That the number of specific forms has notindefinitely increased, geology plainly tells us; and we shall presently attempt to show why it it is that the

number of species throughout the world has not become immeasurably great.

We have seen that the species which are most numerous in individuals have the best chance of producing

favourable variations within any given period. We have evidence of this, in the facts stated in the second

chapter showing that it is the common and diffused or dominant species which offer the greatest number

of recorded varieties. Hence, rare species will be less quickly modified or improved within any given

period; they will consequently be beaten in the race for life by the modified and improved descendants of

the commoner species.

From these several considerations I think it inevitably follows, that as new species in the course of timeare formed through natural selection, others will become rarer and rarer, and finally extinct. The forms

which stand in closest competitionwith those undergoing modificationand improvement will naturally

suffer most. And we have seen in the chapter on the Struggle for Existence that it is the most

closely-allied forms,—varieties of the same species, and species of the same genus or of related

genera,—which, from having nearly the same structure, constitution, and habits, generally come into the

severest competition with each other; consequently, each new variety or species, during the progress of

its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. We see

the same process of extermination amongst our domesticated productions, through the selectionof

improved forms by man. Many curious instances could be given showing how quickly new breeds of

cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. InYorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that

these "were swept away by the short-horns" (I quote the words of an agricultural writer) "as if by some

murderous pestilence."

Divergence of Character .

The principle, which I have designated by this term, is of high importance, and explains, as I believe,

several important facts. In the first place, varieties, even strongly-marked ones, though having somewhat

of the character of species—as is shown by the hopeless doubts in many cases how to rank them—yet

certainly differ far less from each other than do good and distinct species. Nevertheless, according to my

view, varieties are species in the process of formation, or are, as I have called them, incipient species.





How, then, does the lesser difference between varieties become augmented into the greater difference

between species? That this does habitually happen, we must infer from most of the innumerable species

throughout nature presenting well-marked differences; whereas varieties, the supposed prototypes and

parents of future well-marked species, present slight and ill-defined differences. Mere chance, as we may

call it, might cause one variety to differ in some character from its parents, and the offspring of this variety

again to differ from its parent in the very same character and in a greater degree; but this alone would

never account for so habitual and large a degree of difference as that between the species of the samegenus.

As has always been my practice, I have sought light on this head from our domestic productions. We

shall here find something analogous. It will be admitted that the production of races so different as

short-horn and Hereford cattle, race and cart horses, the several breeds of pigeons, &c., could never

have been effected by the mere chance accumulation of similar variations during many successive

generations. In practice, a fancier is, for instance, struck by a pigeon having a slightly shorter beak;

another fancier is struck by a pigeon having a rather longer beak; and on the acknowledged principle that

"fanciers do not and will not admire a medium standard, but like extremes," they both go on (as has

actually occurred with the sub-breeds of the tumbler-pigeon) choosing andbreeding from birds withlonger and longer beaks, or with shorter and shorter beaks. Again, we may suppose that at an early

period of history, the men of one nation or district required swifter horses, whilst those of another

required stronger and bulkier horses. The early differences would be very slight; but, in the course of

time, from the continued selection of swifter horses in the one case, and of stronger ones in the other, the

differences would become greater, and would be noted as forming two sub-breeds. Ultimately, after the

lapse of centuries, these sub-breeds would become converted into two well-established and distinct

breeds. As the differences became greater, the inferior animals with intermediate characters, beingneither

very swift nor very strong, would not have been used for breeding, and will thus have tended to

disappear. Here, then, we see in man's productions the action of what may be called the principle of

divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge

in character, both from each other and from their common parent.

But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply

most efficiently (though it was a long time before I saw how), from the simple circumstance that the more

diversified the descendants from any one species become in structure, constitution, and habits, by so

much will they be better enabled to seize on many and widely diversified places in the polity of nature,

and so be enabled to increase in numbers.

We can clearly discern this in the case of animals with simple habits. Take the case of a carnivorous

quadruped, of which the number that can be supported in any country has long ago arrived at its full

average. If its natural power of increase be allowed to act, it can succeed in increasing (the country notundergoing any change in conditions) only by itsvarying descendants seizing onplaces at present

occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either

dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps

becoming less carnivorous. The more diversified in habits and structure the descendants of our

carnivorous animals become, the more places they will be enabled to occupy. What applies to one animal

will apply throughout all time to all animals—that is, if they vary—for otherwise natural selection can

affect nothing. So it will be with plants. It has been experimentally proved, that if a plot of ground be

sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a

greater number of plants and a greater weight of dry herbage can be raised in the latter than in the former

case. The same has been found to hold good when one variety and several mixed varieties of wheat have

been sown on equal spaces of ground. Hence, if any one species of grass were to go on varying, and the

varieties were continually selected which differed from each other in the same manner, though in a very

slight degree, as do the distinct species and genera of grasses, a greater number of individual plants of this





species, including its modified descendants, would succeed in living on the same piece of ground. And we

know that each species and each variety of grass is annually sowing almost countless seeds; and is thus

striving, as it may be said, to the utmost to increase in number. Consequently, in the course of many

thousand generations, the most distinct varieties of any one species of grass would have the best chance

of succeeding and of increasing in numbers, and thus of supplanting the less distinct varieties; and

varieties, when rendered very distinct from each other, take the rank of species.

The truth of the principle that the greatest amount of life can be supported by great diversification of

structure, is seen under manynatural circumstances. In an extremely small area, especially if freelyopen

to immigration, and where the contest between individual and individual must be very severe, we always

find great diversity in its inhabitants. For instance, I found that a piece of turf, three feet by four in size,

which had been exposed for many years to exactly the same conditions, supported twenty species of

plants, and these belonged to eighteen genera and to eight orders, which shows how much these plants

differed from each other. So it is with the plants and insects on small and uniform islets: also in small

ponds of fresh water. Farmers find that they can raise most food by a rotation of plants belonging to the

most different orders: nature follows what may be called a simultaneous rotation. Most of the animals and

plants which live close round any small piece of ground, could live on it (supposing its nature not to be inany way peculiar), and may be said to be striving to the utmost to live there; but, it is seen, that where

they come into theclosest competition, theadvantages of diversificationof structure, with the

accompanying differencesof habit and constitution, determine that the inhabitants,which thus jostle each

other most closely, shall, as a general rule, belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through man's agency in foreign lands. It might

have been expected that the plants which would succeed in becoming naturalised in any land would

generally have been closely allied to the indigenes; for these are commonly looked at as specially created

and adapted for their own country. It might also, perhaps, have been expected that naturalised plants

would have belonged to a few groups more especially adapted to certain stations in their new homes. But

the case is very different; and Alph. de Candolle has well remarked, in his great and admirable work, thatfloras gain by naturalisation, proportionally with the number of the native genera and species far more in

new genera than in new species. To give a single instance: in the last edition of Dr. Asa Gray's 'Manual of

the Flora of the Northern United States,' 260 naturalised plants are enumerated, and these belong to 162

genera. We thus see that these naturalised plants are of a highly diversified nature. They differ, moreover,

to a large extent, from the indigenes, for out of the 162 naturalised genera, no less than 100 genera are

not there indigenous, and thus a large proportional addition is made to the genera now living in the United

States.

By considering the nature of the plants or animals which have in any country struggled successfully with

the indigenes and have there become naturalised, we may gain some crude idea in what manner some of the natives would have to be modified, in order to gain an advantage over their compatriots; and we may

at least infer that diversification of structure, amounting to new generic differences, would be profitable to

them.

The advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as

that of the physiological division of labour in the organs of the same individual body—a subject so well

elucidated by Milne Edwards. No physiologist doubts that a stomach adapted to digest vegetable matter

alone, or flesh alone, draws most nutriment from these substances. So in the general economy of any

land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will

a greater number of individuals be capable of there supporting themselves. A set of animals, with their

organisation but littlediversified, could hardly competewith a set more perfectly diversified in structure. It

maybe doubted, for instance, whether the Australianmarsupials, which are divided into groups differing

but little from each other, and feebly representing, as Mr. Waterhouse and others have remarked, our





carnivorous, ruminant, and rodent mammals, could successfully competewith these well-developed

orders. In the Australian mammals, we see the process of diversification in an early and incomplete stage

of development.

The Probable Effects of the Action of Natural Selection through Divergence of Character and

Extinction, on the Descendants of a Common Ancestor.

After the foregoing discussion, which has been much compressed, we may assume that the modified

descendants of any one species will succeed so much the better as they become more diversified in

structure, and are thus enabled to encroach on places occupied by other beings. Now let us see how this principle of benefit being derived from divergence of character, combinedwith the principles of natural

selection and of extinction, tends to act.

The accompanying diagram will aid us in understanding this rather perplexing subject. Let A to L

represent the species of a genus large in its own country; these species are supposed to resemble each

other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the

letters standing at unequal distances. I have said a large genus, because as we saw in the second chapter,

on an average more species vary in large genera than in small genera; and the varying species of the large

genera present a greater number of varieties. We have, also, seen that the species, which are the

commonest and the most widely diffused, vary more than do the rare and restricted species. Let (A) be a

common, widely-diffused, and varying species, belonging to a genus large in its own country. The

branching and diverging dotted lines of unequal lengths proceeding from (A),may represent its varying

offspring. The variations are supposed to be extremely slight, but of the most diversified nature; they are

not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed

to endure for equal periods. Only those variations which are in some way profitable will be preserved or

naturally selected. And here the importance of the principle of benefit derived from divergence of

character comes in; for this will generally lead to the most different or divergent variations (represented

by the outer dotted lines) being preserved and accumulated by natural selection. When a dotted line

reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of

variation is supposed to have been accumulated to form it into a fairly well-marked variety, such as

would be thought worthy of record in a systematic work.

The intervals between the horizontal lines in the diagram, may represent each a thousand or more





generations. After a thousand generations, species (A) is supposed to have produced two fairly

well-marked varieties, namelya ?1; andm ?1;. These two varieties will generally still be exposed to the

same conditionswhichmade their parents variable, and the tendency to variability is in itself hereditary;

consequently they will likewise tend to vary, and commonly in nearly the same manner as did their

parents. Moreover, these two varieties, beingonly slightly modified forms, will tend to inherit those

advantages which made their parent (A) more numerous than most of the other inhabitants of the same

country; they will also partake of those more general advantages which made the genus to which the parent-species belonged, a large genus in its own country. And all these circumstances are favourable to

theproduction of newvarieties.

If, then, these two varieties be variable, the most divergent of their variations will generally be preserved

during the next thousandgenerations. And after this interval, varietya ?1; is supposed in the diagram to

have produced varietya ?2;, which will, owing to the principle of divergence, differ more from (A) than

did varietya ?1;. Varietym?1; is supposed to have produced two varieties, namelym ?2; and s ?2;,

differing from each other, and more considerably from their common parent (A). We may continue the

process by similar steps for any length of time; some of the varieties, after each thousand generations,

producing only a single variety, but in a more and more modified condition, some producing two or threevarieties, and some failing to produce any. Thus the varieties or modified descendants of the common

parent (A), will generally go on increasing in number and diverging in character. In the diagram the

process is represented up to the ten-thousandth generation, and under a condensed and simplified form

up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes on so regularly as is

represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is

far more probable that each form remains for long periods unaltered, and then again undergoes

modification. Nor do I suppose that the most divergent varieties are invariably preserved: a medium form

may often long endure, and may or may not produce more than one modified descendant; for natural

selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. But as a general

rule, the more diversified in structure the descendants from any one species can be rendered, the more

places they will be enabled to seize on, and the more their modified progeny will increase. In our diagram

the line of succession is broken at regular intervals by small numbered letters marking the successive

forms which have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary,

and might have been inserted anywhere, after intervals long enough to allow the accumulation of a

considerable amount of divergent variation.

As all the modified descendants from a common and widely-diffused species, belonging to a large genus,

will tend to partake of the same advantages which made their parent successful in life, they will generallygo on multiplying in number as well as diverging in character: this is represented in the diagram by the

several divergent branches proceeding from (A). The modified offspring from the later and more highly

improved branches in the lines of descent, will, it is probable, often take the place of, and so destroy, the

earlier and less improved branches: this is represented in the diagram by some of the lower branches not

reaching to the upper horizontal lines. In some cases no doubt the process of modification will be

confined to a single line of descent, and the number of modified descendants will not be increased;

although the amount of divergent modificationmay have been augmented. This case would be

represented in the diagram, if all the lines proceeding from (A) were removed, excepting that froma?1; to

a?1;?dm; . In the same way the English race-horse and English pointer have apparently both gone on

slowly diverging in character from their original stocks, without either havinggiven off any fresh branches

or races.

After ten thousand generations, species (A) is supposed to have produced three forms,a ?1;?dm; , f





?1;?dm;, andm ?1;?dm;, which, from having diverged in character during the successive generations, will

have come to differ largely, but perhaps unequally, from each other and from their common parent. If we

suppose the amount of change between each horizontal line in our diagram to be excessively small, these

three forms may still be only well-marked varieties; but we have only to suppose the steps in the process

of modification to be more numerous or greater in amount, to convert these three forms into doubtful or

at last into well-defined species. Thus the diagram illustrates the steps by which the small differences

distinguishingvarieties are increased into the larger differences distinguishing species.By continuing thesame process for a greater number of generations (as shown in the diagram in a condensed and simplified

manner), we get eight species, marked by the letters betweena14 andm14 , all descended from (A).

Thus, as I believe, species are multiplied and genera are formed.

In a large genus it is probable that more than one species would vary. In the diagram I have assumed

that a second species (I) has produced, by analogous steps, after ten thousand generations, either two

well-marked varieties(w10 and z 10 ) or two species, according to the amount of change supposed to be

represented between the horizontal lines.After fourteen thousandgenerations, six newspecies,marked

by the lettersn14 to z 14 , are supposed to have been produced. In any genus, the species which are

already very different in character from each other, will generally tend to produce the greatest number of modified descendants; for these will have the best chance of seizing on new and widely different places in

the polity of nature: hence in the diagram I have chosen the extreme species (A), and the nearly extreme

species (I), as those which have largely varied, and have given rise to new varieties and species. The

other nine species (marked by capital letters) of our original genus, may for long but unequal periods

continue to transmit unaltered descendants; and this is shown in the diagram by the dotted lines unequally

prolonged upwards.

But during the process of modification, represented in the diagram, another of our principles, namely that

of extinction, will have played an important part. As in each fully stocked country natural selection

necessarily acts by the selected form having some advantage in the struggle for life over other forms,

there will be a constant tendency in the improved descendants of any one species to supplant andexterminate in each stage of descent their predecessors and their original progenitor. For it should be

remembered that the competition will generally be most severe between those forms which are most

nearly related to each other in habits, constitution, and structure. Hence all the intermediate forms

between the earlier and later states, that is between the less and more improved states of the same

species, as well as the original parent-species itself, will generally tend to become extinct. So it probably

will be with many whole collateral lines of descent, which will be conquered by later and improved lines.

If, however, the modified offspring of a species get into some distinct country, or become quickly

adapted to some quite new station, in which offspring and progenitor do not come into competition, both

may continue to exist.

If, then, our diagram be assumed to represent a considerable amount of modification, species (A) and all

the earlier varieties will have become extinct, being replaced by eight new species (a14tom14 ); and

species (I) will be replaced by six (n14to z 14 ) new species.

But we may go further than this. The original species of our genus were supposed to resemble each

other in unequal degrees, as is so generally the case in nature; species (A) being more nearly related to B,

C, and D, than to the other species; and species (I) more to G, H, K, L, than to the others. These two

species (A) and (I) were also supposed to be very common and widely diffused species, so that they

must originally have had some advantage over most of the other species of the genus. Their modified

descendants, fourteen in number at the fourteen-thousandth generation,will probablyhave inherited some

of the same advantages: they have also been modified and improved in a diversified manner at each stage

of descent, so as to have become adapted to many related places in the natural economy of their country.

It seems, therefore, extremely probable that they will have taken the places of, and thus exterminated not





only their parents (A) and (I), but likewise some of the original species which were most nearly related to

their parents. Hence very few of the original species will have transmitted offspring to the

fourteen-thousandth generation. We may suppose that only one, (F), of the two species (E and F) which

were least closely related to the other nine original species, has transmitted descendants to this late stage

of descent.

The new species in our diagram descended from the original eleven species, will now be fifteen innumber.Owing to thedivergent tendency of natural selection, the extreme amount of difference in

character between speciesa14 and z 14 will be much greater than that between the most distinct of the

original eleven species. The new species, moreover, will be allied to each other in a widely different

manner. Of the eight descendants from (A) the three markeda14 , q14, p14 , will be nearly related from

having recently branched off froma10 , b14 , and f 14 , from having diverged at an earlier period froma5 ,

will be in some degree distinct from the three first-named species; and lastly,o14 ,e14 , andm14 , will be

nearly related one to the other, but, from having diverged at the first commencement of the process of

modification, will be widely different from the other five species, and may constitute a sub-genus or a

distinct genus.

The six descendants from (I) will form two sub-genera or genera. But as the original species (I) differed

largely from (A), standing nearly at the extreme end of the original genus, the six descendants from (I)

will, owing to inheritance alone, differ considerably from the eight descendants from (A); the twogroups,

moreover, are supposed to have gone on diverging in different directions. The intermediate species, also

(and this is a very important consideration), which connected the original species (A) and (I), have all

become, excepting (F), extinct, and have left no descendants. Hence the six new species descended from

(I), and the eight descendants from (A), will have to be ranked as very distinct genera, or even as distinct

sub-families.

Thus it is, as I believe, that two or more genera are produced by descent with modification, from two or

more species of the same genus. And the two or more parent-species are supposed to be descendedfrom some one species of an earlier genus. In our diagram, this is indicated by the broken lines, beneath

the capital letters, converging in sub-branches downwards towards a single point; this point represents a

species, the supposed progenitor of our several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the new species F14, which is supposed not

to have diverged much in character, but to have retained the form of (F), either unaltered or altered only

in a slight degree. In this case, its affinities to the other fourteen new species will be of a curious and

circuitous nature. Being descended from a form which stood between the parent-species (A) and (I),

now supposed to be extinct and unknown, it will be in some degree intermediate in character between

the two groups descended from these two species. But as these two groups have gone on diverging incharacter from the type of their parents, the new species (F14) will not be directly intermediate between

them, but rather between types of the two groups; and every naturalist will be able to call such cases

before his mind.

In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but

each may represent a million or more generations; it may also represent a section of the successive strata

of the earth's crust including extinct remains. We shall, when we come to our chapter on Geology, have

to refer again to this subject, and I think we shall then see that the diagram throws light on the affinities of

extinct beings, which, thoughgenerally belonging to the same orders, families, or genera,with those now

living, yet are often, in some degree, intermediate in character between existing groups; and we can

understand this fact, for the extinct species lived at various remote epochs when the branching lines of

descent had diverged less.







as Milne Edwards would express it, the completeness of the division of physiological labour. But we shall

see how obscure this subject is if we look, for instance, to fishes, amongst which some naturalists rank

those as highest which, like the sharks, approach nearest to amphibians; whilst other naturalists rank the

common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ

most from the other vertebrate classes. We see still more plainly the obscurity of the subject by turning to

plants, amongst which the standard of intellect is of course quite excluded; and here some botanists rank

those plants as highest which have every organ, as sepals, petals, stamens, and pistils, fully developed ineach flower; whereas other botanists, probably with more truth, look at the plants which have their

several organs much modified and reduced in number as the highest.

If we take as the standard of high organisation, the amount of differentiation and specialisation of the

several organs in each being when adult (and this will include the advancement of the brain for intellectual

purposes), natural selection clearly leads towards this standard: for all physiologists admit that the

specialisation of organs, inasmuch as in this state they perform their functions better, is an advantage to

each being; andhence the accumulation of variations tending towards specialisation is within the scope of

natural selection. On the other hand, we can see, bearing in mind that all organic beings are striving to

increase at a high ratio and to seize on every unoccupied or less well occupied place in the economy of nature, that it is quite possible for natural selection gradually to fit a being to a situation in which several

organs would be superfluous or useless: in such cases there would be retrogression in the scale of

organisation. Whether organisation on thewhole has actually advanced from the remotest geological

periods to the present day will be more conveniently discussed in our chapter on Geological Succession.

But it may be objected that if all organic beings thus tend to rise in the scale, how is it that throughout the

world a multitude of the lowest forms still exist; and how is it that in each great class some forms are far

more highly developed than others? Why have not the more highly developed forms everywhere

supplanted andexterminated the lower?Lamarck,who believed in an innate and inevitable tendency

towards perfection in all organic beings, seems to have felt this difficulty so strongly, that he was led to

suppose that newand simple forms are continually being produced by spontaneous generation. Sciencehas not as yet proved the truth of this belief, whatever the future may reveal. On our theory the continued

existence of lowly organisms offers no difficulty; for natural selection, or the survival of the fittest, does

not necessarily include progressive development—it only takes advantage of such variations as arise and

are beneficial to each creature under its complex relations of life. And it may be asked what advantage,

as far as we can see, would it be to an infusorian animalcule—to an intestinal worm—or even to an

earthworm, to be highly organised. If it were no advantage, these forms would be left, by natural

selection, unimproved or but little, improved, and might remain for indefinite ages in their present lowly

condition. And geology tells us that some of the lowest forms, as the infusoria and rhizopods, have

remained for an enormous period in nearly their present state. But to suppose that most of the many now

existing low forms have not in the least advanced since the first dawn of life would be extremely rash; for every naturalist who has dissected some of the beings now ranked as very low in the scale, must have

been struck with their reallywondrous and beautiful organisation.

Nearly the same remarks are applicable if we look to the different grades of organisation within the same

great group; for instance, in the vertebrata, to thecoexistence ofmammals and fish—amongst mammalia,

to the co-existence of man and the ornithorhynchus—amongst fishes, to the co-existence of the shark

and the lancelet (Amphioxus),which latter fish in the extreme simplicityof its structure approaches the

invertebrate classes. But mammals and fish hardly come into competition with each other; the

advancement of the whole class of mammals, or of certain members in this class, to the highest grade

would not lead to their taking the place of fishes. Physiologists believe that the brain must be bathed by

warm blood to be highly active, and this requires aërial respiration; so that warm-blooded mammals

when inhabiting the water lie under a disadvantage in having to come continually to the surface to breathe.

With fishes, members of the shark family would not tend to supplant the lancelet; for the lancelet, as I





hear from Fritz Müller, has as sole companion and competitor on the barren sandy shore of South Brazil,

an anomalous annelid. The three lowest orders of mammals, namely, marsupials, edentata, and rodents,

co-exist in South America in the same regionwith numerous monkeys, and probably interfere little with

each other. Although organisation, on the whole, mayhave advancedandbe still advancing throughout

the world, yet the scale will always present many degrees of perfection; for the high advancement of

certain whole classes, or of certain members of each class, does not at all necessarily lead to the

extinction of those groups with which they do not enter into close competition. In some cases, as we shallhereafter see, lowly organised forms appear to have been preserved to the present day, from inhabiting

confined or peculiar stations, where they have been subjected to less severe competition, and where their

scantynumbers have retarded the chance of favourable variations arising.

Finally, I believe that many lowly organised forms now exist throughout the world, from various causes.

In some cases variations or individual differences of a favourable nature may never have arisen for natural

selection to act on and accumulate. In no case, probably, has time sufficed for the utmost possible

amount of development. In some few cases there has been what we must call retrogression of

organisation. But the main cause lies in the fact that under very simple conditions of life a high organisation

would be of no service,—possibly would be of actual disservice, as being of a more delicate nature, andmore liable to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest

structure, how, it has been asked, could the first steps in the advancement or differentiation of parts have

arisen? Mr. Herbert Spencer would probably answer that, as soon as simple unicellular organism came

by growth or division to be compounded of several cells, or became attached to any supporting surface,

his law "that homologous units of any order become differentiated in proportion as their relations to

incident forces become different" would come into action. But as we have no facts to guide us,

speculation on the subject is almost useless. It is, however, an error to suppose that there would be no

struggle for existence, and, consequently, no natural selection, untilmany formshad been produced:

variations in a single species inhabiting an isolated station might be beneficial, and thus the whole mass of individuals might be modified, or two distinct forms might arise. But, as I remarked towards the close of

the Introduction, no one ought to feel surprise at much remaining as yet unexplained on the origin of

species, if we make due allowance for our profound ignorance on the mutual relations of the inhabitants

of the world at the present time, and still more so during past ages.

Convergence of Character .

Mr. H. C. Watson thinks that I have overrated the importance of divergence of character (in which,however, he apparently believes), and that convergence, as it may be called, has likewise played a part.

If two species, belonging to two distinct though allied genera, had both produced a large number of new

and divergent forms, it is conceivable that these might approach each other so closely that they would

have all to be classed under the same genus; and thus the descendants of two distinct genera would

converge into one. But it would in most cases be extremely rash to attribute to convergence a close and

general similarity of structure in the modified descendants ofwidely distinct forms.The shape of a crystal

is determined solely by the molecular forces, and it is not surprising that dissimilar substances should

sometimes assume the same form; but with organic beings we should bear in mind that the form of each

depends on an infinitude of complex relations, namely on the variationswhichhave arisen, these being

due to causes far too intricate to be followed out,—on the nature of the variations which have been

preserved or selected, and this depends on the surrounding physical conditions, and in a still higher

degree on the surrounding organismswithwhich each being has come into competition,—and lastly, on

inheritance (in itself a fluctuating element) from innumerable progenitors, all ofwhich havehad their forms







welfare, in the same manner as so many variations have occurred useful to man. But if variations useful to

any organic being ever do occur, assuredly individuals thus characterised will have the best chance of

being preserved in the struggle for life; and from the strong principle of inheritance, these will tend to

produceoffspring similarly characterised.This principle of preservation, or the survival of the fittest, I

have called Natural Selection. It leads to the improvement of each creature in relation to its organic and

inorganic conditions of life; and consequently, in most cases, to what must be regarded as an advance in

organisation.Nevertheless, low and simple forms will long endure ifwell fitted for their simple conditionsof life.

Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg,

seed, or young, as easily as the adult. Amongst many animals, sexual selection will have given its aid to

ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of

offspring.Sexual selection will also give characters useful to the males alone, in their strugglesor rivalry

with other males; and these characters will be transmitted to one sex or to both sexes, according to the

form of inheritance which prevails.

Whether natural selection has really thus acted in adapting the various forms of life to their severalconditions and stations, must be judged by the general tenor and balance of evidence given in the

following chapters. But we have already seen how it entails extinction; and how largely extinction has

acted in the world's history, geology plainly declares. Natural selection, also, leads to divergence of

character; for the more organic beings diverge in structure, habits, and constitution, by so much the more

can a large number be supported on the area,—of which we see proof by looking to the inhabitants of

any small spot, and to the productions naturalised in foreign lands. Therefore, during the modification of

the descendants of any one species, and during the incessant struggle of all species to increase in

numbers, the more diversified the descendants become, the better will be their chance of success in the

battle for life. Thus the small differences distinguishingvarieties of the same species, steadily tend to

increase, till they equal the greater differences between species of the same genus, or even of distinct

genera.

We have seen that it is the common, the widely-diffused and widely-ranging species, belonging to the

larger genera within each class, which vary most; and these tend to transmit to their modified offspring

that superiority which now makes them dominant in their own countries. Natural selection, as has just

been remarked, leads to divergence of character and to much extinction of the less improved and

intermediate forms of life.On these principles, the nature of the affinities, and the generallywell-defined

distinctions between the innumerable organic beings in each class throughout the world, maybe

explained. It is a truly wonderful fact—the wonder of which we are apt to overlook from familiarity—that

all animals and all plants throughout all time and space should be related to each other in groups,

subordinate to groups, in the manner which we everywhere behold—namely, varieties of the samespecies most closely related, species of the same genus less closely and unequally related, forming

sections and sub-genera, species of distinct genera much less closely related, and genera related in

different degrees, forming sub-families, families, orders, sub-classes andclasses.The several subordinate

groups in any class cannot be ranked in a single file, but seem clustered round points, and these round

other points, and so on in almost endless cycles. If species had been independently created, no

explanation would have been possible of this kind of classification; but it is explained through inheritance

and the complex action of natural selection, entailing extinction and divergence of character, as we have

seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been represented by a great tree. I

believe this simile largely speaks the truth. The greenandbudding twigsmayrepresent existing species;

and those produced during former years may represent the long succession of extinct species. At each

period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the









structures which can be best explained by the effects of disuse. As Professor Owen has remarked, there

is no greater anomaly in nature than a bird that cannot fly; yet there are several in this state. The

logger-headed duck of South America can only flap along the surface of the water, and has its wings in

nearly the same condition as the domestic Aylesbury duck: it is a remarkable fact that the young birds,

according to Mr. Cunningham, can fly, while the adults have lost this power. As the larger ground-feeding

birds seldom take flight except to escape danger, it is probable that the nearly wingless condition of

several birds, now inhabiting or which lately inhabited several oceanic islands, tenanted by no beast of prey, has been caused by disuse. The ostrich indeed inhabits continents, and is exposed to danger from

which it cannot escape by flight, but it can defend itself by kicking its enemies, as efficiently as many

quadrupeds. We may believe that the progenitor of the ostrich genus had habits like those of the bustard,

and that, as the size and weight of its body were increased during successive generations, its legs were

used more, and its wings less, until they became incapable of flight.

Kirby has remarked (and I have observed the same fact) that the anterior tarsi, or feet, of many male

dung-feeding beetles are often broken off; he examined seventeen specimens in his own collection, and

not one had even a relic left. In the Onites apelles the tarsi are so habitually lost, that the insect has been

described as not having them. In some other genera they are present, but in a rudimentary condition. Inthe Ateuchus or sacred beetle of the Egyptians, they are totally deficient. The evidence that accidental

mutilations can be inherited is at present not decisive; but the remarkable cases observed by

Brown-Séquard in guinea-pigs, of the inherited effects of operations, should make us cautious in denying

this tendency. Hence it will perhaps be safest to look at the entire absence of the anterior tarsi in

Ateuchus, and their rudimentary condition in some othergenera, not as cases of inherited mutilations, but

as due to the effects of long-continued disuse; for as many dung-feeding beetles are generally found with

their tarsi lost, this must happen early in life; therefore the tarsi cannot be of much importance or be much

used by these insects.

In some cases we might easily put down to disuse modifications of structure which are wholly, or mainly,

due to natural selection. Mr. Wollaston has discovered the remarkable fact that 200 beetles, out of the550 species (but more are now known) inhabiting Madeira, are so far deficient in wings that they cannot

fly; and that, of the twenty-nine endemic genera, no less than twenty-three have all their species in this

condition! Several facts,—namely, that beetles in many parts of the world are frequently blown to sea

and perish; that the beetles in Madeira, as observed by Mr. Wollaston, lie much concealed, until the wind

lulls and the sun shines; that the proportion of wingless beetles is larger on the exposed Desert as than in

Madeira itself; and especially the extraordinary fact, so strongly insisted on by Mr. Wollaston, that certain

large groups of beetles, elsewhere excessivelynumerous,which absolutely require the use of their wings,

are here almost entirely absent;—these several considerations make me believe that the wingless

condition of so many Madeira beetles is mainly due to the action of natural selection, combined probably

with disuse. For during many successive generations each individual beetle which flew least, either fromits wings having been ever so little less perfectly developed or from indolent habit, will have had the best

chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most

readily took to flight would oftenest have been blown to sea, and thus destroyed.

The insects in Madeira which are not ground-feeders, and which, as certain flower-feeding coleoptera

and lepidoptera, must habitually use their wings to gain their subsistence, have, as Mr. Wollaston

suspects, their wings not at all reduced, but even enlarged. This is quite compatible with the action of

natural selection. For when a new insect first arrived on the island, the tendency of natural selection to

enlarge or to reduce the wings, would depend on whether a greater number of individuals were saved by

successfully battling with the winds, or by giving up the attempt and rarely or never flying. As with

mariners shipwrecked near a coast, it would have been better for the good swimmers if they had been

able to swim still further, whereas it would have been better for the bad swimmers if they had not been

able to swim at all and had stuck to the wreck.





The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite

covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but

aided perhaps by natural selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys,

is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often

caught them, that they were frequently blind. One which I kept alive was certainly in this condition, the

cause, as appeared on dissection, havingbeen inflammation of thenictitating membrane. As frequentinflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to

animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth

of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of

disuse.

It is well known that several animals, belonging to the most different classes, which inhabit the caves of

Carniola and of Kentucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the

eye is gone;—the stand for the telescope is there, though the telescope with its glasses has been lost. As

it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in

darkness, their loss may be attributed to disuse. In one of the blind animals, namely, the cave-rat(Neotoma), two of which were captured by Professor Silliman at above half a mile distance from the

mouth of the cave, and therefore not in the profoundest depths, the eyes were lustrous and of large size;

and these animals, as I am informed by Professor Silliman, after having been exposed for about a month

to a graduated light, acquired a dim perception of objects.

It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar

climate; so that, in accordance with the old view of the blind animals having been separately created for

the American and European caverns, very close similarity in their organisation and affinitiesmight have

been expected. This is certainly not the case if we look at the two whole faunas; and with respect to the

insects alone, Schiödte has remarked, "We are accordingly prevented from considering the entire

phenomenon in any other light than something purely local, and the similaritywhich is exhibited in a fewforms between the Mammoth cave (in Kentucky) and the caves in Carniola, otherwise than as a very

plain expression of that analogy which subsists generally between the fauna of Europe and of North

America." On my view we must suppose that American animals, having in most cases ordinary powers of

vision, slowly migrated by successive generations from the outer world into the deeper and deeper

recesses of the Kentucky caves, as did European animals into the caves of Europe. We have some

evidence of this gradation of habit; for, as Schiödte remarks, "We accordingly look upon the

subterranean faunas as small ramifications whichhave penetrated into the earth from the geographically

limited faunas of the adjacent tracts, and which, as they extended themselves into darkness, have been

accommodated to surrounding circumstances.Animals not far remote from ordinary forms, prepare the

transition from light to darkness. Next follow those that are constructed for twilight; and, last of all, thosedestined for total darkness, and whose formation is quite peculiar." These remarks of Schiödte's, it

should be understood, apply not to the same, but to distinct species. By the time that an animal had

reached, after numberless generations, the deepest recesses, disuse will on this view have more or less

perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an

increase in the length of the antennæ or palpi, as a compensation for blindness. Notwithstanding such

modifications, we might expect still to see in the cave-animals of America, affinities to the other

inhabitants of that continent, and in those of Europe to the inhabitants of the European continent. And this

is the case with some of the American cave-animals, as I hear from Professor Dana; and some of the

European cave-insects are very closely allied to those of the surrounding country. It would be difficult to

give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two

continents on the ordinary view of their independent creation. That several of the inhabitants of the caves

of the Old and New Worlds should be closely related, we might expect from the well-known relationship

of most of their other productions. As a blind species of Bathyscia is found in abundance on shady rocks







climate of Faroe in the north and of the Falklands in the south, and on many an island in the torrid zones.

Hence adaptation to any special climate may be looked at as a quality readily grafted on an innate wide

flexibility of constitution, common tomost animals.On this view, the capacity of enduring the most

different climates by man himself and by his domestic animals, and the fact of the extinct elephant and

rhinoceros having formerly endured a glacial climate,whereas the living species arenow all tropical or

sub-tropical in their habits, ought not to be looked at as anomalies, but as examples of a very common

flexibility of constitution, brought, under peculiar circumstances, into action.

How much of the acclimatisation of species to any peculiar climate is due to mere habit, and how much

to the natural selection of varieties having different innate constitutions, and how much to bothmeans

combined, is an obscure question. That habit or custom has some influence, I must believe, both from

analogy and from the incessant advice given in agriculturalworks, even in the ancient Encyclopædias of

China, to be very cautious in transporting animals from one district to another. And as it is not likely that

man should have succeeded in selecting somany breeds and sub-breedswith constitutions specially fitted

for their own districts, the result must, I think, be due to habit. On the other hand, natural selection would

inevitably tend to preserve those individuals which were born with constitutions best adapted to any

country which they inhabited. In treatises on many kinds of cultivated plants, certain varieties are said towithstandcertain climates better than others; this is strikingly shown inworks on fruit-trees published in

the United States, in which certain varieties are habitually recommended for the northern and others for

the southern States; and as most of these varieties are of recent origin, they cannot owe their

constitutional differences to habit. The case of the Jerusalem artichoke, which is never propagated in

England by seed, and of which consequently new varieties have not been produced, has even been

advanced, as proving that acclimatisation cannot be effected, for it is now as tender as ever it was! The

case, also, of the kidney-bean has been often cited for a similar purpose, and with much greater weight;

but until someone will sow, during a score of generations, his kidney-beans so early that a very large

proportion are destroyed by frost, and then collect seed from the few survivors, with care to prevent

accidental crosses, and then again get seed from these seedlings, with the same precautions, the

experiment cannot be said to have been tried. Nor let it be supposed that differences in the constitution of seedling kidney-beans never appear, for an account has been published how much more hardy some

seedlings are than others; and of this fact I have myself observed striking instances.

On the whole, we may conclude that habit, or use and disuse, have, in some cases, played a

considerable part in the modification of the constitution and structure; but that the effects have often been

largely combined with, andsometimes overmastered by, the natural selection of innate variations.

Correlated Variation.

I mean by this expression that the whole organisation is so tied together during its growth and

development, that when slight variations in any one part occur, and are accumulated through natural

selection, other parts becomemodified. This is a very important subject, most imperfectly understood,

and no doubt wholly different classes of facts may be here easily confounded together. We shall presently

see that simple inheritance often gives the false appearance of correlation. One of the most obvious real

cases is, that variations of structure arising in the young or larvæ naturally tend to affect the structure of

the mature animal. The several parts of the body which are homologous, and which, at an early

embryonic period, are identical in structure, andwhich arenecessarily exposed to similar conditions,

seem eminently liable to vary in a like manner: we see this in the right and left sides of the body varying in

the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the

lower jaw is believed by some anatomists to be homologous with the limbs. These tendencies, I do not

doubt, may be mastered more or less completely by natural selection; thus a family of stags once existed





with an antler only on one side; and if this had been of any great use to the breed, it might probably have

been rendered permanent by selection.

Homologous parts, as has been remarked by some authors, tend to cohere; this is often seen in

monstrousplants: and nothing ismore common than the union of homologous parts in normal structures,

as in the union of the petals into a tube. Hard parts seem to affect the form of adjoining soft parts; it is

believed by some authors that with birds the diversity in the shape of the pelvis causes the remarkablediversity in the shape of their kidneys. Others believe that the shape of the pelvis in the human mother

influences by pressure the shape of the head of the child. In snakes, according to Schlegel, the form of

the body and the manner of swallowing determine the position and form of several of the most important

viscera.

The nature of the bond is frequently quite obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked,

that certain malconformations frequently, and that others rarely, co-exist, without our being able to assign

any reason. What can be more singular than the relation in cats between complete whiteness and blue

eyes with deafness, or between the tortoise-shell colour and the female sex; or in pigeons between their

feathered feet and skin betwixt the outer toes, or between the presence of more or less down on theyoung pigeon when first hatched, with the future colour of its plumage; or, again, the relation between the

hair and teeth in the naked Turkish dog, though here no doubt homology comes into play? With respect

to this latter case of correlation, I think it can hardly be accidental, that the two orders of mammals which

are most abnormal in their dermal covering, viz., Cetacea (whales) and Edentata (armadilloes, scaly

ant-eaters, &c.,) are likewise on the whole the most abnormal in their teeth; but there are so many

exceptions to this rule, as Mr. Mivart has remarked, that it has little value.

I know of no case better adapted to show the importance of the laws of correlation and variation,

independently of utility and therefore of natural selection, than that of the difference between the outer

and inner flowers in someCompositous and Umbelliferous plants. Every one is familiar with the

difference between the ray and central florets of, for instance, the daisy, and this difference is oftenaccompanied with the partial or complete abortion of the reproductive organs. But in some of these

plants, the seeds also differ in shape and structure. These differences have sometimes been attributed to

the pressure of the involucra on the florets, or to their mutual pressure, and the shape of the seeds in the

ray-florets of some Compositæ countenances this idea; but with the Umbelliferæ, it is by no means, as

Dr. Hooker informs me, the species with the densest heads which most frequently differ in their inner and

outer flowers. It might have been thought that the development of the ray-petals by drawing nourishment

from the reproductive organs causes their abortion; but this can hardly be the sole cause, for in some

Compositæ the seeds of the outer and inner florets differ, without any difference in the corolla. Possibly

these several differences may be connected with the different flow of nutriment towards the central and

external flowers: we know, at least, that with irregular flowers, those nearest to the axis are most subjectto peloria, that is to become abnormally symmetrical. I may add, as an instance of this fact, and as a

striking case of correlation, that in many pelargoniums, the two upper petals in the central flower of the

truss often lose their patches of darker colour; and when this occurs, the adherent nectary is quite

aborted; the central flower thus becoming peloric or regular. When the colour is absent from only one of

the two upper petals, the nectary is not quite aborted but is much shortened.

With respect to the development of the corolla, Sprengel's idea that the ray-florets serve to attract

insects, whose agency is highly advantageous or necessary for the fertilisation of these plants, is highly

probable; and if so, natural selection may have come into play. But with respect to the seeds, it seems

impossible that their differences in shape, which are not always correlated with any difference in the

corolla, can be in any way beneficial: yet in the Umbelliferæ these differences are of such apparent

importance—the seeds being sometimes orthospermous in the exterior flowers and cælospermous in the

central flowers,—that the elder De Candolle founded his main divisions in the order on such characters.





Hence modifications of structure, viewed by systematists as of high value, may be wholly due to the laws

of variation and correlation, without being, as far as we can judge, of the slightest service to the species.

We may often falsely attribute to correlated variation structures which are common to whole groups of

species, and which in truth are simply due to inheritance; for an ancient progenitor may have acquired

throughnatural selection some one modification in structure, and, after thousands of generations, some

other and independent modification; and these two modifications, having been transmitted to a wholegroup of descendants with diverse habits, would naturally be thought to be in some necessary manner

correlated. Some other correlations are apparently due to the manner in which natural selection can alone

act. For instance, Alph. de Candolle has remarked that winged seeds are never found in fruits which do

not open; I should explain this rule by the impossibilityof seeds gradually becomingwinged through

natural selection, unless the capsules were open; for in this case alone could the seeds, which were a little

better adapted to be wafted by the wind, gain an advantage over others less well fitted for wide

dispersal.

Compensation and Economy of Growth.

The elder Geoffroy and Goethe propounded, at about the same time, their law of compensation or

balancement of growth; or, as Goethe expressed it, "in order to spend on one side, nature is forced to

economise on the other side." I think this holds true to a certain extent with our domestic productions: if

nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it

is difficult to get a cow to give much milk and to fatten readily. The same varieties of the cabbage do not

yield abundant and nutritious foliage and a copious supply of oil-bearing seeds. When the seeds in our

fruits become atrophied, the fruit itself gains largely in size and quality. In our poultry, a large tuft of

feathers on the head is generally accompanied by a diminished comb, and a large beard by diminished

wattles. With species in a state of nature it can hardly be maintained that the law is of universalapplication; but many good observers, more especially botanists, believe in its truth. I will not, however,

here give any instances, for I see hardly any way of distinguishing between the effects, on the one hand,

of a part being largely developed through natural selection and another and adjoining part being reduced

by this same process or by disuse, and, on the other hand, the actual withdrawal of nutriment from one

part owing to the excess of growth in another and adjoining part.

I suspect, also, that some of the cases of compensation which have been advanced, and likewise some

other facts, may be merged under a more general principle, namely, that natural selection is continually

trying to economise every part of the organisation. If under changed conditions of life a structure, before

useful, becomes less useful, its diminutionwill be favoured, for itwill profit the individual not to have itsnutriment wasted in building up an useless structure. I can thus only understand a fact with which I was

much struckwhen examining cirripedes, and ofwhich many analogous instancescould be given: namely,

that when a cirripede is parasitic within another cirripede and is thus protected, it loses more or less

completely its own shell or carapace. This is the case with the male Ibla, and in a truly extraordinary

manner with the Proteolepas: for the carapace in all other cirripedes consists of the three highly-important

anterior segments of the head enormously developed, and furnished with great nerves and muscles; but in

the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the merest

rudiment attached to the bases of the prehensile antennæ. Now the saving of a large and complex

structure, when rendered superfluous, would be a decided advantage to each successive individual of the

species; for in the struggle for life to which every animal is exposed, each would have a better chance of

supporting itself, by less nutriment being wasted.

Thus, as I believe, natural selection will tend in the long run to reduce any part of the organisation, as





soon as it becomes, through changed habits, superfluous, without by any means causing some other part

to be largely developed in a corresponding degree. And, conversely, that natural selection may perfectly

well succeed in largely developing an organwithout requiring as a necessary compensation the reduction

of some adjoining part.

Multiple, Rudimentary, and Lowly-organised Structures are Variable.

It seems to be a rule, as remarked by Is. Geoffry St. Hilaire, both with varieties and species, that when

any part or organ is repeated many times in the same individual (as the vertebræ in snakes, and the

stamens in polyandrous flowers) the number is variable; whereas the same part or organ, when it occurs

in lesser numbers, is constant. The same author as well as some botanists have further remarked that

multiple parts are extremely liable to vary in structure. As "vegetative repetition," to use Prof. Owen's

expression, is a sign of low organisation, the foregoing statements accord with the common opinion of

naturalists, that beings which stand low in the scale of nature are more variable than those which are

higher. I presume that lowness here means that the several parts of the organisation have been but littlespecialised for particular functions; and as long as the same part has to perform diversified work, we can

perhaps see why it should remain variable, that is, why natural selection should not have preserved or

rejected each little deviation of form so carefully as when the part has to serve for some one special

purpose. In the same way that a knife which has to cut all sorts of things may be of almost any shape;

whilst a tool for some particular purpose must be of some particular shape. Natural selection, it should

never be forgotten, can act solely through and for the advantage of each being.

Rudimentary parts, as it is generally admitted, are apt to be highly variable. We shall have to recur to this

subject; and I will here only add that their variability seems to result from their uselessness, and

consequently from natural selection havinghad no power to check deviations in their structure.

A Part developed in any Species in an extraordinary degree or manner, in comparison with the

same Part in allied Species, tends to be highly variable.

Several years ago I was much struck by a remark, to the above effect, made by Mr. Waterhouse.

Professor Owen, also, seems to have come to a nearly similar conclusion. It is hopeless to attempt to

convince any one of the truth of the above proposition without giving the long array of facts which I have

collected, and which cannot possibly be here introduced. I can only state my conviction that it is a rule of

high generality. I am aware of several causes of error, but I hope that I have made due allowance for them. It should be understood that the rule by no means applies to any part, however unusually

developed, unless it be unusually developed in one species or in a few species in comparison with the

same part in many closely allied species. Thus, the wing of a bat is a most abnormal structure in the class

of mammals, but the rule would not apply here, because the whole group of bats possesses wings; it

would apply only if some one species had wings developed in a remarkable manner in comparison with

the other species of the same genus. The rule applies very strongly in the case of secondary sexual

characters, when displayed in any unusual manner. The term, secondary sexual characters, used by

Hunter, relates to characters which are attached to one sex, but are not directly connected with the act of

reproduction. The rule applies to males and females; but more rarely to the females, as they seldom offer

remarkable secondary sexual characters. The rule being so plainly applicable in the case of secondary

sexual characters, may be due to the great variability of these characters, whether or not displayed in any

unusual manner—of which fact I think there can be little doubt. But that our rule is not confined to

secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes; I particularly









species branched off from a common progenitor, it is probable that they should still often be in some

degree variable,—at least more variable than those parts of the organisation which have for a very long

period remained constant.

Secondary Sexual Characters Variable.— I think itwill be admitted by naturalists, without myentering

on details, that secondary sexual characters are highly variable. It will also be admitted that species of the

same group differ from each other more widely in their secondary sexual characters, than in other parts of their organisation: compare, for instance, the amount of difference between the males of gallinaceous

birds, in which secondary sexual characters are strongly displayed,with theamount of difference

between the females. The cause of the original variability of these characters is not manifest; but we can

see why they should not have been rendered as constant and uniform as others, for they are accumulated

by sexual selection, which is less rigid in its action than ordinary selection, as it does not entail death, but

only gives fewer offspring to the less favoured males. Whatever the cause may be of the variability of

secondary sexual characters, as they are highly variable, sexual selection will have had a wide scope for

action, and may thus have succeeded in giving to the species of the same group a greater amount of

difference in these than in other respects.

It is a remarkable fact, that the secondary differences between the two sexes of the same species are

generally displayed in the very same parts of the organisation in which the species of the same genus

differ from each other. Of this fact I will give in illustration the two first instances which happen to stand

on my list; and as the differences in these cases are of a very unusual nature, the relation can hardly be

accidental. The same number of joints in the tarsi is a character common to very large groups of beetles,

but in the Engidæ, as Westwood has remarked, the number varies greatly; and the number likewise

differs in the two sexes of the same species. Again in the fossorial hymenoptera, the neuration of the

wings is a character of the highest importance, because common to large groups; but in certain genera the

neuration differs in the different species, and likewise in the two sexes of the same species. Sir J.

Lubbockhas recently remarked, that several minute crustaceansoffer excellent illustrations of this law.

"In Pontella, for instance, the sexual characters are afforded mainly by the anterior antennæ and by thefifth pair of legs: the specific differences also are principally given by these organs." This relation has a

clear meaning on my view: I look at all the species of the same genus as having as certainly descended

from a common progenitor, as have the two sexes of any one species. Consequently, whatever part of

the structure of the common progenitor, or of its early descendants, became variable, variations of this

part would, it is highly probable, be taken advantage of by natural and sexual selection, in order to fit the

several species to their several places in the economy of nature, and likewise to fit the two sexes of the

same species to each other, or to fit the males to struggle with other males for the possession of the

females.

Finally, then, I conclude that the greater variability of specific characters, or those whichdistinguish

species from species, than of generic characters, or those which are possessed by all the species;—that

the frequent extreme variability of any part which is developed in a species in an extraordinary manner in

comparison with the same part in its congeners; and the slight degree of variability in a part, however

extraordinarily it may be developed, if it be common to a whole group of species;—that the great

variability of secondary sexual characters, and their great difference in closely allied species;—that

secondary sexual and ordinary specific differences are generally displayed in the same parts of the

organisation,—are all principles closely connected together.All beingmainlydue to the species of the

same group being the descendants of a common progenitor, from whom they have inherited much in

common,—to parts which have recently and largely varied being more likely still to go on varying than

parts which have long been inherited and have not varied—to natural selection having more or less

completely, according to the lapse of time, overmastered the tendency to reversion and to further

variability,—to sexual selection being less rigid than ordinary selection,—and to variations in the same





parts having been accumulated by natural and sexual selection, and having been thus adapted for

secondary sexual, and for ordinary purposes.

Distinct Species present analogous Variations, so that a Variety of one Species often assumes a

Character proper to an allied Species, or reverts to some of the Characters of an early

Progenitor.— These propositions will be most readily understood by looking to our domestic races. The

most distinct breeds of the pigeon, in countries widely apart, present sub-varieties with reversed featherson the head, and with feathers on the feet,—characters not possessed by the aboriginal rock-pigeon;

these then are analogous variations in two or more distinct races. The frequent presence of fourteen or

even sixteen tail-feathers in the pouter may be considered as a variation representing the normal structure

of another race, the fantail. I presume that no one will doubt that all such analogous variations are due to

the several races of the pigeon having inherited from a common parent the same constitution and

tendency to variation, when acted on by similar unknown influences. In the vegetable kingdom we have a

case of analogous variation, in the enlarged stems, or as commonly called roots, of the Swedish turnip

and Ruta baga, plants which several botanists rank as varieties produced by cultivation from a common

parent: if this be not so, the case will then be one of analogous variation in two so-called distinct species;

and to these a third may be added, namely, the common turnip. According to the ordinary view of eachspecies havingbeen independently created,we should have to attribute this similarity in the enlarged

stems of these three plants, not to thevera causa of community of descent, and a consequent tendency to

vary in a like manner, but to three separate yet closely related acts of creation. Many similar cases of

analogous variation have been observed by Naudin in the great gourd-family, and by various authors in

our cereals. Similar cases occurringwith insects undernatural conditions have latelybeen discussed with

much ability by Mr. Walsh, who has grouped them under his law of Equable Variability.

With pigeons, however, we have another case, namely, the occasional appearance in all the breeds, of

slaty-blue birds with two black bars on the wings, white loins, a bar at the end of the tail, with the outer

feathers externally edged near their basis with white. As all these marks are characteristic of the parent

rock-pigeon, I presume that no one will doubt that this is a case of reversion, and not of a new yetanalogous variation appearing in the several breeds. We may, I think, confidently come to this conclusion,

because, as we have seen, these coloured marks are eminently liable to appear in the crossed offspring of

two distinct and differently coloured breeds; and in this case there is nothing in the external conditions of

life to cause the reappearance of the slaty-blue, with the several marks, beyond the influence of the mere

act of crossing on the laws of inheritance.

No doubt it is a very surprising fact that characters should reappear after having been lost for many,

probably for hundreds of generations. But when a breed has been crossed only once by some other

breed, the offspring occasionally show for many generations a tendency to revert in character to the

foreign breed—some say, for a dozen or even a score of generations. After twelve generations, the proportion of blood, to use a common expression, from one ancestor, is only 1 in 2048; and yet, as we

see, it is generally believed that a tendency to reversion is retained by this remnant of foreign blood. In a

breed which has not been crossed, but in whichboth parents have lost some character which their

progenitor possessed, the tendency, whether strong or weak, to reproduce the lost character might, as

was formerly remarked, for all that we can see to the contrary, be transmitted for almost any number of

generations. When a character which has been lost in a breed, reappears after a great number of

generations, the most probable hypothesis is, not that one individual suddenly takes after an ancestor

removed by some hundred generations, but that in each successive generation the character in question

has been lying latent, and at last, under unknown favourable conditions, is developed. With the

barb-pigeon, for instance, which very rarely produces a blue bird, it is probable that there is a latent

tendency in each generation to produce blue plumage. The abstract improbability of such a tendency

being transmitted through a vast number of generations, is not greater than that of quite useless or

rudimentary organs being similarly transmitted. A mere tendency to produce a rudiment is indeed





sometimes thus inherited.

As all the species of the same genus are supposed to be descended from a common progenitor, it might

be expected that they would occasionally vary in an analogous manner; so that the varieties of two or

more species would resemble each other, or that a variety of one species would resemble in certain

characters another and distinct species,—this other species being, according to our view, only a

well-marked andpermanent variety.But characters exclusively due to analogous variation would probably be of an unimportant nature, for the preservation of all functionally important characterswill

have been determined through natural selection, in accordance with the different habits of the species. It

might further be expected that the species of the same genus would occasionally exhibit reversions to long

lost characters. As, however, we do not know the common ancestor of any natural group, we cannot

distinguish between reversionary and analogous characters. If, for instance, we did not know that the

parent rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether such

characters in our domestic breeds were reversions or only analogous variations; but we might have

inferred that the blue colour was a case of reversion from the number of the markings, which are

correlated with this tint, and which would not probably have all appeared together from simple variation.

More especially we might have inferred this, from the blue colour and the several marks so oftenappearingwhen differently coloured breeds are crossed.Hence, although undernature it must generally

be left doubtful, what cases are reversions to formerly existing characters, and what are new but

analogous variations, yet we ought, on our theory, sometimes to find the varying offspring of a species

assuming characters which are already present in other members of the same group. And this

undoubtedly is the case.

The difficulty in distinguishingvariable species is largely due to the varietiesmocking, as itwere, other

species of the same genus. A considerable catalogue, also, could be given of forms intermediate between

two other forms, which themselves can only doubtfully be ranked as species; and this shows, unless all

these closely allied forms be considered as independently created species, that they have in varying

assumed some of the characters of the others. But the best evidence of analogous variations is afforded by parts or organs which are generally constant in character, but which occasionally vary so as to

resemble, in some degree, the same part or organ in an allied species. I have collected a long list of such

cases; but here, as before, I lie under the great disadvantage of not being able to give them. I can only

repeat that such cases certainly occur, and seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as affecting any important character, but

from occurring in several species of the same genus, partly under domestication and partly under nature.

It is a case almost certainly of reversion. The ass sometimes has very distinct transverse bars on its legs,

like those on the legs of the zebra: it has been asserted that these are plainest in the foal, and, from

inquiries which I have made, I believe this to be true. The stripe on the shoulder is sometimes double, andis very variable in length and outline. A white ass, butnot an albino, has been described without either

spinal or shoulder stripe: and these stripes are sometimes very obscure, or actually quite lost, in

dark-coloured asses. The koulan of Pallas is said to have been seen with a double shoulder-stripe. Mr.

Blyth has seen a specimen of the hemionus with a distinct shoulder-stripe, though it properly has none;

and I have been informed by Colonel Poole that the foals of this species are generally striped on the legs,

and faintly on the shoulder. The quagga, though so plainly barred like a zebra over the body, is without

bars on the legs; but Dr. Gray has figured one specimen with very distinct zebra-like bars on the hocks.

With respect to the horse, I have collected cases in England of the spinal stripe in horses of the most

distinct breeds, and of all colours: transverse bars on the legs are not rare in duns, mouse-duns, and in

one instance in a chestnut: a faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace

in a bay horse. My son made a careful examination and sketch for me of a dun Belgian cart-horse with a

double stripe on each shoulder and with leg-stripes; I have myself seen a dun Devonshire pony, and a









found—that is,where there has been much former variationanddifferentiation, or where themanufactory

of new specific forms has been actively at work—in that district and amongst these species, we now find,

on an average, most varieties. Secondary sexual characters are highly variable, and such characters differ

much in the species of the same group. Variability in the same parts of the organisation has generally been

taken advantage of in giving secondary sexual differences to the two sexes of the same species, and

specific differences to the several species of the same genus. Any part or organ developed to an

extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the alliedspecies, must have gone through an extraordinary amount of modification since the genus arose; and thus

we can understand why it should often still be variable in a much higher degree than other parts; for

variation is a long-continued and slow process, and natural selection will in such cases not as yet have

had time to overcome the tendency to further variability and to reversion to a less modified state. But

when a species with any extraordinarily-developed organ has become the parent of many modified

descendants—which on our view must be a very slow process, requiring a long lapse of time—in this

case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a

manner it mayhave been developed. Species inheriting nearly the same constitution from a common

parent, and exposed to similar influences, naturally tend to present analogous variations, or these same

species may occasionally revert to some of the characters of their ancient progenitors. Although new andimportant modificationsmay not arise from reversion and analogous variation, suchmodificationswill add

to the beautiful and harmonious diversityof nature.

Whatever the cause may be of each slight difference between the offspring and their parents—and a

cause for each must exist—we have reason to believe that it is the steady accumulation of beneficial

differences which has given rise to all the more important modifications of structure in relation to the

habits of each species.

|Go to Contents |

Chapter VI

Difficulties of the Theory.

Difficulties of the theory of descent withmodification—Absence or rarity of transitional

varieties—Transitions in habits of life—Diversifiedhabits in the same species—Species with habits

widely different from those of their allies—Organs of extreme perfection—Modes of transition—Cases

of difficulty—Natura non facit saltum—Organs of small importance—Organs not in all cases absolutely

perfect—The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural

Selection.

LONG before the reader has arrived at this part of my work, a crowd of difficulties will have occurred

to him. Some of them are so serious that to this day I can hardly reflect on them without being in some







confounded me. But I think it can be in large part explained.

In the first place we should be extremely cautious in inferring, because an area is now continuous, that it

has been continuous during a long period. Geology would lead us to believe that most continents have

been broken up into islands even during the later tertiary periods; and in such islands distinct species

might havebeen separately formedwithout the possibility of intermediate varieties existing in the

intermediate zones. By changes in the form of the land and of climate, marine areas now continuous mustoften have existed within recent times in a far less continuous and uniform condition than at present. But I

will pass over this way of escaping from the difficulty; for I believe that many perfectly defined species

have been formed on strictly continuous areas; though I do not doubt that the formerly broken condition

of areas now continuous, has played an important part in the formation of new species, more especially

with freely-crossing andwandering animals.

In looking at species as they are now distributed over a wide area, we generally find them tolerably

numerous over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, and

finally disappearing. Hence the neutral territory, between two representative species is generallynarrow

in comparison with the territory proper to each. We see the same fact in ascending mountains, andsometimes it is quite remarkable how abruptly, as Alph. de Candolle has observed, a common alpine

species disappears. The same fact has been noticed by E. Forbes in sounding the depths of the sea with

the dredge. To those who look at climate and the physical conditions of life as the all-important elements

of distribution, these facts ought to cause surprise, as climate and height or depth graduate away

insensibly. But when we bear in mind that almost every species, even in its metropolis, would increase

immensely in numbers, were it not for other competing species; that nearly all either prey on or serve as

prey for others; in short, that each organic being is either directly or indirectly related in the most

important manner to other organic beings,—we see that the range of the inhabitants of any country by no

meansexclusively depends on insensibly changing physical conditions, but in a large part on the presence

of other species, on which it lives, or by which it is destroyed, or with which it comes into competition;

and as these species are already defined objects, not blending one into another by insensible gradations,the range of any one species, depending as it does on the range of others, will tend to be sharply defined.

Moreover, each species on the confines of its range, where it exists in lessened numbers, will, during

fluctuations in the number of its enemies or of its prey, or in the nature of the seasons, be extremely liable

to utter extermination; and thus its geographical range will come to be still more sharply defined.

As allied or representative species, when inhabiting a continuous area, are generally distributed in such a

manner that each has a wide range, with a comparatively narrow neutral territory between them, in which

they become rather suddenly rarer and rarer; then, as varieties do not essentially differ from species, the

same rule will probably apply to both; and if we take a varying species inhabiting a very large area, we

shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone.The intermediatevariety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser

area; and practically, as far as I can make out, this rule holds good with varieties in a state of nature. I

have met with striking instances of the rule in the case of varieties intermediate between well-marked

varieties in the genus Balanus. And it would appear from information given me by Mr. Watson, Dr. Asa

Gray, and Mr. Wollaston, that generally, when varieties intermediate between two other forms occur,

they are much rarer numerically than the forms which they connect. Now, if we may trust these facts and

inferences, and conclude that varieties linking two other varieties together generally have existed in lesser

numbers than the formswhich they connect, then we canunderstand why intermediate varieties should

not endure for very long periods:—why, as a general rule, they should be exterminated and disappear,

sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run a greater chance of being

exterminated than one existing in large numbers; and in this particular case the intermediate formwould





be eminently liable to the inroads of closely-allied forms existing on both sides of it. But it is a far more

important consideration, that during theprocess of further modification, bywhich twovarieties are

supposed to be converted and perfected into two distinct species, the two which exist in larger numbers,

from inhabiting larger areas, will have a great advantage over the intermediate variety, which exists in

smaller numbers in a narrow and intermediate zone. For forms existing in larger numbers will have a

better chance, within any givenperiod, of presenting further favourable variations fornatural selection to

seize on, than will the rarer forms which exist in lesser numbers. Hence, the more common forms, in therace for life, will tend to beat and supplant the less common forms, for these will be more slowly modified

and improved. It is the same principle which, as I believe, accounts for the common species in each

country, as shown in the second chapter, presenting on an average a greater number of well-marked

varieties than do the rarer species. I may illustrate what I mean by supposing three varieties of sheep to

be kept, one adapted to an extensive mountainous region; a second to a comparatively narrow, hilly

tract; and a third to the wide plains at the base; and that the inhabitants are all trying with equal steadiness

and skill to improve their stocks by selection; the chances in this case will be strongly in favour of the

great holders on the mountains or on the plains, improving their breeds more quickly than the small

holders on the intermediate narrow, hilly tract; and consequently the improvedmountain or plain breed

will soon take the place of the less improved hill breed; and thus the two breeds, which originally existedin greater numbers, will come into close contact with each other, without the interposition of the

supplanted, intermediate hill variety.

To sum up, I believe that species come to be tolerably well-defined objects, and do not at any one

periodpresent an inextricable chaos of varying and intermediate links: first, because newvarieties are

very slowly formed, for variation is a slow process, and natural selection can do nothing until favourable

individual differences or variations occur, and until a place in the natural polity of the country can be

better filled by some modification of some one or more of its inhabitants. And such new places will

depend on slow changes of climate, or on the occasional immigration of new inhabitants, and, probably,

in a still more important degree, on some of the old inhabitants becoming slowly modified, with the new

forms thus produced, and the old ones acting and reacting on each other. So that, in any one region andat any one time, we ought to see only a few species presenting slight modifications of structure in some

degree permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within the recent period as isolated portions, in

which many forms, more especially amongst the classes which unite for each birth and wander much, may

have separately been rendered sufficiently distinct to rank as representative species. In this case,

intermediate varieties between the several representative species and their common parent, must formerly

have existed within each isolated portion of the land, but these links during the process of natural

selection will have been supplanted and exterminated, so that they will no longer be found in a living state.

Thirdly, when two or more varieties have been formed in different portions of a strictly continuous area,

intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will

generallyhave had a short duration. For these intermediate varietieswill, from reasons already assigned

(namely from what we know of the actual distribution of closely allied or representative species, and

likewise of acknowledged varieties), exist in the intermediate zones in lesser numbers than thevarieties

which they tend to connect. From this cause alone the intermediate varieties will be liable to accidental

extermination; and during the process of further modification through natural selection, theywill almost

certainly be beaten and supplanted by the forms which they connect; for these from existing in greater

numberswill, in the aggregate, present more varieties, and thus be further improved through natural

selection and gain further advantages.

Lastly, looking not to any one time, but to all time, if my theory be true, numberless intermediate

varieties, linking closely together all the species of the same group, must assuredly have existed; but the







see traces of an apparatus originally fitted for gliding through the air rather than for flight.

If about a dozen genera of birds were to become extinct, who would have ventured to surmise that birds

might have existed which used their wings solely as flappers, like the logger-headed duck (Micropterus of

Eyton); as fins in the water and as front-legs on the land, like the penguin; as sails, like the ostrich; and

functionally for no purpose, like the Apteryx? Yet the structure of each of these birds is good for it, under

the conditions of life to which it is exposed, for each has to live by a struggle; but it is not necessarily the best possible under all possible conditions. It must not be inferred from these remarks that any of the

grades of wing-structure here alluded to, which perhaps may all be the result of disuse, indicate the steps

by which birds actually acquired their perfect power of flight; but they serve to show what diversified

means of transition are at least possible.

Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are adapted

to live on the land; and seeing that we have flying birds and mammals, flying insects of the most diversified

types, and formerly had flying reptiles, it is conceivable that flying-fish,whichnow glide far through the

air, slightly risingand turning by the aid of their fluttering fins, might havebeenmodified intoperfectly

winged animals. If this had been effected, who would have ever imagined that in an early transitional statethey had been the inhabitants of the open ocean, and had used their incipient organs of flight exclusively,

as far as we know, to escape being devoured by other fish?

When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we

should bear inmind that animals displaying early transitional grades of the structure will seldomhave

survived to the present day, for they will have been supplanted by their successors, which were gradually

rendered more perfect through natural selection. Furthermore, we mayconclude that transitional states

between structures fitted for very different habits of life will rarely have been developed at an early period

in great numbers and under many subordinate forms. Thus, to return to our imaginary illustration of the

flying-fish, it does not seem probable that fishes capable of true flight would have been developed under

many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water, untiltheir organs of flight had come to a high stage of perfection, so as to have given them a decided

advantage over other animals in the battle for life. Hence the chance of discovering species with

transitional grades of structure in a fossil conditionwill always be less, from their having existed in lesser

numbers, than in the case of species with fully developed structures.

I will now give two or three instances both of diversified and of changed habits in the individuals of the

same species. In either case it would be easy for natural selection to adapt the structure of the animal to

its changed habits, or exclusively to one of its several habits. It is, however, difficult to decide, and

immaterial for us, whether habits generally change first and structure afterwards; orwhether slight

modifications of structure lead to changedhabits; both probably oftenoccurring almost simultaneously.Of cases of changed habits it will suffice merely to allude to that of the many British insects which now

feed on exotic plants, or exclusively on artificial substances.Of diversified habits innumerable instances

could be given: I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,

hovering over one spot and then proceeding to another, like a kestrel, and at other times standing

stationary on the margin of water, and then dashing into it like a kingfisher at a fish. In our own country

the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper; it sometimes, like

a shrike, kills small birds by blows on the head; and I have many times seen and heard it hammering the

seeds of the yew on a branch, and thus breaking them like a nuthatch. In North America the black bear

was seen by Hearne swimming for hours with widely open mouth, thus catching, almost like a whale,

insects in the water.

As we sometimes see individuals following habits different from those proper to their species and to the

other species of the same genus, we might expect that such individuals would occasionally give rise to





new species, having anomalous habits, and with their structure either slightly or considerably modified

from that of their type. And such instances occur in nature. Can a more striking instance of adaptation be

given than that of a woodpecker for climbing trees and seizing insects in the chinks of the bark? Yet in

North America there are woodpeckers which feed largely on fruit, and others with elongated wings

which chase insects on the wing. On the plains of La Plata, where hardly a tree grows, there is a

woodpecker (Colaptes campestris) which has two toes before and two behind, a long pointed tongue,

pointed tail-feathers, sufficiently stiff to support the bird in a vertical position on a post, but not so stiff asin the typical woodpeckers, and a straight strong beak. The beak, however, is not so straight or so strong

as in the typical woodpeckers, but it is strong enough to bore into wood. Hence this Colaptes in all the

essential parts of its structure is a woodpecker. Even in such trifling characters as the colouring, the harsh

tone of the voice, and undulatory flight, its close blood-relationship to our common woodpecker is plainly

declared; yet, as I can assert, not only from my own observations, but from those of the accurate Azara,

in certain large districts it does not climb trees, and it makes its nest in holes in banks! In certain other

districts, however, this same woodpecker, as Mr. Hudson states, frequents trees, and bores holes in the

trunk for its nest. I may mention as another illustration of the varied habits of this genus, that a Mexican

Colaptes has been described by De Saussure as boring holes into hard wood in order to lay up a store of

acorns.

Petrels are the most aërial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the

Puffinuria berardi, in its general habits, in its astonishing power of diving, in its manner of swimmingand of

flying when made to take flight, would be mistaken by any one for an auk or a grebe; nevertheless it is

essentially a petrel, but with many parts of its organisation profoundly modified in relation to its new

habits of life; whereas the woodpecker of La Plata has had its structure only slightly modified. In the case

of the water-ouzel, the acutest observer by examining its dead body would never have suspected its

sub-aquatichabits; yet this bird, which is allied to the thrush family, subsists by diving—using itswings

under water, and grasping stones with its feet. All the members of the great order of Hymenopterous

insects are terrestrial, excepting the genus Proctotrupes, which Sir John Lubbock has discovered to be

aquatic in its habits; it often enters the water and dives about by the use not of its legs but of its wings,and remains as long as four hours beneath the surface; yet it exhibits no modification in structure in

accordance with its abnormal habits.

He who believes that each being has been created as we now see it, must occasionally have felt surprise

when he has met with an animal having habits and structure not in agreement. What can be plainer than

that the webbed feet of ducks and geese are formed for swimming? Yet there are upland geese with

webbed feet which rarely go near the water; and no one except Audubon has seen the frigate-bird, which

has all its four toes webbed, alight on the surface of the ocean. On the other hand, grebes and coots are

eminently aquatic, although their toes are only bordered by membrane. What seems plainer than that the

long toes, not furnished with membrane of the Grallatores are formed for walking over swamps andfloating plants?—the water-hen and landrail are members of this order, yet the first is nearly as aquatic as

the coot, and the second nearly as terrestrial as the quail or partridge. In such cases, and many others

could be given, habits have changed without a corresponding change of structure. The webbed feet of the

upland goose may be said to have become almost rudimentary in function, though not in structure. In the

frigate-bird, the deeply scooped membrane between the toes shows that structure has begun to change.

He who believes in separate and innumerable acts of creation may say, that in these cases it has pleased

the Creator to cause a being of one type to take the place of one belonging to another type; but this

seems to me only re-stating the fact in dignified language. He who believes in the struggle for existence

and in the principle of natural selection, will acknowledge that everyorganic being is constantly

endeavouring to increase in numbers; and that if any one being varies ever so little, either in habits or

structure, and thus gains an advantage over some other inhabitant of the same country, it will seize on the

place of that inhabitant, however different that may be from its own place. Hence it will cause him no







main class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and

graduated range of structure in the eyes of the lower animals; and when we bear in mind how small the

number of all living forms must be in comparisonwith those which havebecomeextinct, the difficulty

ceases to be very great in believing that natural selection may have converted the simple apparatus of an

optic nerve, coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the Articulate Class.

He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume

that large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through

natural selection; he ought to admit that a structure even as perfect as an eagle's eye might thus be

formed, although in this case he does not know the transitional states. It has been objected that in order

to modify the eye and still preserve it as a perfect instrument, many changes would have to be effected

simultaneously, which, it is assumed, could not be done through natural selection; but as I have attempted

to show in my work on the variation of domestic animals, it is not necessary to suppose that the

modificationswere all simultaneous, if theywere extremely slight and gradual.Different kinds of modification would, also, serve for the same general purpose: as Mr. Wallace has remarked, "if a lens

has too short or too long a focus, it may be amended either by an alteration of curvature, or an alteration

of density; if the curvature be irregular, and the rays do not converge to a point, then any increased

regularity of curvature will be an improvement. So the contraction of the iris and the muscular movements

of the eye are neither of them essential to vision, but only improvements which might have been added

and perfected at any stage of the construction of the instrument." Within the highest division of the animal

kingdom, namely, the Vertebrata, we can start from an eye so simple, that it consists, as in the lancelet, of

a little sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other

apparatus. In fishes and reptiles, as Owen has remarked, "the range of gradations of dioptric structures is

very great." It is a significant fact that even in man, according to the high authority of Virchow, the

beautiful crystalline lens is formed in the embryobyan accumulation of epidermic cells, lying in asack-like fold of the skin; and the vitreous body is formed from embryonic sub-cutaneous tissue. To

arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not

absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but I

have felt the difficulty far too keenly to be surprised at others hesitating to extend the principle of natural

selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has

been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the

eye has been formed by a somewhat analogous process. But may not this inference be presumptuous?

Have we any right to assume that the Creator works by intellectual powers like those of man? If we mustcompare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent

tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every

part of this layer to be continually changing slowly in density, so as to separate into layers of different

densities and thicknesses, placed at different distances from each other, and with the surfaces of each

layer slowly changing in form. Further we must suppose that there is a power, represented by natural

selection or the survival of the fittest, always intently watching each slight alteration in the transparent

layers; and carefully preserving each which, under varied circumstances, in any way or in any degree,

tends to produce a distincter image. We must suppose each new state of the instrument to be multiplied

by the million; each to be preserved until a better one is produced, and then the old ones to be all

destroyed. In livingbodies, variation will cause the slight alterations, generationwillmultiply themalmost

infinitely, and natural selectionwill pick out with unerring skill each improvement. Let this process go on

for millions of years; and during each year on millions of individuals of many kinds; and may we not

believe that a living optical instrument might thus be formed as superior to one of glass, as the works of





the Creator are to those of man?

Modes of Transition.

If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slightmodifications,my theory would absolutely breakdown. But I can find out

no such case. No doubt many organs exist of which we do not know the transitional grades, more

especially if we look to much-isolated species, round which, according to the theory, there has been

much extinction. Or again, if we take an organ common to all the members of a class, for in this latter

case the organ must have been originally formed at a remote period, since which all the many members of

the class have been developed; and in order to discover the early transitional grades through which the

organ has passed, we should have to look to very ancient ancestral forms, long since become extinct.

We should be extremely cautious in concluding that an organ could not have been formed by transitional

gradations of some kind. Numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions; thus in the larva of the dragon-fly and in the fish

Cobites the alimentary canal respires, digests, and excretes. In the Hydra, the animal may be turned

inside out, and the exterior surface will then digest and the stomach respire. In such cases natural

selection might specialise, if any advantage were thus gained, the whole or part of an organ, which had

previouslyperformed two functions, for one function alone, and thus by insensible steps greatly change its

nature. Many plants are known which regularlyproduce at the same time differently constructed flowers;

and if such plants were to produce one kind alone, a great change would be effected with comparative

suddenness in the character of the species. It is, however, probable that the two sorts of flowers borne

by the sameplant were originally differentiatedby finely graduated steps,which may still be followed in

some few cases.

Again, two distinct organs, or the same organ under two very different forms, may simultaneously

perform in the same individual the same function, and this is an extremely important means of transition: to

give one instance,—there are fish with gills or branchiæ that breathe the air dissolved in the water, at the

same time that they breathe free air in their swimbladders, this latter organ being divided by highly

vascular partitions and having a ductus pneumaticus for the supply of air. To give another instance from

the vegetable kingdom: plants climb by three distinct means, by spirally twining, by clasping a support

with their sensitive tendrils, and by the emission of aërial rootlets; these three means are usually found in

distinct groups, but some few species exhibit two of the means, or even all three, combined in the same

individual. In all such cases one of the two organs might readily be modified and perfected so as to

perform all the work, being aided during the progress of modification by the other organ; and then thisother organ might be modified for some other and quite distinct purpose, or be wholly obliterated.

The illustration of the swimbladder in fishes is a good one, because it shows us clearly the highly

important fact that an organ originally constructed for one purpose, namely, flotation, maybe converted

into one for a widely different purpose, namely, respiration. The swimbladder has, also, been worked in

as an accessory to the auditory organs of certain fishes. All physiologists admit that the swimbladder is

homologous, or "ideally similar" in position and structure with the lungs of the higher vertebrate animals:

hence there is no reason to doubt that the swimbladder has actually been converted into lungs, or an

organ used exclusively for respiration.

According to this view it may be inferred that all vertebrate animals with true lungs are descended by

ordinary generation froman ancient and unknown prototype,whichwas furnished with a floating

apparatus or swimbladder. We can thus, as I infer from Owen's interesting description of these parts,







Although we must be extremely cautious in concluding that any organ could not have been produced by

successive, small, transitional gradations, yet undoubtedly serious cases of difficultyoccur.

One of the most serious is that of neuter insects, which are often differently constructed from either the

males or fertile females; but this case will be treated of in the next chapter. The electric organs of fishes

offer another case of special difficulty; for it is impossible to conceive by what steps these wondrous

organs have been produced. But this is not surprising, for we do not even know of what use they are. Inthe Gymnotus and Torpedo they no doubt serve as powerful means of defence, and perhaps for securing

prey; yet in the Ray, as observed by Matteucci, an analogous organ in the tail manifests but little

electricity, even when the animal is greatly irritated; so little, that it can hardly be of any use for the above

purposes. Moreover, in the Ray, besides the organ just referred to, there is, as Dr. R. M'Donnell has

shown, another organ near the head, not known to be electrical, but which appears to be the real

homologue of the electric battery in the Torpedo. It is generally admitted that there exists between these

organs and ordinary muscle a close analogy, in intimate structure, in the distribution of the nerves, and in

the manner in which they are acted on by various reagents. It should, also, be especially observed that

muscular contraction is accompaniedby an electrical discharge; and, asDr.Radcliffe insists, "in the

electrical apparatus of the torpedo during rest, there would seem to be a charge in every respect like thatwhich is met with in muscle and nerve during rest, and the discharge of the torpedo, instead of being

peculiar, may be only another form of the discharge which depends upon the action of muscle and motor

nerve." Beyond this we cannot at present go in the way of explanation; but as we know so little about the

uses of these organs, and as we know nothing about the habits and structure of the progenitors of the

existing electric fishes, it would be extremely bold tomaintain that no serviceable transitions are possible

bywhich these organs might have been gradually developed.

These organs appear at first to offer another and far more serious difficulty; for they occur in about a

dozen kinds of fish, of which several are widely remote in their affinities. When the same organ is found in

several members of the same class, especially if in members having very different habits of life, we may

generally attribute its presence to inheritance from a common ancestor; and its absence in some of themembers to loss through disuse or natural selection. So that, if the electric organs had been inherited from

some one ancient progenitor, we might have expected that all electric fishes would have been specially

related to each other; but this is far from the case. Nor does geology at all lead to the belief that most

fishes formerly possessed electric organs, which their modifieddescendants have now lost.But when we

look at the subject more closely, we find in the several fishes provided with electric organs, that these are

situated in different parts of the body,—that they differ in construction, as in the arrangement of the

plates, and, according to Pacini, in the process or means by which the electricity is excited—and lastly, in

being supplied with nerves proceeding from different sources, and this is perhaps the most important of

all the differences. Hence in the several fishes furnished with electric organs, these cannot be considered

as homologous, but only as analogous in function. Consequently there is no reason to suppose that theyhave been inherited from a common progenitor; for had this been the case they would have closely

resembled each other in all respects. Thus the difficulty of an organ, apparently the same, arising in

several remotely allied species, disappears, leaving only the lesser yet still great difficulty; namely, bywhat

graduated steps these organs have been developed in each separate group of fishes.

The luminous organs which occur in a few insects, belonging to widely different families, and which are

situated in different parts of the body, offer, under our present state of ignorance, a difficulty almost

exactly parallel with that of the electric organs. Other similar cases could be given; for instance in plants,

the very curious contrivance of a mass of pollen-grains, borne on a foot-stalk with an adhesive gland, is

apparently the same in Orchis and Asclepias,—genera almost as remote as is possible amongst flowering

plants; but here again the parts are not homologous. In all cases of beings, far removed from each other

in the scale of organisation, which are furnished with similar and peculiar organs, it will be found that

although the general appearance and function of the organs may be the same, yet fundamental differences







Another distinguished zoologist, the late Professor Claparède, has argued in the same manner, and has

arrived at the same result. He shows that there are parasitic mites (Acaridæ), belonging to distinct

subfamilies and families, which are furnished with hair-claspers. These organsmust havebeen

independently developed, as they could not have been inherited from a common progenitor; and in the

several groups they are formed by the modification of the fore-legs,—of the hind-legs,—of the maxillæ or

lips,—and of appendages on the under side of the hind part of the body.

In the foregoing cases, we see the same end gained and the same function performed, in beings not at all

or only remotely allied, by organs in appearance, though not in development, closely similar. On the other

hand, it is a common rule throughout nature that the same end should be gained, even sometimes in the

case of closely-related beings, by themost diversified means. How differently constructed is the

feathered wing of a bird and the membrane-covered wing of a bat; and still more so the four wings of a

butterfly, the two wings of a fly, and the two wings with the elytra of a beetle. Bivalve shells are made to

open and shut, but on what a number of patterns is the hinge constructed,—from the long row of neatly

interlocking teeth in a Nucula to the simple ligament of a Mussel! Seeds are disseminated by their minuteness,—by their capsule being converted into a light balloon-like envelope,—by being embedded in

pulp or flesh, formed of the most diverse parts, and rendered nutritious, as well as conspicuously

coloured, so as to attract and be devoured by birds,—by having hooks and grapnels of many kinds and

serrated awns, so as to adhere to the fur of quadrupeds,—and by being furnished with wings and plumes,

as different in shape as they are elegant in structure, so as to be wafted by every breeze. I will give one

other instance; for this subject of the same end being gained by the most diversified means well deserves

attention. Some authors maintain that organic beings have been formed in many ways for the sake of

mere variety, almost like toys in a shop, but such a view of nature is incredible.With plants having

separated sexes, and with those in which, though hermaphrodites, the pollen does not spontaneously fall

on the stigma, some aid is necessary for their fertilisation. With several kinds this is effected by the

pollen-grains, which are light and incoherent, being blown by the wind through mere chance on to thestigma; and this is the simplest plan which can well be conceived. An almost equally simple, though very

different, plan occurs in many plants in which a symmetrical flower secretes a few drops of nectar, and is

consequently visited by insects; and these carry the pollen from the anthers to the stigma.

From this simple stage we may pass through an inexhaustible number of contrivances, all for the same

purpose and effected in essentially the same manner, but entailing changes in every part of the flower. The

nectar may be stored in variously shaped receptacles, with the stamens and pistils modified in many

ways, sometimes forming trap-like contrivances, and sometimes capable of neatly adapted movements

through irritability or elasticity. From such structures we may advance till we come to such a case of

extraordinary adaptation as that lately described by Dr. Crüger in the Coryanthes. This orchid has part of its labellum or lower lip hollowed out into a great bucket, into which drops of almost pure water

continually fall from two secreting horns which stand above it; and when the bucket is half full, the water

overflows by a spout on one side. The basal part of the labellum stands over the bucket, and is itself

hollowed out into a sort of chamber with two lateral entrances; within this chamber there are curious

fleshy ridges. The most ingenious man, if he had not witnessed what takes place, could never have

imagined what purpose all these parts serve. But Dr. Crüger saw crowds of large humble-bees visiting

the gigantic flowers of this orchid, not in order to suck nectar, but to gnaw off the ridges within the

chamber above the bucket; in doing this they frequently pushed each other into the bucket, and their

wings being thus wetted they could not fly away, but were compelled to crawl out through the passage

formed by the spout or overflow. Dr. Crüger saw a "continual procession" of bees thus crawling out of

their involuntary bath. The passage is narrow, and is roofed over by the column, so that a bee, in forcing

its way out, first rubs its back against the viscid stigma and then against the viscid glands of the

pollen-masses. The pollen-masses are thus glued to the back of the bee which first happens to crawl out





through the passage of a lately expanded flower, and are thus carried away. Dr. Crüger sent me a flower

in spirits of wine, with a bee which he had killed before it had quite crawled out with a pollen-mass still

fastened to its back. When the bee, thus provided, flies to another flower, or to the same flower a second

time, and is pushed by its comrades into the bucket and then crawls out by the passage, the pollen-mass

necessarily comes first into contact with the viscid stigma, and adheres to it, and the flower is fertilised.

Now at last we see the full use of every part of the flower, of the water-secreting horns, of the bucket

half full of water, which prevents the bees from flying away, and forces them to crawl out through thespout, and rub against the properly placed viscid pollen-masses and the viscid stigma.

The construction of the flower in another closely allied orchid, namely the Catasetum, is widely different,

though serving the same end; and is equally curious. Bees visit these flowers, like those of the

Coryanthes, in order to gnaw the labellum; in doing this they inevitably touch a long, tapering, sensitive

projection, or, as I have called it, the antenna. This antenna, when touched, transmits a sensation or

vibration to a certain membrane which is instantly ruptured; this sets free a spring by which the

pollen-mass is shot forth, like an arrow, in the right direction, and adheres by its viscid extremity to the

back of the bee. The pollen-mass of the male plant (for the sexes are separate in this orchid) is thus

carried to the flower of the female plant, where it is brought into contact with the stigma, which is viscidenough to break certain elastic threads, and retaining the pollen, fertilisation is effected.

How, it may be asked, in the foregoing and in innumerable other instances, can we understand the

graduated scale of complexity and the multifarious means for gaining the same end. The answer no doubt

is, as already remarked, that when two forms vary, which already differ from each other in some slight

degree, the variability will not be of the same exact nature, and consequently the results obtained through

natural selection for the same general purpose will not be the same. We should also bear in mind that

everyhighly developed organismhaspassed throughmany changes; and that each modified structure

tends to be inherited, so that each modification will not readily be quite lost, but may be again and again

further altered. Hence the structure of each part of each species, for whatever purpose it may serve, is

the sum of many inherited changes, through which the species has passed during its successiveadaptations to changed habits andconditions of life.

Finally then, although in many cases it is most difficult even to conjecture by what transitions organs have

arrived at their present state; yet, considering how small the proportion of living and known forms is to

the extinct and unknown, I have been astonished how rarely an organ can be named, towards which no

transitional grade is known to lead. It certainly is true, that new organs appearing as if created for some

special purpose, rarely or never appear in any being;—as indeed is shown by that old, but somewhat

exaggerated, canon in natural history of "Natura non facit saltum." We meet with this admission in the

writings of almost every experienced naturalist; or as Milne Edwards has well expressed it, Nature is

prodigal in variety, but niggard in innovation. Why, on the theory of Creation, should there be so muchvariety and so little real novelty? Why should all the parts and organs of many independent beings, each

supposed to have been separately created for its proper place in nature, be so commonly linked together

by graduated steps? Why should not Nature take a sudden leap from structure to structure? On the

theory of natural selection, we can clearly understand why she should not; for natural selection acts only

by taking advantage of slight successive variations; she can never take a great and sudden leap, but must

advance by short and sure, though slow steps.

Organs of little apparent Importance, as affected by Natural Selection.

As natural selection acts by life and death,—by the survival of the fittest, and by the destruction of the

less well-fitted individuals,—I have sometimes felt great difficulty in understanding theoriginor formation





of parts of little importance; almost as great, though of a very different kind, as in the case of the most

perfect and complex organs.

In the first place, we are much too ignorant in regard to the whole economy of any one organic being, to

say what slight modifications would be of importance or not. In a former chapter I have given instances of

very trifling characters, such as the down on fruit and the colour of its flesh, the colour of the skin and hair

of quadrupeds,which, from being correlatedwith constitutional differences or from determining theattacks of insects, might assuredly be acted on by natural selection. The tail of the giraffe looks like an

artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its

present purpose by successive slight modifications, each better and better fitted, for so trifling an object

as to drive away flies; yet we should pause before being too positive even in this case, for we know that

the distribution and existence of cattle and other animals in South America absolutely depend on their

power of resisting the attacks of insects: so that individuals which could by any means defend themselves

from these small enemies, would be able to range into new pastures and thus gain a great advantage. It is

not that the larger quadrupeds are actually destroyed (except in some rare cases) by flies, but they are

incessantly harassed and their strength reduced, so that they are more subject to disease, or not so well

enabled in a coming dearth to search for food, or to escape from beasts of prey.

Organs now of trifling importance have probably in some cases been of high importance to an early

progenitor, and, after having been slowly perfected at a former period, have been transmitted to existing

species in nearly the same state, although now of very slight use; but any actually injurious deviations in

their structure would of course have been checked by natural selection. Seeing how important an organ

of locomotion the tail is in most aquatic animals, its general presence and use for many purposes in so

many land animals, which in their lungs ormodified swimbladders betray their aquatic origin, may perhaps

be thus accounted for. A well-developed tail having been formed in an aquatic animal, it might

subsequently come to be worked in for all sorts of purposes,—as a fly-flapper, an organ of prehension,

or as an aid in turning, as in the case of the dog, though the aid in this latter respect must be slight, for the

hare, with hardly any tail, can double still more quickly.

In the second place, we may easily err in attributing importance to characters, and in believing that they

have been developed through natural selection. We must by no means overlook the effects of the definite

action of changed conditions of life,—of so-called spontaneous variations, which seem to depend in a

quite subordinate degree on the nature of the conditions,—of the tendency to reversion to long-lost

characters,—of the complex laws of growth, such as of correlation, compensation, of the pressure of one

part on another, &c.,—and finally of sexual selection, by which characters of use to one sex are often

gained and then transmitted more or less perfectly to the other sex, though of no use to this sex. But

structures thus indirectly gained, although at first of no advantage to a species, may subsequently have

been taken advantage of by its modified descendants, under new conditions of life and newly acquiredhabits.

If green woodpeckers alone had existed, and we did not know that there were many black and pied

kinds, I dare say that we should have thought that the green colour was a beautiful adaptation to conceal

this tree-frequenting bird from its enemies; and consequently that it was a character of importance, and

had been acquired through natural selection; as it is, the colour is probably in chief part due to sexual

selection. A trailing palm in the Malay Archipelago climbs the loftiest trees by the aid of exquisitely

constructed hooks clustered around the ends of the branches, and this contrivance, no doubt, is of the

highest service to the plant; but as we see nearly similar hooks on many trees which are not climbers, and

which, as there is reason to believe from the distribution of the thorn-bearing species in Africa and South

America, serve as a defence against browsing quadrupeds, so the spikes on the palm may at first have

been developed for this object, and subsequently have been improved and taken advantage of by the

plant, as it underwent further modification and became a climber. The naked skin on the head of a vulture











not perfected for its present purpose, with the poison originally adapted for some other object, such as to

produce galls, since intensified, we can perhaps understand how it is that the use of the sting should so

often cause the insect's own death: for if on the whole the power of stinging be useful to the social

community, itwill fulfil all the requirements of natural selection, though itmay cause the deathof some few

members. If we admire the truly wonderful power of scent by which the males of many insects find their

females, can we ad mire the production for this single purpose of thousands of drones, which are utterly

useless to the community for anyother purpose, and whichare ultimately slaughtered by their industriousand sterile sisters? It may be difficult, but we ought to admire the savage instinctive hatred of the

queen-bee, which urges her to destroy the young queens, her daughters, as soon as they are born, or to

perish herself in the combat; for undoubtedly this is for the good of the community; and maternal love or

maternal hatred, though the latter fortunately is most rare, is all the same to the inexorable principle of

natural selection. If we admire the several ingenious contrivances, bywhichorchids and many other

plants are fertilised through insect agency, can we consider as equally perfect the elaboration of dense

clouds of pollen by our fir-trees, so that a few granules may be wafted by chance on to the ovules?

Summary: the Law of Unity of Type and of the Conditions of Existence embraced by the Theoryof Natural Selection.

We have in this chapter discussed some of the difficulties and objections which may be urged against the

theory. Many of them are serious; but I think that in the discussion light has been thrown on several facts,

which on the belief of independent acts of creation are utterly obscure. We have seen that species at any

one period are not indefinitely variable, and are not linked together by a multitude of intermediate

gradations, partly because the process of natural selection is always very slow, and at any one time acts

only on a few forms; and partly because the very process of natural selection implies the continual

supplanting and extinction of preceding and intermediategradations.Closely allied species, now living on

a continuous area, must often have been formed when the area was not continuous, and when theconditions of life did not insensibly graduate away from one part to another. When two varieties are

formed in two districts of a continuous area, an intermediate variety will often be formed, fitted for an

intermediate zone; but from reasons assigned, the intermediate varietywill usuallyexist in lesser numbers

than the two forms which it connects; consequently the two latter, during the course of further

modification, from existing in greater numbers, will have a great advantage over the less numerous

intermediate variety, and will thusgenerally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding that the most different habits of

life could not graduate into each other; that a bat, for instance, could not have been formed by natural

selection froman animalwhich at first only glided through the air.

We have seen that a species under new conditions of life may change its habits; or it may have

diversified habits, with some very unlike those of its nearest congeners. Hence we can understand,

bearing in mind that each organic being is trying to live wherever it can live, how it has arisen that there

are upland geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of

auks.

Although the belief that an organ so perfect as the eye could have been formed by natural selection, is

enough to stagger any one; yet in the case of any organ, if we know of a long series of gradations in

complexity, each good for its possessor, then, under changing conditions of life, there is no logical

impossibility in the acquirement of any conceivable degree of perfection through natural selection. In the

cases in which we know of no intermediate or transitional states, we should be extremely cautious in

concluding that none can have existed, for the metamorphoses of many organs show what wonderful





changes in function are at least possible. For instance, a swimbladder has apparently been converted into

an air-breathing lung. The sameorgan having performed simultaneously very different functions, and then

having been in part or in whole specialised for one function; and two distinct organs having performed at

the same time the same function, the one having been perfected whilst aided by the other, must often have

largely facilitated transitions.

We have seen that in two beings widely remote from each other in the natural scale, organs serving for the same purpose and in external appearance closely similar may have been separately and independently

formed; but when such organs areclosely examined, essential differences in their structure can almost

always be detected; and this naturally follows from the principle of natural selection. On the other hand,

the common rule throughout nature is infinite diversity of structure for gaining the sameend; and this again

naturally follows from the samegreat principle.

In many cases we are far too ignorant to be enabled to assert that a part or organ is so unimportant for

the welfare of a species, that modifications in its structure could not have been slowly accumulated by

means of natural selection. In many other cases, modifications are probably the direct result of the laws of

variation or of growth, independently of any good having been thus gained. But even such structures haveoften, as we may feel assured, been subsequently taken advantage of, and still further modified, for the

good of species under new conditions of life. We may, also, believe that a part formerly of high

importance has frequently been retained (as the tail of an aquatic animal by its terrestrial descendants),

though it has become of such small importance that it could not, in its present state, have been acquired

bymeans of natural selection.

Natural selection can produce nothing in one species for the exclusive good or injury of another; though

it may well produce parts, organs, and excretions highly useful or even indispensable, or againhighly

injurious to another species, but in all cases at the same time useful to the possessor. In each

well-stocked country natural selection acts through the competition of the inhabitants, and consequently

leads to success in the battle for life, only in accordance with the standard of that particular country.Hence the inhabitants of one country, generally the smaller one, often yield to the inhabitants of another

and generally the larger country. For in the larger country there will have existed more individuals and

more diversified forms, and the competition will have been severer, and thus the standard of perfection

will have been rendered higher. Natural selection will not necessarily lead to absolute perfection; nor, as

far as we can judge by our limited faculties, can absolute perfection be everywhere predicated.

On the theory of natural selection we can clearly understand the full meaning of that old canon in natural

history, "Natura non facit saltum." This canon, if we look to the present inhabitants alone of the world, is

not strictly correct; but if we include all those of past times, whether known or unknown, it must on this

theory be strictly true.

It is generally acknowledged that all organic beings have been formed on two great laws—Unity of

Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure

which we see in organic beings of the same class, and which is quite independent of their habits of life.

On my theory, unity of type is explained by unity of descent. The expression of conditions of existence,

so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For

natural selection acts by either now adapting the varying parts of each being to its organic and inorganic

conditions of life; or by having adapted them during past periods of time: the adaptations being aided in

many cases by the increased use or disuse of parts, being affected by the direct action of the external

conditions of life, and subjected in all cases to the several laws of growth and variation. Hence, in fact,

the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former

variations and adaptations, that of Unity of Type.





|Go to Contents |

Chapter VII

Miscellaneous Objections to the Theory of

Natural Selection.

Longevity—Modifications not necessarily simultaneous—Modifications apparently of no direct

service—Progressive development—Characters of small functional importance, the most

constant—Supposed incompetence of natural selection to account for the incipient stagesof useful

structures—Causeswhich interfere with the acquisition through natural selection of useful

structures—Gradations of structure with changed functions—Widelydifferent organs inmembers of the

same class, developed from one and the same source—Reasons for disbelieving in great and abrupt

modifications.

I WILL devote this chapter to the consideration of various miscellaneous objections which have been

advanced against my views, as some of the previous discussions may thus be made clearer; but it would

be useless to discuss all of them, as many have been made by writers who have not taken the trouble to

understand the subject. Thus a distinguished German naturalist has asserted that the weakest part of my

theory is, that I consider all organic beings as imperfect: what I have really said is, that all are not as

perfect as they might have been in relation to their conditions; and this is shown to be the case by so

many native forms in many quarters of the world having yielded their places to intruding foreigners. Nor

can organic beings, even if they were at any one time perfectly adapted to their conditions of life, have

remained so, when their conditions changed, unless they themselves likewise changed; and no one will

dispute that the physical conditions of each country, as well as the numbers and kinds of its inhabitants,

have undergonemany mutations.

A critic has lately insisted, with some parade of mathematical accuracy, that longevity is a great

advantage to all species, so that he who believes in natural selection "must arrange his genealogical tree"

in such a manner that all the descendants have longer lives than their progenitors! Cannot our critic

conceive that a biennial plant or one of the lower animals might range into a cold climate and perish there

every winter; and yet, owing to advantages gained through natural selection, survive from year to year by

means of its seeds or ova? Mr. E. Ray Lankester has recently discussed this subject, and he concludes,

as far as its extreme complexity allows him to form a judgment, that longevity is generally related to the

standard of each species in the scale of organisation, as well as to the amount of expenditure in

reproduction and in general activity.And these conditions have, it is probable, been largely determined

through natural selection.

It has been argued that, as none of the animals and plants of Egypt, of which we know anything, have









suspended within the sameovarium, would follow from the selection of any slight deviations in position

which favoured their fertilisation, and the production of seed.

Several plants belonging to distinct orders habitually produce flowers of two kinds,—the one open of the

ordinary structure, the other closed and imperfect. These two kinds of flowers sometimes differ

wonderfully in structure, yet may be seen to graduate into each other on the same plant. The ordinary and

open flowers can be intercrossed; and the benefits which certainly are derived from this process are thussecured. The closed and imperfect flowers are, however, manifestly of high importance, as they yield with

the utmost safety a large stock of seed, with the expenditure of wonderfully little pollen. The two kinds of

flowers often differ much, as just stated, in structure. The petals in the imperfect flowers almost always

consist of mere rudiments, and the pollen-grains are reduced in diameter. In Ononis columnæ five of the

alternate stamens are rudimentary; and in some species of Viola three stamens are in this state, two

retaining their proper function, but being of very small size. In six out of thirty of the closed flowers in an

Indian violet (name unknown, for the plants have never produced with me perfect flowers), the sepals are

reduced from the normal number of five to three. In one section of the Malpighiaceæ the closed flowers,

according to A. de Jussieu, are still further modified, for the five stamens which stand opposite to the

sepals are all aborted, a sixth stamen standing opposite to a petal being alone developed; and this stamenis not present in the ordinary flowers of these species; the style is aborted; and the ovaria are reduced

from three to two. Now although natural selection may well have had the power to prevent some of the

flowers from expanding, and to reduce the amount of pollen, when rendered by the closure of the flowers

superfluous, yet hardly any of the above special modifications can have been thus determined, but must

have followed from the laws of growth, including the functional inactivityof parts, during the progress of

the reduction of the pollen and the closure of the flowers.

It is so necessary to appreciate the important effects of the laws of growth, that I will give some

additional cases of another kind, namely of differences in the same part or organ, due to differences in

relative position on the same plant. In the Spanish chestnut, and in certain fir-trees, the angles of

divergence of the leaves differ, according to Schacht, in the nearly horizontal and in the upright branches.In the common rue and some other plants, one flower, usually the central or terminal one, opens first, and

has five sepals and petals, and five divisions to the ovarium; whilst all the other flowers on the plant are

tetramerous. In the British Adoxa the uppermost flower generally has two calyx-lobes with the other

organs tetramerous, whilst the surrounding flowers generallyhave three calyx-lobes with the other organs

pentamerous. In manyCompositæandUmbelliferæ (and in some other plants) the circumferential flowers

have their corollas much more developed than those of the centre; and this seems often connected with

the abortion of the reproductive organs. It is a more curious fact, previously referred to, that the achenes

or seeds of the circumference and centre sometimes differ greatly in form, colour, and other characters.

In Carthamus and some other Compositæ the central achenes alone are furnished with a pappus; and in

Hyoseris the same head yields achenes of three different forms. In certain Umbelliferæ the exterior seeds,according to Tausch, are orthospermous, and the central one cælospermous, and this is a character

which was considered by De Candolle to be in other species of the highest systematic importance. Prof.

Braun mentions a Fumariaceous genus, in which the flowers in the lower part of the spike bear oval,

ribbed, one-seeded nutlets; and in the upper part of the spike, lanceolate, two-valved, and two-seeded

siliques. In these several cases, with the exception of that of the well developed ray-florets, which are of

service in making the flowers conspicuous to insects, natural selection cannot, as far as we can judge,

have come into play, or only in a quite subordinate manner. All these modifications follow from the

relative position and inter-action of the parts; and it can hardly be doubted that if all the flowers and

leaves on the same plant had been subjected to the same external and internal condition, as are the

flowers and leaves in certain positions, all would have been modified in the same manner.

In numerous other cases we find modifications of structure, which are considered by botanists to be

generally of a highly important nature, affecting only some of the flowers on the same plant, or occurring





on distinct plants, which grow close together under the same conditions. As these variations seem of no

special use to the plants, they cannot have been influenced by natural selection. Of their cause we are

quite ignorant; we cannot even attribute them, as in the last class of cases, to any proximate agency, such

as relative position. I will give only a few instances. It is so common to observe on the same plant,

flowers indifferently tetramerous, pentamerous, &c., that I need not give examples; but as numerical

variations are comparatively rare when the parts are few, I may mention that, according to De Candolle,

the flowers of Papaver bracteatum offer either two sepals with four petals (which is the common typewith poppies), or three sepals with six petals. The manner in which the petals are folded in the bud is in

most groups a very constant morphological character; but Professor Asa Gray states that with some

species of Mimulus, the æstivation is almost as frequently that of the Rhinanthideæ as of the

Antirrhinideæ, towhich latter tribe the genus belongs.Aug. St. Hilaire gives the following cases: the genus

Zanthoxylon belongs to a division of the Rutaceæ with a single ovary, but in some species flowers may be

found on the same plant, and even in the same panicle, with either one or two ovaries. In Helianthemum

the capsule has been described as unilocular or 3-locular; and in H. mutabile, "Une lame, plus ou moins

large , s'étend entire le pericarpe et le placenta." In the flowers of Saponaria officinalis, Dr. Masters has

observed instances of both marginal and free central placentation. Lastly, St. Hilaire found towards the

southern extreme of the range of Gomphia oleæformis two forms which he did not at first doubt weredistinct species, but he subsequently saw them growing on the same bush; and he then adds, "Voilà done

dans un même individu des loges et un style qui se rattachent tantôt à un axe verticale et tantôt à un

gynobase."

We thus see that with plants many morphological changes may be attributed to the laws of growth and

the inter-action of parts, independently of natural selection. But with respect to Nägeli's doctrine of an

innate tendency towards perfection or progressive development, can it be said in the case of these

strongly pronounced variations, that the plants have been caught in the act of progressing towards a

higher state of development? On the contrary, I should infer from the mere fact of the parts in question

differing or varying greatly on the sameplant, that suchmodificationswere of extremelysmall importance

to the plants themselves, of whatever importance they may generally be to us for our classifications. Theacquisition of a useless part can hardly be said to raise an organism in the natural scale; and in the case of

the imperfect, closed flowers above described, if any new principle has to be invoked, it must be one of

retrogression rather than of progression; and so it must be with many parasitic and degraded animals. We

are ignorant of the exciting cause of the above specified modifications; but if the unknown cause were to

act almost uniformly for a length of time, we may infer that the result would be almost uniform; and in this

case all the individuals of the species would be modified in the same manner.

From the fact of the above characters being unimportant for the welfare of the species, any slight

variationswhichoccurred in themwould not have been accumulated and augmented through natural

selection. A structure which has been developed through long-continued selection, when it ceases to beof service to a species, generally becomes variable, as we see with rudimentary organs; for it will no

longer be regulated by this same power of selection. But when, from the nature of the organism and of

the conditions, modifications have been inducedwhich areunimportant for the welfare of the species,

they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise

modified, descendants. It cannot have been of much importance to the greater number of mammals,

birds, or reptiles, whether they were clothed with hair, feathers, or scales; yet hair has been transmitted to

almost all mammals, feathers to all birds, and scales to all true reptiles. A structure, whatever it may be,

which is common to many allied forms, is ranked by us as of high systematic importance, and

consequently is often assumed to be of high vital importance to the species. Thus, as I am inclined to

believe, morphological differences, whichwe consider as important—suchas the arrangementof the

leaves, the divisions of the flower or of the ovarium, the position of the ovules, &c.—first appeared in

many cases as fluctuating variations, which sooner or later became constant through the nature of the

organismand of the surrounding conditions, aswell as through the intercrossingof distinct individuals, but





not through natural selection; for as these morphological characters do not affect the welfare of the

species, any slight deviations in them could not have been governed or accumulated through this latter

agency. It is a strange result which we thus arrive at, namely that characters of slight vital importance to

the species, are the most important to the systematist; but, as we shall hereafter see when we treat of the

genetic principle of classification, this is by no means so paradoxical as it may at first appear.

Although we have no good evidence of the existence in organic beings of an innate tendency towards progressive development, yet this necessarily follows, as I have attempted to show in the fourth chapter,

through the continued action of natural selection. For the best definition which has ever been given of a

high standard of organisation, is the degree to which the parts have been specialised or differentiated; and

natural selection tends towards this end, inasmuch as the parts are thus enabled to perform their functions

more efficiently.

A distinguished zoologist, Mr. St. George Mivart, has recently collected all the objections which have

ever been advanced by myself and others against the theory of natural selection, as propounded by Mr.

Wallace and myself, and has illustrated them with admirable art and force. When thus marshalled, theymake a formidable array; and as it forms no part of Mr. Mivart's plan to give the various facts and

considerations opposed to his conclusions, no slight effort of reason and memory is left to the reader,

who may wish to weigh the evidence on both sides. When discussing special cases, Mr. Mivart passes

over the effects of the increased use and disuse of parts, which I have always maintained to be highly

important, and have treated in my 'Variation under Domestication' at greater length than, as I believe, any

otherwriter. He likewise often assumes that I attribute nothing to variation, independentlyof natural

selection, whereas in the work just referred to I have collected a greater number of well-established

cases than can be found in any other work known to me. My judgment may not be trustworthy, but after

reading with care Mr. Mivart's book, and comparing each section with what I have said on the same

head, I never before felt so strongly convinced of the general truth of the conclusions here arrived at,

subject, of course, in so intricate a subject, to much partial error.

All Mr. Mivart's objections will be, or have been, considered in the present volume. The one new point

which appears to have struck many readers is, "that natural selection is incompetent to account for the

incipient stages of useful structures." This subject is intimately connectedwith that of the gradation of

characters, often accompanied by a change of function,—for instance, the conversion of a swim-bladder

into lungs,—points which were discussed in the last chapter under two headings. Nevertheless, I will here

consider in some detail several of the cases advanced by Mr. Mivart, selecting those which are the most

illustrative, aswant of spaceprevents me from considering all.

The giraffe, by its lofty stature, much elongated neck, fore-legs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the

reach of the other Ungulata or hoofed animals inhabiting the same country; and this must be a great

advantage to it during dearths. The Niata cattle in S. America show us how small a difference in structure

may make, during such periods, a great difference in preserving an animal's life. These cattle can browse

as well as others on grass, but from the projection of the lower jaw they cannot, during the often

recurrent droughts, browse on the twigs of trees, reeds, &c., to which food the common cattle and

horses are then driven; so that at these times the Niatas perish, if not fed by their owners. Before coming

to Mr. Mivart's objections, it may be well to explain once again how natural selection will act in all

ordinary cases. Manhas modified some of his animals,without necessarilyhavingattended to special

points of structure, by simply preserving andbreeding from the fleetest individuals, aswith the race-horse

and greyhound, or as with the game-cock, by breeding from the victorious birds. So under nature with

the nascent giraffe, the individuals which were the highest browsers, and were able during dearths to

reach even an inch or two above the others, will often have been preserved; for they will have roamed







favourable to its increase in some new country. We can, however, see in a general manner that various

causes might have interfered with the development of a long neck or proboscis. To reach the foliage at a

considerable height (without climbing, forwhich hoofed animals are singularly ill-constructed) implies

greatly increased bulk of body; and we know that some areas support singularly few large quadrupeds,

for instance S. America, though it is so luxuriant; whilst S. Africa abounds with them to an unparalleled

degree. Why this should be so, we do not know; nor why the later tertiary periods should have been

much more favourable for their existence than the present time. Whatever the causes may have been, wecan see that certain districts and times would have been much more favourable than others for the

development of so large a quadruped as the giraffe.

In order that an animal should acquire some structure specially and largely developed, it is almost

indispensable that several other parts should be modified and co-adapted. Although every part of the

body varies slightly, it does not follow that the necessary parts should always vary in the right direction

and to the right degree. With the different species of our domesticated animals we know that the parts

vary in a different manner and degree; and that some species are much more variable than others. Even if

the fitting variations did arise, it does not follow that natural selection would be able to act on them, and

produce a structure which apparently would be beneficial to the species. For instance, if the number of individuals existing in a country is determined chiefly through destruction bybeasts of prey,—byexternal

or internal parasites, &c.,—as seems often to be the case, then natural selection will be able to do little,

orwill be greatly retarded, inmodifying any particular structure for obtaining food. Lastly, natural

selection is a slow process, and the same favourable conditions must long endure in order that any

marked effect should thus be produced. Except by assigning such general and vague reasons, we cannot

explain why, in many quarters of the world, hoofed quadrupeds have not acquired much elongated necks

or other means for browsing on the higher branches of trees.

Objections of the same nature as the foregoing have been advanced by many writers. In each case

various causes, besides the general ones just indicated, have probably interfered with the acquisition

through natural selection of structures, which it is thought would be beneficial to certain species. Onewriter asks, why has not the ostrich acquired the power of flight? But a moment's reflection will show

what an enormous supply of food would be necessary to give to this bird of the desert force to move its

huge body through the air. Oceanic islands are inhabited by bats and seals, but by no terrestrial

mammals; yet as some of these bats are peculiar species, they must have long inhabited their present

homes. Therefore Sir C. Lyell asks, and assigns certain reasons in answer, why have not seals and bats

given birth on such islands to forms fitted to live on the land? But seals would necessarily be first

converted into terrestrial carnivorous animals of considerable size, andbats into terrestrial insectivorous

animals; for the former there would be no prey; for the bats ground-insects would serve as food, but

these would already be largely preyed on by the reptiles or birds, which first colonise and abound on

most oceanic islands. Gradations of structure, with each stage beneficial to a changing species, will befavouredonly under certain peculiar conditions.A strictly terrestrial animal, by occasionallyhunting for

food in shallow water, then in streams or lakes, might at last be converted into an animal so thoroughly

aquatic as to brave the open ocean. But seals would not find on oceanic islands the conditions favourable

to their gradual reconversion into a terrestrial form. Bats, as formerly shown, probably acquired their

wings by at first gliding through the air from tree to tree, like the so-called flying squirrels, for the sake of

escaping from their enemies, or for avoiding falls; but when the power of true flight had once been

acquired, it would never be reconverted back, at least for the above purposes, into the less efficient

power of gliding through the air. Bats might, indeed, like many birds, have had their wings greatly

reduced in size, or completely lost, through disuse; but in this case it would be necessary that they should

first have acquired the power of running quickly on the ground, by the aid of their hind legs alone, so as

to compete with birds or other ground animals; and for such a change a bat seems singularly ill-fitted.

These conjectural remarks have been made merely to show that a transition of structure, with each step

beneficial, is a highly complex affair; and that there is nothing strange in a transition not having occurred in





anyparticular case.

Lastly, more than one writer has asked, why have some animals had their mental powers more highly

developed than others, as such development would be advantageous to all?Why have not apes acquired

the intellectual powers of man? Various causes could be assigned; but as they are conjectural, and their

relative probability cannot be weighed, it would be useless to give them. A definite answer to the latter

question ought not to be expected, seeing that no one can solve the simpler problem why, of two races of savages, one has risen higher in the scale of civilisation than the other; and this apparently implies

increased brain-power.

We will return to Mr. Mivart's other objections. Insects often resemble for the sake of protection various

objects, such as green or decayed leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds,

and living insects; but to this latter point I shall hereafter recur. The resemblance is often wonderfully

close, and is not confined to colour, but extends to form, and even to the manner in which the insects

hold themselves. The caterpillars which project motionless like dead twigs from the bushes on which they

feed, offer an excellent instance of a resemblance of this kind. The cases of the imitation of such objects

as the excrement of birds, are rare and exceptional. On this head, Mr. Mivart remarks, "As, according toMr. Darwin's theory, there is a constant tendency to indefinite variation, and as the minute incipient

variationswill be inall directions , they must tend to neutralise each other, and at first to form such

unstablemodifications that it is difficult, if not impossible, to see how such indefinite oscillationsof

infinitesimal beginnings canever build up a sufficiently appreciable resemblance to a leaf, bamboo, or

other object, for Natural Selection to seize upon and perpetuate."

But in all the foregoing cases the insects in their original state no doubt presented some rude and

accidental resemblance to an object commonly found in the stations frequented by them. Nor is this at all

improbable, considering the almost infinitenumber of surrounding objects and the diversity in formand

colour of the hosts of insects which exist. As some rude resemblance is necessary for the first start, we

can understand how it is that the larger and higher animals do not (with the exception, as far as I know, of one fish) resemble for the sake of protection special objects, but only the surface which commonly

surrounds them, and this chiefly in colour. Assuming that an insect originally happened to resemble in

some degree a dead twig or a decayed leaf, and that it varied slightly in many ways, then all the variations

which rendered the insect at all more like any such object, and thus favoured its escape, would be

preserved, whilst other variations would be neglected and ultimately lost; or, if they rendered the insect at

all less like the imitated object, they would be eliminated. There would indeed be force in Mr. Mivart's

objection, if we were to attempt to account for the above resemblances, independently of natural

selection, throughmere fluctuating variability; but as the case stands there is none.

Nor can I see any force in Mr. Mivart's difficulty with respect to "the last touches of perfection in themimicry;" as in the case given by Mr. Wallace, of a walking-stick insect (Ceroxylus laceratus), which

resembles "a stick grown over by a creeping moss or jungermannia." So close was this resemblance, that

a native Dyak maintained that the foliaceous excrescences were really moss. Insects are preyed on by

birds and other enemies, whose sight is probably sharper than ours, and every grade in resemblance

which aided an insect to escape notice or detection, would tend towards its preservation; and the more

perfect the resemblance so much the better for the insect. Considering the nature of the differences

between the species in the group which includes the above Ceroxylus, there is nothing improbable in this

insect having varied in the irregularities on its surface, and in these having become more or less

green-coloured; for in every group the characters which differ in the several species are the most apt to

vary, whilst the generic characters, or those common to all the species, are the most constant.

The Greenland whale is one of the most wonderful animals in the world, and the baleen, or whale-bone,





one of its greatest peculiarities. The baleen consists of a row, on each side, of the upper jaw, of about

300 plates or laminæ, which stand close together transversely to the longer axis of the mouth. Within the

main row there are some subsidiary rows. The extremities and inner margins of all the plates are frayed

into stiff bristles, which clothe the whole gigantic palate, and serve to strain or sift the water, and thus to

secure the minute prey on which these great animals subsist. The middle and longest lamina in the

Greenland whale is ten, twelve, or even fifteen feet in length; but in the different species of Cetaceans

there are gradations in length; the middle lamina being in one species, according to Scoresby, four feet, inanother three, in another eighteen inches, and in the Balænoptera rostrata only about nine inches in length.

The quality of the whalebone also differs in the different species.

With respect to the baleen, Mr. Mivart remarks that if it "had once attained such a size and development

as to be at all useful, then its preservation and augmentation within serviceable limits would be promoted

by natural selection alone. But how to obtain the beginning of such useful development?" In answer, it

may be asked, why should not the early progenitors of the whales with baleen have possessed a mouth

constructed something like the lamellated beak of a duck? Ducks, like whales, subsist by sifting the mud

and water; and the family has sometimes been calledCriblatores , or sifters. I hope that I may not be

misconstrued into saying that the progenitors ofwhales did actually possess mouths lamellated like the beak of a duck. I wish only to show that this is not incredible, and that the immense plates of baleen in

the Greenland whale might have been developed from such lamellæ by finely graduated steps, each of

service to its possessor.

The beak of a shoveller-duck (Spatula clypeata) is a more beautiful and complex structure than the

mouth of a whale. The upper mandible is furnished on each side (in the specimen examined by me) with a

row or comb formed of 188 thin, elastic lamellæ, obliquely bevelled so as to be pointed, and placed

transversely to the longer axis of the mouth. They arise from the palate, and are attached by flexible

membrane to the sides of the mandible. Those standing towards the middle are the longest, being about

one-third of an inch in length, and they project .14 of an inch beneath the edge. At their bases there is a

short subsidiary row of obliquely transverse lamellæ. In these several respects they resemble the plates of baleen in the mouth of a whale. But towards the extremity of the beak they differ much, as they project

inwards, instead of straight downwards. The entire head of the shoveller, though incomparably less bulky,

is about one-eighteenth of the length of the head of a moderately large Balænoptera rostrata, in which

species the baleen is only nine inches long; so that if we were to make the head of the shoveller as long as

that of the Balænoptera, the lamellæ would be six inches in length,—that is, two-thirds of the length of the

baleen in this species of whale. The lower mandible of the shoveller-duck is furnished with lamellæ of

equal length with those above, but finer; and in being thus furnished it differs conspicuously from the

lower jaw of a whale, which is destitute of baleen. On the other hand the extremities of these lower

lamellæ are frayed into fine bristly points, so that they thus curiously resemble the plates of baleen. In the

genus Prion, a member of the distinct family of the Petrels, the upper mandible alone is furnished withlamellæ, which are well developed and project beneath the margin; so that the beak of this bird

resembles in this respect the mouth of a whale.

From the highly developed structure of the shoveller's beak we may proceed (as I have learnt from

information and specimens sent to me by Mr. Salvin), without any great break, as far as fitness for sifting

is concerned, through the beak of the Merganetta armata, and in some respects through that of the Aix

sponsa, to the beak of the common duck. In this latter species, the lamellæ are much coarser than in the

shoveller, and are firmly attached to the sides of the mandible; they are only about 50 in number on each

side, and do not project at all beneath the margin. They are square-topped, and are edged with

translucent hardish tissue, as if for crushing food. The edges of the lower mandible are crossed by

numerous fine ridges, which project very little. Although the beak is thus very inferior as a sifter to that of

the shoveller, yet this bird, as every one knows, constantly uses it for this purpose. There are other

species, as I hear from Mr. Salvin, in which the lamellæ are considerably less developed than in the







have been liable to be abraded by the sandy bottom. That the Pleuronectidæ are admirably adapted by

their flattened and asymmetrical structure for their habits of life, is manifest from several species, such as

soles, flounders, &c., being extremely common. The chief advantages thus gained seem to be protection

from their enemies, and facility for feeding on the ground. The differentmembers, however, of the family

present, as Schiödte remarks, "a long series of formsexhibiting a gradual transition from Hippoglossus

pinguis, which does not in any considerable degree alter the shape in which it leaves the ovum, to the

soles, which are entirely thrown to one side."

Mr. Mivart has taken up this case, and remarks that a sudden spontaneous transformation in the position

of the eyes is hardly conceivable, in which I quite agree with him. He then adds: "if the transit was

gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the

head could benefit the individual is, indeed, far from clear. It seems, even, that such an incipient

transformation must rather have been injurious." But he might have found an answer to this objection in

the excellent observations published in 1867byMalm.The Pleuronectidæ,whilst very young andstill

symmetrical, with their eyes standing on opposite sides of the head, cannot long retain a vertical position,

owing to the excessive depth of their bodies, the small size of their lateral fins, and to their being destitute

of a swimbladder. Hence soon growing tired, they fall to the bottom on one side. Whilst thus at rest theyoften twist, as Malm observed, the lower eye upwards, to see above them; and they do this so vigorously

that the eye is pressed hard against the upper part of the orbit. The forehead between the eyes

consequently becomes, as could be plainly seen, temporarily contracted in breadth. On one occasion

Malm saw a young fish raise and depress the lower eye through an angular distance of about seventy

degrees.

We should remember that the skull at this early age Is cartilaginous and flexible, so that it readily yields

to muscular action. It is also known with the higher animals, even after early youth, that the skull yields

and is altered in shape, if the skin or muscles be permanently contracted through disease or some

accident. With long-eared rabbits, if one ear lops forwards and downwards, its weight drags forward all

the bones of the skull on the same side, of which I have given a figure. Malm states that thenewly-hatched young of perches, salmon, and several other symmetrical fishes, have the habit of

occasionally resting on one side at the bottom; and he has observed that they often then strain their lower

eyes so as to look upwards; and their skulls are thus rendered rather crooked. These fishes, however,

are soon able to hold themselves in a vertical position, and no permanent effect is thus produced. With

the Pleuronectidæ, on the other hand, the older they grow the more habitually they rest on one side,

owing to the increasing flatness of their bodies, and a permanent effect is thus produced on the form of

the head, and on the position of the eyes. Judging from analogy, the tendency to distortion would no

doubt be increased through the principle of inheritance. Schiödte believes, in opposition to some other

naturalists, that the Pleuronectidæ are not quite symmetrical even in the embryo; and if this be so, we

could understand how it is that certain species, whilst young, habitually fall over and rest on the left side,and other species on the right side. Malm adds, in confirmation of the above view, that the adult

Trachypterus arcticus, which is not a member of the Pleuronectidæ, rests on its left side at the bottom,

and swims diagonally through the water; and in this fish, the two sides of the head are said to be

somewhat dissimilar. Our great authority on Fishes, Dr. Günther, concludes his abstract of Malm's paper,

by remarking that "the author gives a very simple explanation of the abnormal condition of the

Pleuronectoids."

We thus see that the first stages of the transit of the eye from one side of the head to the other, which

Mr. Mivart considers would be injurious, may be attributed to the habit, no doubt beneficial to the

individual and to the species, of endeavouring to look upwards with both eyes, whilst resting on one side

at the bottom. We may also attribute to the inherited effects of use the fact of the mouth in several kinds

of flat-fish being bent towards the lower surface, with the jaw bones stronger and more effective on this,

the eyeless side of the head, than on the other, for the sake, as Dr. Traquair supposes, of feeding with





ease on the ground. Disuse, on the other hand, will account for the less developed condition of the whole

inferior half of the body, including the lateral fins; though Yarrell thinks that the reduced size of these fins

is advantageous to the fish, as "there is so much less room for their action, than with the larger fins

above." Perhaps the lesser number of teeth in the proportion of four to seven in the upper halves of the

two jaws of the plaice, to twenty-five to thirty in the lower halves, may likewise be accounted for by

disuse. From the colourless state of the ventral surface of most fishes and of many other animals, we may

reasonably suppose that the absence of colour in flat-fish on the side, whether it be the right or left, whichis undermost, is due to the exclusion of light. But it cannot be supposed that the peculiar speckled

appearance of the upper side of the sole, so like the sandy bed of the sea, or the power in some species,

as recently shown by Pouchet, of changing their colour in accordance with the surrounding surface, or the

presence of bony tubercles on the upper side of the turbot, are due to the action of the light. Here natural

selection has probably come into play, as well as in adapting the general shape of the body of these

fishes, and many other peculiarities, to their habits of life. We should keep in mind, as I have before

insisted, that the inherited effects of the increased use of parts, and perhaps of their disuse, will be

strengthened by natural selection. For all spontaneous variations in the right direction will thus be

preserved; as will those individuals which inherit in the highest degree the effects of the increased and

beneficial use of any part. How much to attribute in each particular case to the effects of use, and howmuch to natural selection, it seems impossible to decide.

I may give another instance of a structure which apparently owes its origin exclusively to use or habit.

The extremity of the tail in some American monkeys has been converted into a wonderfully perfect

prehensile organ, and serves as a fifth hand. A reviewer who agrees with Mr. Mivart in every detail,

remarks on this structure: "It is impossible to believe that in any number of ages the first slight incipient

tendency to grasp could preserve the lives of the individuals possessing it, or favour their chance of

having and of rearing offspring." But there is no necessity for any such belief. Habit, and this almost

implies that some benefit great or small is thus derived, would in all probability suffice for the work.

Brehm saw the young of an African monkey (Cercopithecus) clinging to the under surface of their mother

by their hands, and at the same time they hooked their little tails round that of their mother. Professor Henslow kept in confinement some harvest mice (Mus messorius) which do not possess a structurally

prehensile tail; but he frequently observed that they curled their tails round the branches of a bush placed

in the cage, and thus aided themselves in climbing. I have received an analogous account from Dr.

Günther, who has seen a mouse thus suspend itself. If the harvest mouse had been more strictly arboreal,

it would perhaps have had its tail rendered structurally prehensile, as is the case with some members of

the same order. Why Cercopithecus, considering its habits whilst young, has not become thus provided,

it would be difficult to say. It is, however, possible that the long tail of this monkey may be of more

service to it as a balancing organ in making its prodigious leaps, than as a prehensile organ.

The mammary glands are common to the whole class of mammals, and are indispensable for their

existence; they must, therefore, have been developed at an extremely remote period, and we can know

nothing positivelyabout theirmanner of development. Mr. Mivart asks: "Is it conceivable that the young

of any animal was ever saved from destruction by accidentally sucking a drop of scarcely nutritious fluid

from an accidentally hypertrophied cutaneous gland of its mother? And even if one was so, what chance

was there of the perpetuation of such a variation?" But the case is not here put fairly. It is admitted by

most evolutionists that mammals are descended from a marsupial form; and if so, the mammary glands

will have been at first developed within the marsupial sack. In the case of the fish (Hippocampus) the

eggs are hatched, and the young are reared for a time, within a sack of this nature; and an American

naturalist, Mr. Lockwood, believes from what he has seen of the development of the young, that they are

nourished by a secretion from the cutaneous glands of the sack. Now with the early progenitors of

mammals, almost before they deserved to be thus designated, is it not at least possible that the young

might havebeen similarlynourished?And in this case, the individuals which secreted a fluid, in some









now existing between a zooid and an avicularium. It is therefore impossible to conjecture by what

serviceable gradations the one could have been converted into the other: but it by no means follows from

this that such gradations have not existed.

As the chelæ of Crustaceans resemble in some degree the avicularia of Polyzoa, both serving as pincers,

it may be worth while to show that with the former a long series of serviceable gradations still exists. In

the first and simplest stage, the terminal segment of a limb shuts down either on the square summit of the broad penultimate segment, or against one whole side; and is thus enabled to catch hold of an object; but

the limb still serves as an organ of locomotion. We next find one corner of the broad penultimate segment

slightly prominent, sometimes furnished with irregular teeth; and against these the terminal segment shuts

down. By an increase in the size of this projection, with its shape, as well as that of the terminal segment,

slightly modified and improved, the pincers are rendered more and more perfect, until we have at last an

instrument as efficient as the chelæ of a lobster; and all these gradations can be actually traced.

Besides the avicularia, the Polyzoa possess curious organs called vibracula. These generally consist of

long bristles, capable of movement and easily excited. In one species examined by me the vibracula were

slightly curved and serrated along the outer margin; and all of them on the same polyzoary often movedsimultaneously; so that, acting like long oars, they swept a branch rapidly across the object-glass of my

microscope. When a branch was placed on its face, the vibracula became entangled, and they made

violent efforts to free themselves. They are supposed to serve as a defence, and may be seen, as Mr.

Busk remarks, "to sweep slowly and carefully over the surface of the polyzoary, removing what might be

noxious to the delicate inhabitants of the cells when their tentacula are protruded." The avicularia, like the

vibracula, probably serve for defence, but they also catch and kill small living animals, which it is believed

are afterwards swept by the currents within reach of the tentacula of the zooids. Some species are

providedwith avicularia and vibracula; somewith avicularia alone, and a fewwith vibracula alone.

It is not easy to imagine two objects more widely different in appearance than a bristle or vibraculum,

and an avicularium like the head of a bird; yet they are almost certainly homologous and have beendeveloped from the same common source, namely a zooid with its cell. Hence we can understand how it

is that these organs graduate in some cases, as I am informed by Mr. Busk, into each other. Thus with

the avicularia of several species of Lepralia, the moveable mandible is so much produced and is so like a

bristle, that the presence of the upper or fixed beak alone serves to determine its avicularian nature. The

vibracula may have been directly developed from the lips of the cells, without having passed through the

avicularian stage; but it seems more probable that they have passed through this stage, as during the early

stages of the transformation, the other parts of the cell with the included zooid could hardly have

disappeared at once. In many cases the vibracula have a grooved support at the base, which seems to

represent the fixed beak; though this support in some species is quite absent. This view of the

development of thevibracula, if trustworthy, is interesting; for supposing that all the species providedwithavicularia had become extinct, no one with the most vivid imagination would ever have thought that the

vibracula had originally existed as part of an organ, resembling a bird's head or an irregular box or hood.

It is interesting to see two such widely different organs developed from a common origin; and as the

moveable lip of the cell serves as a protection to the zooid, there is no difficulty in believing that all the

gradations, by which the lip became converted first into the lower mandible of an avicularium and then

into an elongated bristle, likewise served as a protection in different ways and under different

circumstances.

In the vegetable kingdomMr. Mivart only alludes to two cases, namely the structure of the flowers of

orchids, and the movements of climbing plants. With respect to the former, he says, "the explanation of

their origin is deemed thoroughlyunsatisfactory—utterly insufficient to explain the incipient, infinitesimal

beginnings of structureswhichareof utility onlywhen they are considerably developed." As I have fully











general reasons, but in some few cases special reasons, can be assigned. Thus to adapt a species to new

habits of life, many co-ordinatedmodifications are almost indispensable, and it may often have happened

that the requisite parts did not vary in the right manner or to the right degree. Many species must have

been prevented from increasing in numbers throughdestructive agencies, which stood in no relation to

certain structures, whichwe imagine would have been gained through natural selection from appearing to

us advantageous to the species. In this case, as the struggle for life did not depend on such structures,

they could not have been acquired through natural selection. Inmany cases complex and long-enduringconditions, often of a peculiar nature, are necessary for the development of a structure; and the requisite

conditions mayseldom have concurred. The belief that any given structure,whichwe think, often

erroneously, would have been beneficial to a species, would have been gained under all circumstances

through natural selection, is opposed to what we can understand of its manner of action. Mr. Mivart does

not deny that natural selection has effected something; but he considers it as "demonstrably insufficient" to

account for the phenomena which I explain by its agency. His chief arguments have now been

considered, and the others will hereafter be considered. They seem to me to partake little of the

character of demonstration, and to have little weight in comparison with those in favour of the power of

natural selection, aided by the other agencies often specified. I am bound to add, that some of the facts

and arguments here used by me, have been advanced for the same purpose in an able article lately published in the 'Medico-Chirurgical Review.'

At the present day almost all naturalists admit evolution under some form. Mr. Mivart believes that

species change through "an internal force or tendency," about which it is not pretended that anything is

known. That species have a capacity for change will be admitted by all evolutionists; but there is no need,

as it seems to me, to invoke any internal force beyond the tendency to ordinary variability, which through

the aid of selection by man has given rise to many well-adapted domestic races, and which through the

aid of natural selection would equally well give rise by graduated steps to natural races or species. The

final result will generally have been, as already explained, an advance, but in some few cases a

retrogression, in organisation.

Mr. Mivart is further inclined to believe, and some naturalists agree with him, that new species manifest

themselves "with suddenness and by modifications appearing at once." For instance, he supposes that the

differences between the extinct three-toed Hipparion and the horse arose suddenly. He thinks it difficult

to believe that the wing of a bird "was developed in any other way than by a comparatively sudden

modification of a marked and important kind;" and apparently he would extend the same view to the

wings of bats andpterodactyles. This conclusion, which implies great breaks or discontinuity in the series,

appears to me improbable in the highest degree.

Every one who believes in slow and gradual evolution, will of course admit that specific changes may

have been as abrupt and as great as any single variation which we meet with under nature, or even under domestication. But as species are more variable when domesticated or cultivated than under their natural

conditions, it is not probable that such great and abrupt variations have often occurred under nature, as

are knownoccasionally to arise under domestication. Of these latter variations several maybe attributed

to reversion; and the characters which thus reappear were, it is probable, in many cases at first gained in

a gradual manner. A still greater number must be called monstrosities, such as six-fingered men,

porcupine men, Ancon sheep, Niata cattle, &c.; and as they are widely different in character from natural

species, they throw very little light on our subject. Excluding such cases of abrupt variations, the few

which remain would at best constitute, if found in a state of nature, doubtful species, closely related to

their parental types.

My reasons for doubting whether natural species have changed as abruptly as have occasionally

domestic races, and for entirelydisbelieving that they have changed in thewonderful manner indicated by

Mr. Mivart, are as follows. According to our experience, abrupt and strongly marked variations occur in









are commonly embraced by this term; but every one understands what is meant, when it is said that

instinct impels the cuckoo to migrate and to lay her eggs in other birds' nests. An action, which we

ourselves require experience to enable us to perform, when performed by an animal, more especially by

a very young one, without experience, and when performed by many individuals in the same way, without

their knowing for what purpose it is performed, is usually said to be instinctive. But I could show that

none of these characters are universal. A little dose of judgment or reason, as Pierre Huber expresses it,

often comes into play, even with animals low in the scale of nature.

Frederick Cuvier and several of the older metaphysicians have compared instinct with habit. This

comparison gives, I think, an accurate notion of the frame of mind under which an instinctive action is

performed, but not necessarilyof its origin.How unconsciously many habitual actions are performed,

indeed not rarely in direct opposition to our conscious will! yet they may be modified by the will or

reason. Habits easily become associated with other habits, with certain periods of time, and states of the

body. When once acquired, they often remain constant throughout life. Several other points of

resemblance between instincts and habits could be pointed out. As in repeating a well-known song, so in

instincts, one action follows another by a sort of rhythm; if a person be interrupted in a song, or in

repeating anything by rote, he is generally forced to go back to recover the habitual train of thought; so P.Huber found it was with a caterpillar, which makes a very complicated hammock; for if he took a

caterpillar which had completed its hammock up to, say, the sixth stage of construction, and put it into a

hammock completed up only to the third stage, the caterpillar simply re-performed the fourth, fifth, and

sixth stages of construction. If, however, a caterpillar were taken out of a hammock made up, for

instance, to the third stage, and were put into one finished up to the sixth stage, so that much of its work

was already done for it, far from deriving any benefit from this, it was much embarrassed, and in order to

complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried to

complete the already finishedwork.

If we suppose any habitual action to become inherited—and it can be shown that this does sometimes

happen—then the resemblance between what originally was a habit and an instinct becomes so close asnot to be distinguished. If Mozart, instead of playing the pianoforte at three years old with wonderfully

little practice, had played a tune with no practice at all, he might truly be said to have done so

instinctively. But it would be a serious error to suppose that the greater number of instincts have been

acquired by habit in one generation, and then transmitted by inheritance to succeeding generations. It can

be clearly shown that the most wonderful instincts with which we are acquainted, namely, those of the

hive-bee and of many ants, could not possibly have been acquired by habit.

It will be universally admitted that instincts are as important as corporeal structures for the welfare of

each species, under its present conditions of life. Under changed conditions of life, it is at least possible

that slight modifications of instinct might be profitable to a species; and if it can be shown that instincts dovary ever so little, then I can see nodifficulty in natural selectionpreserving and continually accumulating

variations of instinct to any extent that was profitable. It is thus, as I believe, that all the most complex and

wonderful instincts have originated. Asmodifications of corporeal structure arise from, and are increased

by, use or habit, and are diminished or lost by disuse, so I do not doubt it has been with instincts. But I

believe that the effects of habit are in many cases of subordinate importance to the effects of the natural

selection of what may be called spontaneous variations of instincts;—that is of variations produced by the

same unknown causes whichproduce slight deviations of bodily structure.

No complex instinct can possibly be produced through natural selection, except by the slow and gradual

accumulation of numerous slight, yet profitable, variations.Hence, as in the case of corporeal structures,

we ought to find in nature, not the actual transitional gradations by which each complex instinct has been

acquired—for these could be found only in the lineal ancestors of each species—but we ought to find in

the collateral lines of descent some evidence of such gradations; or we ought at least to be able to show





that gradations of some kind are possible; and this we certainly can do. I have been surprised to find,

making allowance for the instincts of animals having been but little observed except in Europe and North

America, and for no instinct being known amongst extinct species, how very generallygradations, leading

to the most complex instincts, can be discovered. Changes of instinct may sometimes be facilitated by the

same species having different instincts at different periods of life, or at different seasons of the year, or

when placed under different circumstances, &c.; in which case either the one or the other instinct might

be preserved by natural selection. And such instances of diversity of instinct in the same species can beshown to occur in nature.

Again, as in the case of corporeal structure, and conformably to my theory, the instinct of each species is

good for itself, but has never, as far as we can judge, been produced for the exclusive good of others.

One of the strongest instances of an animal apparently performing an action for the sole good of another,

with which I am acquainted, is that of aphides voluntarily yielding, as was first observed by Huber, their

sweet excretion to ants: that they do so voluntarily, the following facts show. I removed all the ants from a

group of about a dozen aphides on a dock-plant, and prevented their attendance during several hours.

After this interval, I felt sure that the aphides would want to excrete. I watched them for some time

through a lens, but not one excreted; I then tickled and stroked them with a hair in the same manner, aswell as I could, as the ants do with their antennæ; but not one excreted. Afterwards I allowed an ant to

visit them, and it immediately seemed, by its eager way of running about, to be well aware what a rich

flock it had discovered; it then began to play with its antennæ on the abdomen first of one aphis and then

of another; and each, as soon as it felt the antennæ, immediately lifted up its abdomen and excreted a

limpid drop of sweet juice, which was eagerly devoured by the ant. Even the quite young aphides

behaved in this manner, showing that the action was instinctive, and not the result of experience. It is

certain, from the observations of Huber, that the aphides show no dislike to the ants: if the latter be not

present they are at last compelled to eject their excretion. But as the excretion is extremely viscid, it is no

doubt a convenience to the aphides to have it removed; therefore probably they do not excrete solely for

the good of the ants. Although there is no evidence that any animal performs an action for the exclusive

good of another species, yet each tries to take advantage of the instincts of others, as each takesadvantage of the weaker bodily structure of other species. So again certain instincts cannot be

considered as absolutely perfect; but as details on this and other such points are not indispensable, they

may be here passed over.

As some degree of variation in instincts under a state of nature, and the inheritance of such variations, are

indispensable for the action of natural selection, as many instances as possible ought to be given; but want

of space prevents me. I can only assert that instincts certainly do vary—for instance, the migratory

instinct, both in extent and direction, and in its total loss. So it is with the nests of birds, which vary partly

in dependence on the situations chosen, and on the nature and temperature of the country inhabited, but

often from causes wholly unknown to us: Audubon has given several remarkable cases of differences inthe nests of the same species in the northern and southern United States. Why, it has been asked, if

instinct be variable, has it not granted to the bee "the ability to use some other material when wax was

deficient"? But what other natural material could bees use? They will work, as I have seen, with wax

hardened with vermilion or softened with lard. Andrew Knight observed that his bees, instead of

laboriously collecting propolis, used a cement of wax and turpentine, with which he had covered

decorticated trees. It has lately been shown that bees, instead of searching for pollen, will gladly use a

very different substance, namely oatmeal. Fear of any particular enemy is certainly an instinctive quality,

as may be seen in nestling birds, though it is strengthened by experience, and by the sight of fear of the

same enemy in other animals. The fear of man is slowly acquired, as I have elsewhere shown, by the

various animals which inhabit desert islands; and we see an instance of this even in England, in the greater

wildness of all our large birds in comparison with our small birds; for the large birds have been most

persecuted by man. We may safely attribute the greater wildness of our large birds to this cause; for in

uninhabited islands large birds are not more fearful than small; and the magpie, so wary in England, is







showed a slight tendency to this strange habit, and that the long-continued selection of the best individuals

in successive generations made tumblers what they now are; and near Glasgow there are house-tumblers,

as I hear from Mr. Brent, which cannot fly eighteen inches high without going head over heels. It may be

doubted whether any one would have thought of training a dog to point, had not some one dog naturally

shown a tendency in this line; and this is known occasionally to happen, as I once saw, in a pure terrier:

the act of pointing is probably, as many have thought, only the exaggerated pause of an animal preparing

to spring on its prey. When the first tendency to point was once displayed, methodical selection and theinherited effects of compulsory training in each successive generationwould soon complete thework; and

unconscious selection is still in progress, as each man tries to procure, without intending to improve the

breed, dogs which stand and hunt best. On the other hand, habit alone in some cases has sufficed; hardly

any animal is more difficult to tame than the young of the wild rabbit; scarcely any animal is tamer than the

young of the tame rabbit; but I can hardly suppose that domestic rabbits have often been selected for

tameness alone; so that we must attribute at least the greater part of the inherited change from extreme

wildness to extreme tameness, to habit and long-continuedclose confinement.

Natural instincts are lost under domestication: a remarkable instance of this is seen in those breeds of

fowls which very rarely or never become "broody," that is, never wish to sit on their eggs. Familiarityalone prevents our seeing how largely and how permanently the minds of our domestic animals have been

modified. It is scarcely possible to doubt that the love of man has become instinctive in the dog. All

wolves, foxes, jackals, and species of the cat genus, when kept tame, are most eager to attack poultry,

sheep, and pigs; and this tendency has been found incurable in dogs which have been brought home as

puppies from countries such as Tierra del Fuego and Australia, where the savages do not keep these

domestic animals. How rarely, on the other hand, do our civilised dogs, even when quite young, require

to be taught not to attack poultry, sheep, and pigs! No doubt they occasionally do make an attack, and

are then beaten; and if not cured, they are destroyed; so that habit and some degree of selection have

probably concurred in civilising by inheritance our dogs. On the other hand, young chickens have lost,

wholly by habit, that fear of the dog and cat which no doubt was originally instinctive in them; for I am

informed by Captain Hutton that the young chickens of the parent-stock, the Gallus bankiva, whenreared in India under a hen, are at first excessively wild. So it is with young pheasants reared in England

under a hen. It is not that chickens have lost all fear, but fear only of dogs and cats, for if the hen gives the

danger-chuckle, they will run (moreespecially young turkeys) from under her, andconceal themselves in

the surrounding grass or thickets; and this is evidently done for the instinctive purpose of allowing, as we

see in wild ground-birds, their mother to fly away. But this instinct retained by our chickens has become

useless under domestication, for the mother-hen has almost lost by disuse the power of flight.

Hence, we may conclude, that under domestication instincts have been acquired, and natural instincts

have been lost, partly by habit, and partly by man selecting and accumulating, during successive

generations, peculiar mental habits and actions, which at first appeared from what we must in our ignorance call an accident. In some cases compulsory habit alone has sufficed to produce inherited

mental changes; in other cases, compulsory habit has done nothing, and all has been the result of

selection, pursued bothmethodically and unconsciously: but in most cases habit and selection have

probably concurred.

Special Instincts.

We shall, perhaps, best understand how instincts in a state of nature have become modified by selection

by considering a few cases. I will select only three,—namely, the instinct which leads the cuckoo to lay

her eggs in other birds' nests; the slave-making instinct of certain ants; and the cell-making power of the

hive-bee. These two latter instincts have generally and justly been ranked by naturalists as the most







eggs in an open nest, manifest a decided preference for nests containing eggs similar in colour to their

own. The European species apparently manifests some tendency towards a similar instinct, but not rarely

departs from it, as is shown by her laying her dull and pale-coloured eggs in the nest of the

Hedge-warbler with bright greenish-blue eggs.Hadour cuckoo invariably displayed the above instinct, it

would assuredly have been added to those which it is assumed must all have been acquired together. The

eggs of the Australian Bronze cuckoo vary, according to Mr. Ramsay, to an extraordinary degree in

colour; so that in this respect, as well as in size, natural selection might have secured and fixed anyadvantageous variation.

In the case of the European cuckoo, the offspring of the foster-parents are commonly ejected from the

nest within three days after the cuckoo is hatched; and as the latter at this age is in a most helpless

condition, Mr. Gould was formerly inclined to believe that the act of ejection was performed by the

foster-parents themselves. But he has now received a trustworthy account of a young cuckoo which was

actually seen, whilst still blind and not able even to hold up its own head, in the act of ejecting its

foster-brothers. One of these was replaced in the nest by the observer, and was again thrown out. With

respect to the means by which this strange and odious instinct was acquired, if it were of great

importance for the young cuckoo, as is probably the case, to receive as much food as possible soon after birth, I can see no special difficulty in itshaving gradually acquired, during successive generations, the

blind desire, the strength, and structure necessary for the work of ejection; for those young cuckoos

which had such habits and structure best developed would be the most securely reared. The first step

towards the acquisition of the proper instinct might have been mere unintentional restlessness on the part

of the young bird, when somewhat advanced in age and strength; the habit having been afterwards

improved, and transmitted to an earlier age. I can see no more difficulty in this, than in the unhatched

young of other birds acquiring the instinct to break through their own shells;—or than in young snakes

acquiring in their upper jaws, as Owen has remarked, a transitory sharp tooth for cutting through the

tough egg-shell. For if each part is liable to individual variations at all ages, and the variations tend to be

inherited at a corresponding or earlier age,—propositions which cannot be disputed,—then the instincts

and structure of the young could be slowly modified as surely as those of the adult; and both cases muststand or fall together with the whole theory of natural selection.

Some species of Molothrus, a widely distinct genus of American birds, allied to our starlings, have

parasitic habits like those of the cuckoo; and the species present an interesting gradation in the perfection

of their instincts. The sexes of Molothrus badius are stated by an excellent observer, Mr. Hudson,

sometimes to live promiscuously together in flocks, and sometimes to pair. They either build a nest of

their own, or seize on one belonging to some other bird, occasionally throwing out the nestlings of the

stranger. They either lay their eggs in the nest thus appropriated, or oddly enough build one for

themselves on the top of it. They usually sit on their own eggs and rear their own young; but Mr. Hudson

says it is probable that they are occasionally parasitic, for he has seen the young of this species followingold birds of a distinct kind and clamouring to be fed by them. The parasitic habits of another species of

Molothrus, the M. bonariensis, are much more highly developed than those of the last, but are still far

from perfect. This bird, as far as it is known, invariably lays its eggs in the nests of strangers; but it is

remarkable that several together sometimes commence to build an irregular untidy nest of their own,

placed in singularly ill-adapted situations, as on the leaves of a large thistle. They never, however, as far

as Mr. Hudson has ascertained, complete a nest for themselves. They often lay so many eggs—from

fifteen to twenty—in the same foster-nest, that few or none can possibly be hatched. They have,

moreover, the extraordinary habit of pecking holes in the eggs, whether of their own species or of their

foster-parents, which they find in the appropriated nests. They drop also many eggs on the bare ground,

which are thus wasted. A third species, the M. pecoris of North America, has acquired instincts as

perfect as those of the cuckoo, for it never lays more than one egg in a foster-nest, so that the young bird

is securely reared. Mr. Hudson is a strong disbeliever in evolution, but he appears to have been so much

struck by the imperfect instincts of the Molothrus bonariensis that he quotes my words, and asks, "Must







them away to a place of safety. Hence, it is clear, that the slaves feel quite at home. During the months of

June and July, on three successive years, I watched for many hours several nests in Surrey and Sussex,

and never saw a slave either leave or enter a nest. As, during these months, the slaves are very few in

number, I thought that theymight behavedifferentlywhenmore numerous; but Mr. Smith informs me that

he has watched the nests at various hours during May, June, and August, both in Surrey and Hampshire,

and has never seen the slaves, though present in large numbers in August, either leave or enter the nest.

Hence he considers them as strictly household slaves. The masters, on the other hand, may be constantlyseen bringing in materials for the nest, and food of all kinds. During the year 1860, however, in the month

of July, I came across a community with an unusually large stock of slaves, and I observed a few slaves

mingled with their masters leaving the nest, and marching along the same road to a tall Scotch-fir-tree,

twenty-five yards distant, which they ascended together, probably in search of aphides or cocci.

According to Huber, who had ample opportunities forobservation, the slaves in Switzerlandhabitually

work with their masters in making the nest, and they alone open and close the doors in the morning and

evening; and, as Huber expressly states, their principal office is to search for aphides. This difference in

the usual habits of the masters and slaves in the two countries, probably depends merely on the slaves

being captured in greater numbers in Switzerland than in England.

One day I fortunately witnessed a migration of F. sanguinea from one nest to another, and it was a most

interesting spectacle to behold the masters carefully carrying their slaves in their jaws insteadof being

carried by them, as in the case of F. rufescens. Another day my attention was struck by about a score of

the slave-makers haunting the same spot, and evidently not in search of food; they approached and were

vigorously repulsed by an independent community of the slave species (F. fusca); sometimes as many as

three of these ants clinging to the legs of the slave-making F. sanguinea. The latter ruthlessly killed their

small opponents, and carried their dead bodies as food to their nest, twenty-nine yards distant; but they

were prevented from getting any pupæ to rear as slaves. I then dug up a small parcel of the pupæ of F.

fusca from another nest, and put them down on a bare spot near the place of combat; they were eagerly

seized and carried off by the tyrants, who perhaps fancied that, after all, they had been victorious in their

late combat.

At the same time I laid on the same place a small parcel of the pupæ of another species, F. flava, with a

few of these little yellow ants still clinging to the fragments of their nest. This species is sometimes, though

rarely, made into slaves, as has been described by Mr. Smith. Although so small a species, it is very

courageous, and I have seen it ferociously attack other ants. In one instance I found to my surprise an

independent community of F. flava under a stone beneath a nest of the slave-making F. sanguinea; and

when I had accidentallydisturbed both nests, the little ants attacked their big neighbourswith surprising

courage. Now I was curious to ascertain whether F. sanguinea could distinguish the pupæ of F. fusca,

which they habitually make into slaves, from those of the little and furious F. flava, which they rarely

capture, and it was evident that they did at once distinguish them; for we have seen that they eagerly andinstantly seized the pupæ of F. fusca, whereas they were much terrified when they came across the

pupæ, or even the earth from the nest, of F. flava, and quickly ran away; but in about a quarter of an

hour, shortly after all the little yellow ants had crawled away, they took heart and carried off the pupæ.

One evening I visited another community of F. sanguinea, and found a number of these ants returning

home and entering their nests, carrying the dead bodies of F. fusca (showing that it was not a migration)

and numerous pupæ. I traced a long file of ants burthened with booty, for about forty yards back, to a

very thick clump of heath, whence I saw the last individual of F. sanguinea emerge, carrying a pupa; but I

was not able to find the desolated nest in the thick heath. The nest, however, must have been close at

hand, for two or three individuals of F. fusca were rushing about in the greatest agitation, and one was

perched motionless with its own pupa in its mouth on the top of a spray of heath, an image of despair

over its ravaged home.





Such are the facts, though they did not need confirmation by me, in regard to the wonderful instinct of

making slaves. Let it be observed what a contrast the instinctive habits of F. sanguinea present with those

of the continental F. rufescens. The latter does not build its own nest, does not determine its own

migrations, does not collect food for itself or its young, and cannot even feed itself: it is absolutely

dependent on its numerous slaves. Formica sanguinea, on the other hand, possesses much fewer slaves,

and in the early part of the summer extremely few: the masters determine when and where a new nest

shall be formed, and when they migrate, the masters carry the slaves. Both in Switzerland and Englandthe slaves seem to have the exclusive care of the larvæ, and the masters alone go on slave-making

expeditions. In Switzerland the slaves and masters work together, making andbringing materials for the

nest; both, but chiefly the slaves, tend, and milk, as it may be called their aphides; and thus both collect

food for the community. InEngland the masters aloneusually leave the nest to collect buildingmaterials

and food for themselves, their slaves and larvæ. So that the masters in this country receive much less

service from their slaves than they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not pretend to conjecture. But as ants which

are not slave-makers will, as I have seen, carry off the pupæ of other species, if scattered near their

nests, it is possible that such pupæ originally stored as food might become developed; and the foreignants thus unintentionally reared would then follow their proper instincts, and do what work they could. If

their presence proved useful to the species which had seized them—if it were more advantageous to this

species to capture workers than to procreate them—the habit of collecting pupæ, originally for food,

might by natural selection be strengthened and rendered permanent for the very different purpose of

raising slaves. When the instinct was once acquired, if carried out to a much less extent even than in our

British F. sanguinea, which, as we have seen, is less aided by its slaves than the same species in

Switzerland, natural selectionmight increase andmodify the instinct—always supposing eachmodification

to be of use to the species—until an ant was formed as abjectly dependent on its slaves as is the Formica

rufescens.

Cell-making instinct of the Hive-Bee.— I will not here enter on minute details on this subject, but willmerely give an outline of the conclusions at which I have arrived. He must be a dull man who can examine

the exquisite structure of a comb, so beautifully adapted to its end, without enthusiastic admiration. We

hear from mathematicians that bees have practically solved a recondite problem, and have made their

cells of the proper shape to hold the greatest possible amount of honey, with the least possible

consumption of precious wax in their construction. It has been remarked that a skilful workman with

fitting tools and measures, would find it very difficult to make cells of wax of the true form, though this is

effected by a crowd of bees working in a dark hive. Granting whatever instincts you please, it seems at

first quite inconceivable how they can make all the necessary angles and planes, or even perceive when

they are correctly made. But the difficulty is not nearly so great as it at first appears: all this beautiful work

can be shown, I think, to follow from a few simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown that the form of the cell stands

in close relation to the presence of adjoining cells; and the following view may, perhaps, be considered

only as a modification of his theory. Let us look to the great principle of gradation, and see whether

Nature does not reveal to us her method of work. At one end of a short series we have humble-bees,

which use their old cocoons to hold honey, sometimes adding to them short tubes of wax, and likewise

making separate and very irregular rounded cells of wax. At the other end of the series we have the cells

of the hive-bee, placed in a double layer: each cell, as is well known, is an hexagonal prism, with the

basal edges of its six sides bevelled so as to join an inverted pyramid, of three rhombs. These rhombs

have certain angles, and the three which form the pyramidal base of a single cell on one side of the comb

enter into the composition of the bases of three adjoining cells on the opposite side. In the series between

the extreme perfection of the cells of the hive-bee and the simplicity of those of the humble-bee we have

the cells of the Mexican Melipona domestica, carefully described and figured by Pierre Huber. The

















in some few alone; and that by the survival of the communities with females which produced most neuters

having the advantageous modification, all the neuters ultimately came to be thus characterised.According

to this view we ought occasionally to find in the same nest neuter insects, presenting gradations of

structure; and this we do find, even not rarely, considering how few neuter insects out of Europe have

been carefullyexamined.Mr.F. Smith has shown that the neuters of severalBritish ants differ surprisingly

from each other in size and sometimes in colour; and that the extreme forms can be linked together by

individuals taken out of the same nest: I have myself compared perfect gradations of this kind. Itsometimes happens that the larger or the smaller sized workers are the most numerous; or that both large

and small are numerous, whilst those of an intermediate size are scanty in numbers. Formica flava has

larger and smaller workers, with some few of intermediate size; and, in this species, as Mr. F. Smith has

observed, the larger workers have simple eyes (ocelli), which though small canbe plainly distinguished,

whereas the smaller workers have their ocelli rudimentary.Having carefullydissected several specimens

of these workers, I can affirm that the eyes are far more rudimentary in the smaller workers than can be

accounted for merely by their proportionally lesser size; and I fully believe, though I dare not assert so

positively, that the workersof intermediate size have their ocelli in an exactly intermediate condition. So

that here we have two bodies of sterile workers in the same nest, differing not only in size, but in their

organs of vision, yet connected by some few members in an intermediate condition. I may digress byadding, that if the smaller workers had been the most useful to the community, and those males and

females had been continually selected, which produced more and more of the smaller workers, until all

the workers were in this condition; we should then have had a species of ant with neuters in nearly the

same condition as those of Myrmica. For the workers of Myrmica have not even rudiments of ocelli,

though the male and female ants of this genus have well-developed ocelli.

I may give one other case: so confidently did I expect occasionally to find gradations of important

structures between the different castes of neuters in the same species, that I gladly availed myself of Mr.

F. Smith's offer of numerous specimens from the same nest of the driver ant (Anomma) of West Africa.

The reader will perhaps best appreciate the amount of difference in these workers, by my giving not the

actual measurements, but a strictly accurate illustration: the difference was the same as if we were to seea set of workmen building a house, of whom many were five feet four inches high, and many sixteen feet

high; but we must in addition suppose that the larger workmen had heads four instead of three times as

big as those of the smaller men, and jaws nearly five times as big. The jaws, moreover, of the working

ants of the several sizes differed wonderfully in shape, and in the form and number of the teeth. But the

important fact for us is, that, though the workers can be grouped into castes of different sizes, yet they

graduate insensibly into each other, as does the widely-different structure of their jaws. I speak

confidently on this latter point, as Sir J. Lubbock made drawings for me, with the camera lucida, of the

jaws which I dissected from the workers of the several sizes. Mr. Bates, in his interesting 'Naturalist on

the Amazons,' has described analogous cases.

With these facts before me, I believe that natural selection, by acting on the fertile ants or parents, could

form a species which should regularly produce neuters, all of large size with one form of jaw, or all of

small size with widely different jaws; or lastly, and this is the greatest difficulty, one set of workers of one

size and structure, and simultaneously another set of workers of a different size and structure;—a

graduated series having first been formed, as in the case of the driver ant, and then the extreme forms

having been produced in greater and greater numbers, through the survival of the parents which

generated them, until none with an intermediate structure were produced.

An analogous explanation has been given by Mr. Wallace, of the equally complex case, of certain

MalayanButterflies regularly appearing under twoor even threedistinct female forms; and by Fritz

Müller, of certain Brazilian crustaceans likewise appearingunder two widely distinct male forms. But this

subject need not here be discussed.







|Go to Contents |

Chapter IX

Hybridism

Distinction between the sterility of first crosses and of hybrids-Sterility various in degree, not universal,

affected by close interbreeding, removed by domestication—Laws governing the sterility of

hybrids—Sterility not a special endowment, but incidental on other differences, not accumulated by

natural selection—Causes of the sterility of first crosses and of hybrids—Parallelismbetween the effects

of changed conditionsof life and of crossing—Dimorphism and trimorphism—Fertility of varietieswhen

crossed andof theirmongreloffspring not universal—Hybridsandmongrels compared independently of

their fertility—Summary.

THE view commonly entertained by naturalists is that species, when intercrossed, have been specially

endowed with sterility, in order to prevent their confusion. This view certainly seems at first highly

probable, for species living together could hardly have been kept distinct had they been capable of freely

crossing. The subject is in many ways important for us, more especially as the sterility of species when

first crossed, and that of their hybrid offspring, cannot have been acquired, as I shall show, by the

preservation of successive profitabledegrees of sterility. It is an incidental result of differences in the

reproductive systemsof the parent-species.

In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been

confounded; namely, the sterility of species when first crossed, and the sterility of the hybrids produced

from them.

Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed

they produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs

functionally impotent, as may be clearly seen in the state of the male element in both plants and animals;

though the formative organs themselves are perfect in structure, as far as the microscope reveals. In the

first case the two sexual elements which go to form the embryo are perfect; in the second case they are

either not at all developed, or are imperfectly developed. This distinction is important, when the cause of

the sterility, which is common to the two cases, has to be considered. The distinction probably has been

slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the

province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to be descended from common parents,

when crossed, and likewise the fertility of their mongrel offspring, is, with reference to my theory, of equal





importance with the sterility of species; for it seems to make a broad and clear distinction between

varieties and species.

Degrees of Sterility.— First, for the sterility of species when crossed and of their hybrid offspring. It is

impossible to study the several memoirs and works of those two conscientious and admirable observers,

Kölreuter and Gärtner, who almost devoted their lives to this subject, without being deeply impressed

with the high generality of some degree of sterility. Kölreuter makes the rule universal; but then he cutsthe knot, for in ten cases in which he found two forms, considered by most authors as distinct species,

quite fertile together, he unhesitatingly ranks them as varieties.Gärtner, also,makes the rule equally

universal; and he disputes the entire fertility of Kölreuter's ten cases. But in these and in many other

cases, Gärtner is obliged carefully to count the seeds, in order to show that there is any degree of sterility.

He always compares the maximum number of seeds produced by two species when first crossed, and

the maximum produced by their hybrid offspring, with the average number produced by both pure

parent-species in a state of nature. But causes of serious error here intervene: a plant, to be hybridised,

must be castrated, and, what is often more important, must be secluded in order to prevent pollen being

brought to it by insects from other plants. Nearly all the plants experimented on by Gärtner were potted,

and were kept in a chamber in his house. That these processes are often injurious to the fertility of a plantcannot be doubted; for Gärtner gives in his table about a score of cases of plants which he castrated, and

artificially fertilised with theirown pollen, and (excludingall cases such as the Leguminosæ, inwhich there

is an acknowledged difficulty in the manipulation) half of these twenty plants had their fertility in some

degree impaired. Moreover, as Gärtner repeatedly crossed some forms, such as the common red and

blue pimpernels (Anagallis arvensis and cærulea), which thebest botanists rank as varieties, and found

them absolutely sterile, we may doubt whether many species are really so sterile, when intercrossed, as

he believed.

It is certain, on the one hand, that the sterility of various species when crossed is so different in degree

and graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily

affected by various circumstances, that for all practical purposes it is most difficult to say where perfectfertility ends and sterility begins. I think no better evidence of this can be required than that the two most

experienced observers who have ever lived, namelyKölreuter andGärtner, arrived at diametrically

opposite conclusions in regard to some of the very same forms. It is also most instructive to

compare—but I have not space here to enter on details—the evidence advanced by our best botanists

on the question whether certain doubtful forms should be ranked as species or varieties, with the

evidence from fertility adducedby different hybridisers, or by the same observer from experimentsmade

during different years. It can thus be shown that neither sterility nor fertilityaffords any certain distinction

between species and varieties. The evidence from this source graduates away, and is doubtful in the same

degree as is the evidencederived from other constitutional and structural differences.

In regard to the sterility of hybrids in successive generations; though Gärtner was enabled to rear some

hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for

tengenerations, yet he asserts positively that their fertility never increases, but generally decreases greatly

and suddenly. With respect to this decrease, it may first be noticed that when any deviation in structure or

constitution is common to both parents, this is often transmitted in an augmented degree to the offspring;

and both sexual elements in hybrid plants are already affected in some degree. But I believe that their

fertility has been diminished in nearly all these cases by an independent cause, namely, by too close

interbreeding. I have made so many experiments and collected so many facts, showing on the one hand

that an occasional cross with a distinct individual or variety increases the vigour and fertility of the

offspring, and on the other hand that very close interbreeding lessens their vigour and fertility, that I

cannot doubt the correctness of this conclusion. Hybrids are seldom raised by experimentalists in great

numbers; and as the parent-species, or other allied hybrids, generally grow in the same garden, the visits

of insectsmust be carefullypreventedduring the flowering season: hence hybrids, if left to themselves,







would have been notorious to nurserymen. Horticulturists raise large beds of the same hybrid, and such

alone are fairly treated, for by insect agency the several individuals are allowed to cross freely with each

other, and the injurious influence of close interbreeding is thus prevented. Any one may readily convince

himselfof the efficiency of insect-agencybyexamining the flowers of the more sterilekinds of hybrid

Rhododendrons, which produce no pollen, for he will find on their stigmas plenty of pollen brought from

other flowers.

In regard to animals, much fewer experiments have been carefully tried than with plants. If our systematic

arrangements can be trusted, that is, if the genera of animals are as distinct from each other as are the

genera of plants, then we may infer that animals more widely distinct in the scale of nature can be crossed

more easily than in the case of plants; but the hybrids themselves are, I think, more sterile. It should,

however, be borne in mind that, owing to few animals breeding freely under confinement, few

experiments have been fairly tried: for instance, the canary-bird has been crossed with nine distinct

species of finches, but, as not one of these breeds freely in confinement, we have no right to expect that

the first crosses between them and the canary, or that their hybrids, should be perfectly fertile. Again,

with respect to the fertility in successive generations of the more fertile hybrid animals, I hardly know of

an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding. On the contrary, brothers and sisters have

usually been crossed in each successivegeneration, in opposition to the constantly repeated admonition

of every breeder. And in this case, it is not at all surprising that the inherent sterility in the hybrids should

have gone on increasing.

Although I knowof hardly any thoroughlywell-authenticated cases of perfectly fertile hybrid animals, I

have reason to believe that the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus

colchicus with P. torquatus, are perfectly fertile. M. Quatrefages states that the hybrids from two moths

(Bombyx cynthia and arrindia) were proved in Paris to be fertileinter se for eight generations. It has lately

been asserted that two such distinct species as the hare and rabbit, when they can be got to breed

together, produce offspring, which are highly fertile when crossed with one of the parent-species. Thehybrids from the common and Chinese geese (A. cygnoides), species which are so different that they are

generally ranked in distinct genera, have often bred in this country with either pure parent, and in one

single instance they have bredinter se . This was effected by Mr. Eyton, who raised two hybrids from the

same parents, but from different hatches; and from these two birds he raised no less than eight hybrids

(grandchildren of the pure geese) from one nest. In India, however, these cross-bred geese must be far

more fertile; for I am assured by two eminently capable judges, namely Mr. Blyth and Capt. Hutton, that

whole flocks of these crossed geese are kept in various parts of the country; and as they are kept for

profit, whereneither pure parent-species, exists, they must certainlybe highly or perfectly fertile.

With our domesticated animals, the various races when crossed together are quite fertile; yet in manycases they are descended from two or more wild species. From this fact we must conclude either that the

aboriginal parent-species at first produced perfectly fertile hybrids, or that the hybrids subsequently

rearedunder domestication becamequite fertile. This latter alternative,whichwas first propounded by

Pallas, seems by far the most probable, and can, indeed, hardly be doubted. It is, for instance, almost

certain that our dogs are descended from several wild stocks; yet, with perhaps the exception of certain

indigenous domestic dogs of South America, all are quite fertile together; but analogy makes me greatly

doubt, whether the several aboriginal species would at first have freely bred together and have produced

quite fertile hybrids. So again I have lately acquired decisive evidence that the crossed offspring from the

Indian humped and common cattle areinter se perfectly fertile; and from the observations byRütimeyer

on their important osteological differences, as well as from those by Mr. Blyth on their differences in

habits, voice, constitution, &c., these two forms must be regarded as good and distinct species. The

same remarks may be extended to the two chief races of the pig. We must, therefore, either give up the

belief of the universal sterility of species when crossed; or we must look at this sterility in animals, not as









that when forms, which must be considered as good and distinct species, are united, their fertility

graduates from zero to perfect fertility, or even to fertility under certain conditions in excess; that their

fertility, besides being eminently susceptible to favourable and unfavourable conditions, is innately

variable; that it is by no means always the same in degree in the first cross and in the hybrids produced

from this cross; that the fertility of hybrids is not related to the degree in which they resemble in external

appearance either parent; and lastly, that the facility of making a first cross between any two species is

not always governed by their systematic affinity or degree of resemblance to each other. This latter statement is clearly proved by the difference in the result of reciprocal crosses between the same two

species, for, according as the one species or the other is used as the father or the mother, there is

generally somedifference, and occasionally the widest possible difference, in the facility of effecting an

union. The hybrids,moreover, produced from reciprocal crosses oftendiffer in fertility.

Now do these complex and singular rules indicate that species have been endowed with sterility simply

to prevent their becoming confounded in nature? I think not. For why should the sterility be so extremely

different in degree, when various species are crossed, all of which we must suppose it would be equally

important to keep from blending together? Why should the degree of sterility be innately variable in the

individuals of the same species? Why should some species cross with facility, and yet produce verysterile hybrids; andother species crosswith extreme difficulty, andyetproduce fairly fertile hybrids? Why

should there often be so great a difference in the result of a reciprocal cross between the same two

species? Why, it may even be asked, has the production of hybrids been permitted? To grant to species

the special power of producing hybrids, and then to stop their further propagation by different degrees of

sterility, not strictly related to the facility of the first union between their parents, seems a strange

arrangement.

The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both of

first crosses and of hybrids is simply incidental or dependent on unknown differences in their reproductive

systems; the differences being of so peculiar and limited a nature, that, in reciprocal crosses between the

same two species, the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction. It will be advisable to explain a little more fully by an example

what I mean by sterility being incidental on other differences, and not a specially endowed quality. As the

capacity of one plant to be grafted or budded on another is unimportant for their welfare in a state of

nature, I presume that no one will suppose that this capacity is a specially endowed quality, but will admit

that it is incidental on differences in the laws of growth of the two plants.We can sometimes see the

reason why one tree will not take on another, from differences in their rate of growth, in the hardness of

their wood, in the period of the flow or nature of their sap, &c.; but in a multitude of cases we can assign

no reason whatever. Great diversity in the size of two plants, one being woody and the other herbaceous,

one being evergreen and the other deciduous, and adaptation to widely different climates, do not always

prevent the two grafting together.As in hybridisation, sowith grafting, the capacity is limited bysystematic affinity, for no one has been able to graft together trees belonging to quite distinct families;

and, on the other hand, closely allied species, and varieties of the same species, can usually, but not

invariably, be grafted with ease. But this capacity, as in hybridisation, is by no means absolutely governed

by systematic affinity. Althoughmanydistinct generawithin the same family havebeen grafted together, in

other cases species of the same genus will not take on each other. The pear can be grafted far more

readily on the quince, which is ranked as a distinct genus, than on the apple, which is a member of the

same genus. Even different varieties of the pear take with different degrees of facility on the quince; so do

different varieties of the apricot and peach on certain varieties of the plum.

As Gärtner found that there was sometimes an innate difference in differentindividuals of the same two

species in crossing; so Sageret believes this to be the case with different individuals of the same two

species in being grafted together. As in reciprocal crosses, the facility of effecting an union is often very

far from equal, so it sometimes is in grafting; the common gooseberry, for instance, cannot be grafted on





the currant, whereas the currant will take, thoughwith difficulty, on the gooseberry.

We have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition,

is a different case from the difficulty of uniting two pure species, which have their reproductive organs

perfect; yet these two distinct classes of cases run to a large extent parallel. Something analogous occurs

in grafting; for Thouin found that three species of Robinia, which seeded freely on their own roots, and

which could be grafted with no great difficulty on a fourth species, when thus grafted were rendered barren. On the other hand, certain species of Sorbus, when grafted on other species yielded twice as

much fruit as when on their own roots. We are reminded by this latter fact of the extraordinary cases of

Hippeastrum, Passiflora, &c., which seed much more freely when fertilised with the pollen of a distinct

species, than when fertilised with pollen from the same plant.

We thus see, that, although there is a clear and great difference between the mere adhesion of grafted

stocks, and the union of the male and female elements in the act of reproduction, yet that there is a rude

degree of parallelism in the results of grafting and of crossing distinct species. And as we must look at the

curious and complex laws governing the facility with which trees can be grafted on each other as

incidental on unknown differences in their vegetative systems, so I believe that the stillmore complex lawsgoverning the facilityof first crosses are incidental on unknown differences in their reproductive systems.

These differences in both cases, follow to a certain extent, as might have been expected, systematic

affinity, bywhich term every kind of resemblance and dissimilaritybetweenorganic beings is attempted to

be expressed. The facts by no means seem to indicate that the greater or lesser difficulty of either grafting

or crossing various species has been a special endowment; although in the case of crossing, the difficulty

is as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant

for their welfare.

Origin and Causes of the Sterility of first Crosses and of Hybrids.

At one time it appeared to me probable, as it has to others, that the sterility of first crosses and of

hybrids might have been slowly acquired through the natural selection of slightly lessened degreesof

fertility, which, like any other variation, spontaneously appeared in certain individualsof one varietywhen

crossed with those of another variety. For it would clearly be advantageous to two varieties or incipient

species, if they could be kept from blending, on the same principle that, when man is selecting at the same

time two varieties, it is necessary that he should keep them separate. In the first place, it may be

remarked that species inhabiting distinct regions are often sterile when crossed; now it could clearly have

been of no advantage to such separated species to have been rendered mutually sterile, and consequently

this could not have been effected through natural selection; but it may perhaps be argued, that, if aspecies was rendered sterile with some one compatriot, sterility with other species would follow as a

necessary contingency. In the second place, it is almost as much opposed to the theory of natural

selection as to that of special creation, that in reciprocal crosses the male element of one form should

have been rendered utterly impotent on a second form, whilst at the same time the male element of this

second form is enabled freely to fertilise the first form; for this peculiar state of the reproductive system

could hardly have been advantageous to either species.

In considering the probability of natural selection having come into action, in rendering speciesmutually

sterile, the greatest difficulty will be found to lie in the existence of many graduated steps from slightly

lessened fertility to absolute sterility. It may be admitted that it would profit an incipient species, if it were

rendered in some slight degree sterile when crossed with its parent form or with some other variety; for

thus fewer bastardised and deteriorated offspring would be produced to commingle their blood with the

new species in process of formation. But he who will take the trouble to reflect on the steps by which this





first degree of sterility could be increased through natural selection to that high degree which is common

with so many species, and which is universal with species which have been differentiated to a generic or

family rank,will find the subject extraordinarilycomplex. Aftermature reflection it seems tome that this

could not have been effected through natural selection. Take the case of any two species which, when

crossed, produced few and sterile offspring; now, what is there which could favour the survival of those

individuals which happened tobe endowed in a slightlyhigher degreewithmutual infertility, and which

thus approached by one small step towards absolute sterility? Yet an advance of this kind, if the theory of natural selection be brought to bear, must have incessantly occurred with many species, for a multitude

are mutually quite barren. With sterile neuter insects we have reason to believe that modifications in their

structure and fertilityhave been slowly accumulated by natural selection, from an advantage having been

thus indirectly given to the community towhich theybelonged over other communitiesof the same

species; but an individual animal not belonging to a social community, if rendered slightly sterile when

crossed with some other variety, would not thus itself gain any advantage or indirectly give any advantage

to the other individuals of the same variety, thus leading to their preservation.

But it would be superfluous to discuss this question in detail; for with plants we have conclusive evidence

that the sterility of crossed species must be due to some principle, quite independent of natural selection.Both Gärtner and Kölreuter have proved that in genera including numerous species, a series can be

formed from species which when crossed yield fewer and fewer seeds, to species which never produce a

single seed, but yet are affected by the pollen of certain other species, for the germen swells. It is here

manifestly impossible to select the more sterile individuals, which have already ceased to yield seeds; so

that this acme of sterility, when the germen alone is affected, cannot have been gained through selection;

and from the laws governing the various grades of sterility being so uniform throughout the animal and

vegetable kingdoms, we may infer that the cause, whatever it may be, is the same or nearly the same in all

cases.

We will now look a little closer at the probable nature of the differences between species which inducesterility in first crosses and in hybrids. In the case of first crosses, the greater or less difficulty in effecting

an union and in obtaining offspring apparentlydepends on several distinct causes. Theremust sometimes

be a physical impossibility in the male element reaching the ovule, as would be the case with a plant

having a pistil too long for the pollen-tubes to reach the ovarium. It has also been observed that when the

pollen of one species is placed on the stigma of a distantly allied species, though the pollen-tubes

protrude, they do not penetrate the stigmatic surface. Again, the male element may reach the female

element but be incapable of causing an embryo to be developed, as seems to have been the case with

some of Thuret's experiments on Fuci. No explanation can be given of these facts, any more than why

certain trees cannot be grafted on others. Lastly, an embryo may be developed, and then perish at an

early period. This latter alternative has not been sufficiently attended to; but I believe, from observationscommunicated to me by Mr. Hewitt, who has had great experience in hybridising pheasants and fowls,

that the early death of the embryo is a very frequent cause of sterility in first crosses. Mr. Salter has

recently given the results of an examination of about 500 eggs produced from various crosses between

three species of Gallus and their hybrids; the majority of these eggs had been fertilised; and in the

majority of the fertilised eggs, the embryos had either been partially developed and had then perished, or

had become nearly mature, but the young chickens had been unable to break through the shell. Of the

chickens which were born, more than four-fifths died within the first few days, or at latest weeks,

"without any obvious cause, apparently from mere inability to live;" so that from the 500 eggs only twelve

chickens were reared. With plants, hybridised embryos probably often perish in a like manner; at least it

is known that hybrids raised from very distinct species are sometimes weak and dwarfed, and perish at

an early age; of which fact MaxWichura has recently given some striking cases with hybrid willows. It

may be here worth noticing that in some cases of parthenogenesis, the embryos within the eggs of silk

moths which had not been fertilised, pass through their early stages of development and then perish like





the embryos produced by a cross between distinct species. Until becoming acquainted with these facts, I

was unwilling to believe in the frequent early death of hybrid embryos; for hybrids, when once born, are

generally healthy and long-lived, as we see in the case of the common mule. Hybrids, however, are

differently circumstanced before and after birth: when born and living in a country where their two parents

live, they are generally placed under suitable conditions of life. But a hybrid partakes of only half of the

nature and constitution of its mother; it may therefore before birth, as long as it is nourished within its

mother's womb, or within the egg or seed produced by the mother, be exposed to conditions in somedegree unsuitable, and consequently be liable to perish at an early period; more especially as all very

young beings areeminently sensitive to injurious or unnaturalconditionsof life. But after all, the cause

more probably lies in some imperfection in the original act of impregnation, causing the embryo to be

imperfectly developed, rather than in the conditions to which it is subsequently exposed.

In regard to the sterility of hybrids, in which the sexual elements are imperfectly developed, the case is

somewhat different. I have more than once alluded to a large body of facts showing that, when animals

and plants are removed from their natural conditions, they are extremely liable to have their reproductive

systems seriously affected. This, in fact, is the great bar to the domestication of animals. Between the

sterility thus superinduced and that of hybrids, there are many points of similarity. In both cases thesterility is independent of general health, and is often accompanied by excess of size or great luxuriance.

In both cases the sterility occurs in various degrees; in both, the male element is the most liable to be

affected; but sometimes the female more than the male. In both, the tendency goes to a certain extent

with systematic affinity, for whole groups of animals and plants are rendered impotent by the same

unnatural conditions; and whole groups of species tend to produce sterile hybrids. On the other hand,

one species in a group will sometimes resist great changes of conditionswith unimpaired fertility; and

certain species in a group will produce unusually fertile hybrids. No one can tell, till he tries, whether any

particular animal will breed under confinement, or any exotic plant seed freely under culture; nor can he

tell till he tries, whether any two species of a genus will produce more or less sterile hybrids. Lastly, when

organic beings are placed during several generations under conditions not natural to them, they are

extremely liable to vary, which seems to be partly due to their reproductive systems having been speciallyaffected, though in a lesser degree than when sterility ensues. So it is with hybrids, for their offspring in

successivegenerations are eminently liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids

are produced by the unnatural crossing of two species, the reproductive system, independently of the

general state of health, is affected in a very similar manner. In the one case, the conditions of life have

been disturbed, though often in so slight a degree as to be inappreciable by us; in the other case, or that

of hybrids, the external conditions have remained the same, but the organisation has been disturbed by

twodistinct structures andconstitutions, including of course the reproductive systems, having been

blended into one. For it is scarcely possible that two organizations should be compounded into one,without some disturbance occurring in thedevelopment, or periodical action, ormutual relations of the

different parts and organs one to another or to the conditions of life. When hybrids are able to breedinter

se , they transmit to their offspring from generation to generation the same compounded organisation, and

hence we need not be surprised that their sterility, though in some degree variable, does not diminish; it is

even apt to increase, this being generally the result, as before explained, of too close interbreeding. The

above view of the sterility of hybrids being caused by two constitutions being compounded into one has

been stronglymaintained byMax Wichura.

It must, however, be owned that we cannot understand, on the above or any other view, several facts

with respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from

reciprocal crosses; or the increased sterility in those hybridswhich occasionally and exceptionally

resemble closely either pure parent. Nor do I pretend that the foregoing remarks go to the root of the

matter; no explanation is offered why an organism, when placed underunnatural conditions, is rendered





sterile. All that I have attempted to show is, that in two cases, in some respects allied, sterility is the

common result,—in the one case from the conditions of life having been disturbed, in the other case from

theorganisation havingbeen disturbed by twoorganisations being compounded into one.

A similar parallelism holds good with an allied yet very different class of facts. It is an old and almost

universal belief founded on a considerable body of evidence, which I have elsewhere given, that slight

changes in the conditions of life are beneficial to all living things.We see this acted on by farmers andgardeners in their frequent exchanges of seed, tubers, &c., from one soil or climate to another, and back

again. During the convalescence of animals, great benefit is derived from almost any change in their habits

of life. Again, both with plants and animals, there is the clearest evidence that a cross between individuals

of the same species, which differ to a certain extent, gives vigour and fertility to the offspring; and that

close interbreeding continuedduring several generations between the nearest relations, if these be kept

under the same conditions of life, almost always leads to decreased size, weakness, or sterility.

Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings,

and on the other hand, that slight crosses, that is crosses between the males and females of the same

species, whichhave been subjected to slightly different conditions, orwhichhave slightly varied, givevigour and fertility to the offspring. But, as we have seen, organic beings long habituated to certain

uniform conditions under a state of nature, when subjected, as under confinement, to a considerable

change in their conditions, very frequently are rendered more or less sterile; and we know that a cross

between two forms, that have become widely or specifically different, produce hybrids which are almost

always in some degree sterile. I am fully persuaded that this double parallelism is by no means an

accident or an illusion. He who is able to explain why the elephant and a multitude of other animals are

incapable of breeding when kept under only partial confinement in their native country, will be able to

explain the primary cause of hybrids being so generally sterile. He will at the same time be able to explain

how it is that the races of some of our domesticated animals, which have often been subjected to new

andnot uniform conditions, are quite fertile together, although they are descended from distinct species,

which would probably have been sterile if aboriginally crossed. The above two parallel series of factsseem to be connected together by some common but unknown bond, which is essentially related to the

principle of life; this principle, according to Mr. Herbert Spencer, being that life depends on, or consists

in, the incessant action and reaction of various forces; which, as throughout nature, are always tending

towards an equilibrium; and when this tendency is slightly disturbed by any change, the vital forces gain in

power.

Reciprocal Dimorphism and Trimorphism.

This subject may be here briefly discussed, and will be found to throw some light on hybridism. Several

plants belonging to distinct orders present two forms, which exist in about equal numbers and which differ

in no respect except in their reproductive organs; one form having a long pistil with short stamens, the

other a short pistil with long stamens; the two havingdifferently sizedpollen-grains. With trimorphic plants

there are three forms likewise differing in the lengths of their pistils and stamens, in the size and colour of

the pollen-grains, and in some other respects; and as in each of the three forms there are two sets of

stamens, the three forms possess altogether six sets of stamens and three kinds of pistils. These organs

are so proportioned in length to each other, that half the stamens in two of the forms stand on a level with

the stigma of the third form. Now I have shown, and the result has been confirmed by other observers,

that, in order to obtain full fertility with these plants, it is necessary that the stigma of the one form should

be fertilised by pollen taken from the stamens of corresponding height in another form. So that with

dimorphic species two unions, which may be called legitimate, are fully fertile; and two, which may be

called illegitimate, are moreor less infertile. With trimorphic species six unions are legitimate, or fully





fertile,—and twelve are illegitimate, ormore or less infertile.

The infertility which may be observed in various dimorphic and trimorphic plants, when they are

illegitimately fertilised, that is bypollen taken fromstamens not corresponding in heightwith the pistil,

differs much in degree, up to absolute and utter sterility; just in the same manner as occurs in crossing

distinct species. As the degree of sterility in the latter case depends in an eminent degree on the

conditions of life being more or less favourable, so I have found it with illegitimate unions. It is well knownthat if pollen of a distinct species be placed on the stigma of a flower, and its own pollen be afterwards,

even after a considerable interval of time, placed on the same stigma, its action is so strongly prepotent

that it generally annihilates the effect of the foreign pollen; so it is with the pollen of the several forms of

the same species, for legitimate pollen is stronglyprepotent over illegitimatepollen,when both are placed

on the same stigma. I ascertained this by fertilising several flowers, first illegitimately, and twenty-four

hours afterwards legitimatelywith the pollen taken from a peculiarly coloured variety, and all the seedlings

were similarly coloured; this shows that the legitimatepollen, though applied twenty-four hours

subsequently, had wholly destroyed or prevented theaction of thepreviously applied illegitimate pollen.

Again, as in making reciprocal crosses between the same two species, there is occasionally a great

difference in the result, so the same thing occurs with trimorphic plants; for instance, the mid-styled formofLythrum salicariawas illegitimately fertilisedwith the greatest ease bypollen from the longer stamens of

the short-styled form, and yielded many seeds; but the latter form did not yield a single seed when

fertilised by the longer stamens of the mid-styled form.

In all these respects, and in others which might be added, the forms of the same undoubted species

when illegitimately united behave in exactly the same manner as do two distinct species when crossed.

This ledmecarefully to observe during four years many seedlings, raised from several illegitimate unions.

The chief result is that these illegitimate plants, as they may be called, are not fully fertile. It is possible to

raise from dimorphic species, both long-styled and short-styled illegitimate plants, and from trimorphic

plants all three illegitimate forms. These can then be properly united in a legitimate manner.When this is

done, there is no apparent reason why they should not yield as many seeds as did their parents whenlegitimately fertilised. But such is not the case. They are all infertile, in various degrees; some being so

utterly and incurably sterile that they did not yield during four seasons a single seed or even seed-capsule.

The sterilityof these illegitimate plants, when unitedwith each other in a legitimate manner, may be strictly

compared with that of hybrids when crossedinter se . If, on the other hand, a hybrid is crossed with

either pure parent-species, the sterility is usually much lessened: and so it is when an illegitimate plant is

fertilised by a legitimate plant. In the same manner as the sterility of hybrids does not always run parallel

with the difficulty of making the first cross between the two parent-species, so the sterility of certain

illegitimate plantswas unusually great, whilst the sterility of the union fromwhich theywere derived was

by no means great. With hybrids raised from the same seed-capsule the degree of sterility is innately

variable, so it is in a marked manner with illegitimate plants. Lastly, many hybrids are profuse and persistent flowerers, whilst other and more sterile hybrids produce few flowers, and are weak, miserable

dwarfs; exactly similarcases occur with the illegitimate offspring of various dimorphic and trimorphic

plants.

Altogether there is the closest identity in character andbehaviour between illegitimate plants andhybrids.

It is hardly an exaggeration tomaintain that illegitimateplants arehybrids, producedwithin the limits of the

same species by the improper union of certain forms, whilst ordinary hybrids are produced from an

improper union between so-called distinct species. We have also already seen that there is the closest

similarity in all respectsbetween first illegitimate unions and first crosses betweendistinct species. This

will perhaps be made more fully apparent by an illustration; we may suppose that a botanist found two

well-marked varieties (and such occur) of the long-styled form of the trimorphic Lythrumsalicaria, and

that he determined to try by crossing whether they were specifically distinct. He would find that they

yielded only about one-fifth of the proper number of seeds, and that they behaved in all the other above





specified respects as if they had been two distinct species. But to make the case sure, he would raise

plants from his supposed hybridised seed, and he would find that the seedlings were miserably dwarfed

and utterly sterile, and that they behaved in all other respects like ordinary hybrids. He might then

maintain that he had actually proved, in accordance with the common view, that his two varieties were as

good and as distinct species as any in the world; but he would be completely mistaken.

The facts now given on dimorphic and trimorphic plants are important, because they show us, first, thatthe physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of

specific distinction; secondly, because we may conclude that there is some unknown bond which

connects the infertility of illegitimate unionswith that of their illegitimate offspring, and we are led to

extend the same view to first crosses and hybrids; thirdly, because we find, and this seems to me of

especial importance, that two or three forms of the same species may exist and may differ in no respect

whatever, either in structure or in constitution, relatively to external conditions, and yet be sterile when

united in certain ways. For we must remember that it is the union of the sexual elements of individuals of

the same form, for instance, of two long-styled forms, which results in sterility; whilst it is the union of the

sexual elements proper to two distinct forms which is fertile. Hence the case appears at first sight exactly

the reverse of what occurs, in the ordinary unions of the individuals of the same species and with crosses between distinct species. It is, however, doubtful whether this is really so; but I will not enlarge on this

obscure subject.

We may, however, infer as probable from the consideration of dimorphic and trimorphic plants, that the

sterility of distinct species when crossed and of their hybrid progeny, depends exclusively on the nature of

their sexual elements, and not on any difference in their structure or general constitution. We are also led

to this same conclusion by considering reciprocal crosses, in which the male of one species cannot be

united, or can be united with great difficulty, with the female of a second species, whilst the converse

cross can be effected with perfect facility. That excellent observer, Gärtner, likewise concluded that

species when crossed are sterile owing to differences confined to their reproductive systems.

Fertility of Varieties when Crossed, and of their Mongrel Offspring, not universal .

It may be urged, as an overwhelming argument, that there must be some essential distinction between

species and varieties, inasmuch as the latter, however much they may differ from each other in external

appearance, crosswith perfect facility, andyield perfectly fertile offspring.With some exceptions,

presently to be given, I fully admit that this is the rule. But the subject is surrounded by difficulties, for,

looking to varieties produced under nature, if two forms hitherto reputed to be varieties be found in any

degree sterile together, they are at once ranked by most naturalists as species. For instance, the blue andred pimpernel, which are considered by most botanists as varieties, are said by Gärtner to be quite sterile

when crossed, and he subsequently ranks them as undoubted species. If we thus argue in a circle, the

fertility of all varieties produced under nature will assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced, under domestication, we are still

involved in some doubt. For when it is stated, for instance, that certain South American indigenous

domestic dogs do not readily unite with European dogs, the explanation which will occur to every one,

and probably the true one, is that they are descended from aboriginally distinct species. Nevertheless the

perfect fertility of so many domestic races, differing widely from each other in appearance, for instance

those of the pigeon, or of the cabbage, is a remarkable fact; more especially when we reflect how many

species there are, which, though resembling each other most closely, are utterly sterile when intercrossed.

Several considerations, however, render the fertility of domestic varieties less remarkable. In the first

place, it may be observed that the amount of external difference between two species is no sure guide to





their degree of mutual sterility, so that similar differences in the case of varieties would be no sure guide.

It is certain that with species the cause lies exclusively in differences in their sexual constitution. Now the

varying conditions to which domesticated animals and cultivated plants have been subjected, have had so

little tendency towards modifying the reproductive system in a manner leading tomutual sterility, thatwe

have good grounds for admitting thedirectly opposite doctrineof Pallas, namely, that such conditions

generally eliminate this tendency; so that the domesticated descendants of species, which in their natural

state probably would have been in some degree sterile when crossed, become perfectly fertile together.With plants, so far is cultivation from giving a tendency towards sterility between distinct species, that in

several well-authenticated cases already alluded to, certain plants have been affected in an opposite

manner, for they have becomeself-impotent whilst still retaining the capacity of fertilising, and being

fertilised by, other species. If the Pallasian doctrine of the elimination of sterility through long-continued

domestication be admitted, and it can hardly be rejected, it becomes in the highest degree improbable

that similar conditions long-continued should likewise induce this tendency; though in certain cases, with

species havinga peculiar constitution, sterility might occasionallybe thus caused. Thus, as I believe, we

canunderstand why with domesticated animals varieties have not been produced which aremutually

sterile; and why with plants only a few such cases, immediately to be given, have been observed.

The real difficulty in our present subject is not, as it appears to me, why domestic varieties have not

become mutually infertile when crossed, but why this has so generally occurredwith natural varieties, as

soon as they have been permanently modified in a sufficient degree to take rank as species. We are far

from precisely knowing the cause; nor is this surprising, seeing how profoundly ignorant we are in regard

to the normal and abnormal action of the reproductive system. But we can see that species, owing to

their struggle for existence with numerous competitors, will have been exposed during long periods of

time to more uniform conditions, than have domestic varieties; and this may well make a wide difference

in the result. For we know how commonly wild animals and plants, when taken from their natural

conditions and subjected to captivity, are rendered sterile; and the reproductive functions of organic

beingswhichhave always lived under natural conditionswould probably in likemanner be eminently

sensitive to the influence of an unnatural cross. Domesticated productions, on the other hand, which, asshown by the mere fact of their domestication,were not originally highly sensitive to changes in their

conditions of life, and which can now generally resistwith undiminished fertility repeated changes of

conditions, might be expected to produce varieties, which would be little liable to have their reproductive

powers injuriously affected by the act of crossing with other varieties which had originated in a like

manner.

I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. But

it is impossible to resist the evidence of the existence of a certain amount of sterility in the few following

cases, which I will briefly abstract. The evidence is at least as good as that from which we believe in the

sterility of a multitude of species. The evidence is, also, derived from hostile witnesses, who in all other cases consider fertility andsterility as safe criterions of specific distinction.Gärtner kept during several

years a dwarf kind of maize with yellow seeds, and a tall variety with red seeds growing near each other

in his garden; and although these plants have separated sexes, they never naturally crossed. He then

fertilised thirteen flowers of the one kind with pollen of the other; but only a single head produced any

seed, and this one head produced only five grains. Manipulation in this case could not have been

injurious, as the plants have separated sexes. No one, I believe, has suspected that these varieties of

maize are distinct species; and it is important to notice that the hybrid plants thus raised were themselves

perfectly fertile; so that even Gärtner did not venture to consider the two varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the maize has separated sexes, and

he asserts that their mutual fertilisation is by so much the less easy as their differences are greater. How

far these experiments may be trusted, I know not; but the forms experimented on are ranked by Sageret,

who mainly founds his classification by the test of infertility, as varieties, and Naudin has come to the





same conclusion.

The following case is far more remarkable, and seems at first incredible; but it is the result of an

astonishing number of experiments made during many years on nine species of Verbascum, by so good

an observer and so hostile a witness as Gärtner: namely that the yellow and white varieties when crossed

produce less seed than the similarly coloured varieties of the same species. Moreover, he asserts that,

when yellow and white varieties of one species are crossed with yellow and white varieties of adistinct species, more seed is produced by the crosses between the similarly coloured flowers, than between

those which are differently coloured. Mr. Scott also has experimented on the species and varieties of

Verbascum; and although unable to confirm Gärtner's results on the crossing of the distinct species, he

finds that the dissimilarly coloured varieties of the same species yield fewer seeds, in the proportion of 86

to 100, than the similarly coloured varieties. Yet these varieties differ in no respect except in the colour of

their flowers; and one variety can sometimes be raised from the seed of another.

Kölreuter, whose accuracy has been confirmed by every subsequent observer, has proved the

remarkable fact, that one particular variety of the common tobacco was more fertile than the other

varieties, when crossed with a widely distinct species. He experimented on five forms which arecommonly reputed to be varieties, and which he tested by the severest trial, namely, by reciprocal

crosses, and he found their mongrel offspring perfectly fertile. But one of these five varieties, when used

either as the father or mother, and crossed with the Nicotiana glutinosa, always yielded hybrids not so

sterile as those which were produced from the four other varieties when crossed with N. glutinosa.

Hence the reproductive system of this one variety must have been in some manner and in some degree

modified.

From these facts it can no longer be maintained that varieties when crossed are invariably quite fertile.

From the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed

variety, if proved to be infertile in any degree, would almost universally be ranked as a species;—from

man attending only to external characters in his domestic varieties, and from such varieties not having been exposed for very long periods to uniform conditions of life;—from these several considerationswe

mayconclude that fertilitydoes not constitute a fundamental distinction between varieties and species

when crossed. The general sterility of crossed species may safely be looked at, not as a special

acquirement or endowment, but as incidental on changes of an unknown nature in their sexual elements.

Hybrids and Mongrels compared, independently of their fertility.

Independently of the question of fertility, the offspring of species and of varieties when crossed may becompared in several other respects. Gärtner, whose strong wish it was to draw a distinct line between

species and varieties, could find very few, and, as it seems to me, quite unimportant differences between

the so-called hybrid offspring of species, and the so-called mongrel offspring of varieties. And, on the

other hand, they agree most closely in many important respects.

I shall here discuss this subject with extreme brevity. The most important distinction is, that in the first

generationmongrels aremore variable than hybrids; but Gärtner admits that hybrids from specieswhich

have long been cultivated are often variable in the first generation; and I have myself seen striking

instances of this fact. Gärtner further admits that hybrids between very closely allied species are more

variable than those from very distinct species; and this shows that the difference in the degree of

variability graduates away. When mongrels and the more fertile hybrids are propagated for several

generations, an extreme amount of variability in the offspring in both cases is notorious; but some few

instancesof both hybrids and mongrels long retaining a uniform character could be given.The variability,









why, in the case of distinct species, the sexual elements should so generally have become more or less

modified, leading to their mutual infertility, we do not know; but it seems to stand in some close relation

to species having been exposed for long periods of time to nearly uniform conditions of life.

It is not surprising that the difficulty in crossing any two species, and the sterility of their hybrid offspring,

should in most cases correspond, even if due to distinct causes: for both depend on the amount of

difference between the species which are crossed. Nor is it surprising that the facility of effecting a firstcross, and the fertility of the hybrids thus produced, and the capacity of being grafted together—though

this latter capacity evidently depends onwidelydifferent circumstances—should all run, to a certain

extent, parallel with the systematic affinity of the forms subjected to experiment; for systematic affinity

includes resemblancesof all kinds.

First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and

their mongrel offspring, are very generally, but not, as is so often stated, invariably fertile. Nor is this

almost universal and perfect fertility surprising, when it is remembered how liable we are to argue in a

circle with respect to varieties in a state of nature; and when we remember that the greater number of

varieties have been produced under domestication by the selection of mere external differences, and thatthey have not been long exposed to uniform conditions of life. It should also be especially kept in mind,

that long-continued domestication tends to eliminate sterility, and is therefore little likely to induce this

same quality. Independently of the question of fertility, in all other respects there is the closest general

resemblance between hybrids andmongrels,—in their variability, in their power of absorbing each other

by repeated crosses, and in their inheritance of characters from both parent-forms. Finally, then, although

we are as ignorant of the precise cause of the sterility of first crosses and of hybrids as we are why

animals and plants removed from their natural conditions become sterile, yet the facts given in this chapter

do not seem to me opposed to the belief that species aboriginally existed as varieties.

|Go to Contents |

Contents

Additions and Corrections, to the Sixth Edition

Historical Sketch

Introduction

Chapter I.VariationUnderDomestication. Causes of Variability—Effects of Habit and theuse or disuse

of Parts—Correlated Variation—Inheritance—Character of Domestic Varieties—Difficulty of

distinguishing between Varieties and Species—Origin of domestic varieties from one ormore

species—Domestic Pigeons, their Differences andOrigin—Principles of Selection, anciently followed,

their Effects—Methodical andUnconscious Selection—UnknownOrigin of ourDomestic

Productions—Circumstances favourable to Man's power of Selection

Chapter II. Variation Under Nature.Variability—Individual differences—Doubtful species—Wide

ranging, much diffused, and common species, vary most—Species of the larger genera in each country





vary more frequently than the species of the smaller genera—Many of the species of the larger genera

resemble varieties in being very closely, but unequally, related to each other, and in having restricted

ranges

Chapter III. Struggle for Existence. Its bearing on natural selection—The term used in a wide

sense—Geometrical ratio of increase—Rapid increase of naturalised animals andplants—Nature of the

checks to increase—Competition universal—Effects ofClimate—Protection from the number of individuals—Complex relations of all animals andplants throughout nature—Struggle for lifemost severe

between individuals and varieties of the same species: often severe between species of the same

genus—The relation of organism to organism the most important of all relations

Chapter IV. Natural Selection; or the Survival of the Fittest. Natural Selection—its power compared

with man's selection—its power on characters of trifling importance—its power at all ages and on both

sexes—Sexual selection—On thegenerality of intercrosses between individuals of the same

species—Circumstances favourable and unfavourable to the results of Natural Selection, namely,

intercrossing, isolation, number of individuals—Slow action—Extinction caused by Natural

Selection—Divergence of Character, related to the diversity of inhabitants of any small area, and tonaturalisation—ActionofNatural Selection, through Divergence ofCharacter andExtinction, on the

descendants from a commonparent—Explains the grouping of all organic beings—Advance in

organisation—Low forms preserved—Convergence of Character—Indefinite multiplication of

species—Summary

Chapter V. Laws of Variation. Effects of changed conditions—Use anddisuse, combinedwith natural

selection; organs of flight and of vision—Acclimatisation—Correlated variation—Compensation and

economyof growth—False correlations—Multiple, rudimentary, and lowly organised structures

variable—Parts developed in an unusual manner are highly variable; specific charactersmore variable

than generic: secondary sexual characters variable—Species of the same genus vary in an analogous

manner—Reversions to long-lost characters—Summary

Chapter VI.Difficultiesof the Theory. Difficultiesof the theory of descent withmodification—Absence

or rarity of transitional varieties—Transitions in habits of life—Diversified habits in the same

species—Species with habitswidelydifferent from those of their allies—Organs of extreme

perfection—Modes of transition—Cases of difficulty—Natura non facit saltum—Organs of small

importance—Organs not in all cases absolutely perfect—The law of Unity of Type and of the Conditions

of Existence embraced by the theory of Natural Selection

Chapter VII. Miscellaneous Objections to theTheory ofNatural Selection. Longevity—Modifications

not necessarily simultaneous—Modifications apparently of no direct service—Progressivedevelopment—Characters of small functional importance, the most constant—Supposed incompetence

of natural selection to account for the incipient stages of useful structures—Causeswhich interfere with

the acquisition through natural selection of useful structures—Gradations of structure with changed

functions—Widely different organs in members of the same class, developed from one and the same

source—Reasons for disbelieving in great and abrupt modifications

Chapter VIII. Instinct. Instincts comparable with habits, but different in their origin—Instincts

graduated—Aphides and ants—Instincts variable—Domestic instincts, their origin—Natural instincts of

the cuckoo, Molothrus, ostrich, and parasitic bees—Slave-making ants—Hive-bee, its cell-making

instinct—Changes of instinct and structure not necessarily simultaneous—Difficulties of the theoryof the

Natural Selectionof instincts—Neuter or sterile insects—Summary





Vol. II.

Chapter IXHybridism.Distinction between the sterility of first crosses and of hybrids-Sterility various in

degree, not universal, affected by close interbreeding, removed by domestication—Lawsgoverning thesterility of hybrids—Sterility not a special endowment, but incidental on otherdifferences, not

accumulated by natural selection—Causes of the sterilityof first crosses and of hybrids—Parallelism

between theeffects of changed conditions of life and of crossing—Dimorphismand

trimorphism—Fertilityof varieties when crossed and of their mongrel offspring not universal—Hybrids

andmongrels compared independently of their fertility—Summary

Chapter X. On the Imperfection of the Geological Record. On the absence of intermediate varieties at

the present day—On the nature of extinct intermediate varieties; on their number-On the lapse of time, as

inferred from the rate of denudation and of deposition—On the lapse of time as estimated by years—On

the poorness of our palæntological collections—Onthe intermittence of geological formations—On the

denudation of granitic areas—On the absence of intermediate varieties in anyone formation—On the

sudden appearance of groups of species—On their sudden appearance in the lowest known fossiliferous

strata—Antiquityof the habitable earth

Chapter XI. On the Geological Succession of Organic Beings. On the slow and successive appearance

of new species—On their different rates of change—Species once lost do not reappear—Groups of

species follow the same general rules in their appearance and disappearance as do single species—On

extinction—Onsimultaneous changes in the forms of life throughout the world—On the affinities of

extinct species to each other and to living species—On the state of development of ancient forms—On

the succession of the same types within the same areas—Summary of preceding and present chapter

Chapter XII. GeographicalDistribution— continued

Present distribution cannot be accounted for by differences in physical conditions—Importanceof

barriers—Affinity of the productions of the same continent—Centres of creation—Meansof dispersal by

changes of climate and of the level of the land and by occasional means—Dispersal during the Glacial

period—Alternate Glacial periods in the North and South

Chapter XIII. Geographical Distribution—continued.Distribution of fresh-water productions—On the

inhabitants of oceanic islands—AbsenceofBatrachians and of terrestrial Mammals—On the relation of

the inhabitants of islands to those of the nearest mainland—On colonisation from the nearest source with

subsequentmodification—Summaryof the last andpresent chapter

Chapter XIV. Mutual Affinities of Organic Beings:Morphology: Embryology: RudimentaryOrgans.

Classification, groups subordinate to groups—Natural system-Rules and difficulties in classification,

explained on the theory of descent withmodification—Classification of varieties—Descent always used

in classification—Analogical or adaptive characters—Affinities, general, complex, and

radiating—Extinction separates and defines groups—Morphology, between members of the same class,

between parts of the same individual—Embryology, laws of, explained byvariations not supervening at

an early age, andbeing inherited at a corresponding age—RudimentaryOrgans; their origin

explained—Summary

Chapter XV. Recapitulation and Conclusion. Recapitulation of the objections to the theory of NaturalSelection—Recapitulationof thegeneral and special circumstances in its favour—Causesof the general

belief in the immutability of species—Howfar the theory ofNatural Selection maybe extended—Effects





of its adoption on the study of Natural History—Concluding remarks

Glossary of Scientific Terms

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Book Design by Ericka O'Rourke, Elm Design

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ISBN 0-594-04113-9

Glossary of the

Principal Scientific Terms Used in the

Present Volume.1

ABERRANT.—Forms or groups of animals or plants which deviate in important characters from their

nearest allies, so as not to be easily included in the same group with them, are said to be aberrant.

ABERRATION (in Optics).—In the refraction of light by a convex lens the rays passing throughdifferent parts of the lens are brought to a focus at slightly different distances,—this is called spherical

aberration ; at the same time the coloured rays are separated by the prismatic action of the lens and

likewise brought to a focus at different distances,—this is chromatic aberration.

ABNORMAL.—Contrary to the general rule.

ABORTED.—An organ is said to be aborted, when its development has been arrested at very early

stage.

ALBINISM.—Albinos are animals inwhich theusual colouring matters characteristic of the species

have not been produced in the skin and its appendages. Albinism is the state of being an albino.

ALGAE.—A class of plants including the ordinary sea-weeds and the filamentous fresh-water weeds.





ALTERNATION OF GENERATIONS.—This term is applied to a peculiar mode of reproduction

which prevails among many of the lower animals, in which the egg produces a living form quite different

from its parent, but from which the parent-form is reproduced by a process of budding, or by the division

of the substance of the first product of the egg.

AMMONITES.—A group of fossil, spiral, chambered shells, allied to the existing pearly Nautilus, buthaving the partitions between the chambers waved in complicated patterns at their junction with the outer

wall of the shell.

ANALOGY.—That resemblance of structureswhichdepends upon similarity of function, as in the wings

of insects and birds. Such structures are said to beanalogous , and to beanalogues of each other.

ANIMALCULE.—A minute animal: generally applied to those visible only by the microscope.

ANNELIDS.—A class of worms in which the surface of the body exhibits a more or less distinct

division into rings or segments, generally providedwith appendages for locomotion and with gills. Itincludes the ordinary marine worms, the earthworms, and the leeches.

ANTENNÆ.—Jointed organs appended to the head in Insects, Crustacea and Centipedes, and not

belonging to the mouth.

ANTHERS.—The summits of the stamens of flowers, in which the pollen or fertilising dust is produced.

APLACENTALIA, APLACENTATA or AplacentalMammals. SeeMammalia .

ARCHETYPAL.—Of or belonging to the Archetype, or ideal primitive form upon which all the beings

of a group seem to be organised.

ARTICULATA.—A greatdivision of the Animal Kingdom characterised generallybyhaving the surface

of the body divided into rings called segments, a greater or less number of which are furnished with

jointed legs (such as Insects, Crustaceans and Centipedes).

ASYMMETRICAL.—Having the two sides unlike.

BALANUS.—Arrested in development at a very early stage.

BALANUS.—The genus including the common Acorn-shells which live in abundance on the rocks of

the sea-coast.

BATRACHIANS.—Aclass of animals allied to theReptiles, but undergoinga peculiarmetamorphosis,

in which the young animal is generally aquatic and breathes by gills. ( Examples, Frogs, Toads, and

Newts.)

BOULDERS.—Large transported blocks of stone generally imbedded in clays or gravels.

BRACHIOPODA.—A class ofmarineMollusca, or soft-bodied animals, furnished with a bivalve shell,

attached to submarine objects by a stalk which passes through an aperture in one of the valves, and

furnished with fringed arms, by the action of which food is carried to the mouth.





BRANCHIÆ.—Gills or organs for respiration in water.

BRANCHIAL.—Pertaining to gills or branchiæ.

CAMBRIAN SYSTEM.—A Series of very ancient Palæozoic rocks, between the Laurentian and the

Silurian. Until recently these were regarded as the oldest fossiliferous rocks.

CANIDÆ.—The Dog-family, including the Dog, Wolf, Fox, Jackal, &c.

CARAPACE.—The shell enveloping the anterior part of the body in Crustaceans generally; applied also

to the hard shelly pieces of the Cirripedes.

CARBONIFEROUS.—This term is applied to the great formation which includes, among other rocks,

the coal-measures. It belongs to the oldest, or Palæozoic, system of formations.

CAUDAL.—Of or belonging to the tail.

CEPHALOPODS.—The highest class of the Mollusca, or soft-bodied animals, characterised by having

the mouth surrounded by a greater or less number of fleshy arms or tentacles, which, in most living

species, are furnishedwith sucking-cups. ( Examples, Cuttle-fish, Nautilus.)

CETACEA.—An order of Mammalia, including the Whales, Dolphins, &c., having the form of the body

fish-like, the skin naked, and only the fore-limbs developed.

CHELONIA.—An order of Reptiles including the Turtles, Tortoises, &c.

CIRRIPEDES.—An order of Crustaceans including theBarnacles andAcorn-shells. Their youngresemble those of many other Crustaceans in form; but when mature they are always attached to other

objects, either directly or by means of a stalk, and their bodies are enclosed by a calcareous shell

composed of several pieces, two of which can open to give issue to a bunch of curled, jointed tentacles,

which represent the limbs.

COCCUS.—The genus of Insects including the Cochineal. In these the male is a minute, winged fly, and

the femalegenerallya motionless, berry-like mass.

COCOON.—A case usually of silky material, in which insects are frequently enveloped during the

second or resting-stage (pupa) of their existence. The term "cocoon-stage" is here used as equivalent to"pupa-stage."

CŒLOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which have the seed hollowed

on the inner face.

COLEOPTERA.—Beetles, an order of Insects, having a biting mouth and the first pair of wings more or

less horny, forming sheaths for the second pair, and usually meeting in a straight line down the middle of

the back.

COLUMN.—A peculiar organ in the flowers of Orchids, in which the stamens, style and stigma (or the

reproductive parts) are united.

COMPOSITE or COMPOSITOUS PLANTS.—Plants in which the inflorescence consists of





numerous small flowers (florets) brought together into a dense head, the base of which is enclosed by a

common envelope. ( Examples, the Daisy, Dandelion, &c.)

CONFERVÆ.—The filamentous weeds of fresh water.

CONGLOMERATE.—A rock made up of fragments of rook or pebbles, cemented together by some

other material

COROLLA.—The second envelope of a flower usually composed of coloured, leaf-like organs

(petals), which may be united by their edges either in the basal part or throughout.

CORRELATION.—The normal coincidence of one phenomenon, character,&c., with another.

CORYMB.—A bunch of flowers in which those springing from the lower part of the flower stalk are

supported on long stalks so as to be nearly on a level with the upper ones.

COTYLEDONS.—The first or seed-leaves of plants.

CRUSTACEANS.—A class of articulated animals, having the skin of the body generally more or less

hardened by the deposition of calcareous matter breathing by means of gills. ( Examples, Crab, Lobster,

Shrimp,&c.)

CURCULIO.—The old generic term for the Beetles known as Weevils, characterised by their four

jointed feet, and by the head being produced into a sort of beak, upon the sides of which the antennæ are

inserted.

CUTANEOUS—Of or belonging to the skin.

DEGRADATION.—The wearing down of land by the action of the sea or of meteoric agencies.

DENUDATION.—The wearing away of the surface of the land by water.

DEVONIAN SYSTEM, or formation.—A series of Palæozoic rocks, including the Old Red

Sandstone.

DICOTYLEDONS or DICOTYLEDONOUS PLANTS.—A class of plants characterised by having

two seed-leaves, by the formation of new wood between the bark and the old wood (exogenous growth)and by the reticulation of the veins of the leaves. The parts of the flowers are generally in multiples of five.

DIFFERENTIATION.—The separation or discrimination of parts or organs which in simpler forms of

life are more or less united.

DIMORPHIC.—Having twodistinct forms.—Dimorphism is the conditionof the appearance of the

same species under two dissimilar forms.

DIŒCIOUS.—Having, the organs of the sexesupon distinct individuals.

DIORITE.—A peculiar form of Greenstone.

DORSAL.—Of or belonging to the back.





EDENTATA.—A peculiar order of Quadrupeds, characterised by the absence of at least the middle

incisor (front) teeth in both jaws. ( Examples, the Sloths and Armadillos.)

ELYTRA.—The hardened fore-wings of Beetles, serving as sheaths for the membranous hind-wings,

which constitute the true organs of flight.

EMBRYO.—The young animal undergoingdevelopment within the egg orwomb.

EMBRYOLOGY.—The study of the development of the embryo.

ENDEMIC,—Peculiar to a given locality.

ENTOMOSTRACA.—A division of the class Crustacea, having all the segments of the body usually

distinct, gills attached to the feet or organs of the mouth, and the feet fringed with fine hairs. They are

generallyof small size.

EOCENE.—The earliest of the three divisions of the Tertiary epoch of geologists. Rocks of this age

containa small proportionof shells identicalwith species now living.

EPHEMEROUS INSECTS.—Insects allied to the May-fly.

FAUNA.—The totality of the animalsnaturally inhabiting a certain country or region, orwhich have lived

during a given geological period.

FELIDÆ.—The Cat-family.

FERAL.—Having become wild from a state of cultivation or domestication.

FLORA.—The totality of the plants growing naturally in a country, or during a given geological period.

FLORETS.—Flowers imperfectly developed in some respects, and collected into a dense spike or

head, as in the Grasses, the Dandelion, &c.

FŒTAL.—Of or belonging to the fætus, or embryo in course of development.

FORAMINIFERA.—A class of animals of very low organisation, and generally of small size, having a

jelly-like body, from the surface of which delicate filaments can be given off and retracted for the

prehension of external objects, and having a calcareous or sandy shell, usually divided into chambers, and

perforated with small apertures.

FOSSILIFEROUS.—Containing fossils.

FOSSORIAL.—Having a faculty of digging. The Fossorial Hymenoptera are a group of Wasp-like

Insects, which burrow in sandy soil to make nests for their young.

FRENUM (pl. FRENA).—A small band or fold of skin.

FUNGI (sing. FUNGUS).—A class of cellular plants, of which Mushrooms, Toadstools, and Moulds,





are familiar examples.

FLAURCU.—The forked bone formed by the union of the collarbones in many birds, such as the

commonFowl.

GALLINACEOUS BIRDS.—An order of Birds of which the common Fowl , Turkey, and Pheasant, are well-known examples.

GALLUS.—The genus of birds which includes the common Fowl.

GANGLION.—A swelling or knot from which nerves are given off as from a centre.

GANOID FISHES.—Fishes covered with peculiar enamelled bony scales. Most of them are extinct.

GREMINAL VESICLE.—A minute vesicle in the eggs of animals, from which development of the

embryo proceeds.

GLACIAL PERIOD.—A period of great cold and of enormous extension of ice upon the surface of the

earth. It is believed that glacial periods have occurred repeatedly during the geological history of the

earth, but the term is generally applied to the close of the Tertiary epoch, when nearly the whole of

Europe was subjected to an arctic climate.

GLAND.—An organ which secretes or separates some peculiar product from the blood or sap of

animalsor plants.

GLOTTIS.—The opening of the windpipe into the æsophagus or gullet.

GNEISS.—A rock approaching granite in composition, but more or less laminated, and really produced

by the alteration of a sedimentary deposit after its consolidation.

GRALLATORES.—The so-called Wading-birds (Storks, Cranes, Snipes, &c.), which are generally

furnished with long legs, bare of feathers above the heel, and have no membranes between the toes.

GRANITE.—A rock consisting essentially of crystals of felspar and mica in a mass of quartz.

HABITAT.—The locality inwhich a plant or animal naturally lives.

HEMIPTERA.—An order or sub-order of Insects, characterised by the possession of a jointed beak or

rostrum, and by having the fore-wings horny in the basal portion and membranous at the extremity, where

they cross each other. This group includes the various species of Bugs.

HERMAPHRODITE.—Possessing the organs of both sexes.

HOMOLOGY.—That relation between parts which results from their development from corresponding

embryonic parts, either in different animals, as in the case of the arm of man, the fore-leg of a quadruped,

and the wing of a bird; or in the same individual, as in the case of the fore and hind legs in quadrupeds,

and the segments or rings and their appendages of which the body of a worm, a centipede, &c., is

composed. The latter is called serial homology . The parts which stand in such a relation to each other

are said to behomologous , and one such part or organ is called thehomologue of the other. In different





plants the parts of the flower are homologous, and in general these parts are regarded as homologous

with leaves.

HOMOPTERA.—An order or sub-order of Insects having (like the Hemiptera) a jointed beak, but in

which the fore-wings are either whollymembranous orwholly leathery. TheCicadæ , Frog-hoppers, and

Aphides , are well-known examples.

HYBRID.—The offspring of the union of two distinct species.

HYMENOPTERA.—An orderof insects possessingbiting jaws and usually, four membranouswings in

which there are a few veins. Bees andWasps are familiar examples of this group.

HYPERTROPHIED.—Excessively developed.

ICHNEUMONIDÆ.—A family of Hymenopterous insects, the members of which lay their eggs in the

bodies or eggs of other insects.

IMAGO.—The perfect (generally winged) reproductive state of an insect.

INDIGENS.—The aboriginal animal or vegetable inhabitants of a country or region.

INFLORESCENCE.—The mode of arrangement of the flowers of plants.

INFUSORIA.—A class ofmicroscopic Animalcules, so called from their having originallybeen

observed in infusions of vegetable matters. They consist of a gelatinous material enclosed in a delicate

membrane, the whole or part of which is furnished with short vibrating hairs(called cilia); by means of

which the animalcules swim through the water or convey the minute particles of their food to the orifice of themouth.

INSECTIVOROUS.—Feeding on Insects.

INVERTEBRATA, or INVERTEBRATE ANIMALS.—Those animals which do not possess a

backboneor spinal column.

LACUNÆ.—Spaces left among the tissues in some of the lower animals, and serving in place of vessels

for the circulation of the fluids of the body.

LAMELLATED.—Furnishedwith lamellæor little plates.

LARVA (pl. LARVÆ).-.—The first condition of an insect at its issuing from the egg, when it is usually in

the form of a grub, caterpillar, or maggot.

LARYNX.—The upper part of the windpipe opening into the gullet.

LAURENTIAN.—A group of greatly altered and very ancient rocks, which is greatly developed along

the course of the St. Laurence, whence the name. It is in these that the earliest known traces of organic

bodies have been found.

LEGUMINOSÆ.—An order of plants represented by the common Peas and Beans, having an irregular





flower in which one petal stands up like a wing, and the stamens and pistil are enclosed in a sheath

formed by two other petals. The fruit is a pod (or legume).

LEMURIDÆ.—Agroup of four-handedanimals, distinct from the Monkeys andapproaching the

Insectivorous Quadrupeds in some of their characters and habits. Its members have the nostrils curved or

twisted, and a claw instead of a nail upon the first finger of the hind hands.

LEPIDOPTERA.—An order of Insects, characterised by the possession of a spiral proboscis, and of

four large more f wings. It includes the well-known Butterflies and Moths.

LITTORAL.—Inhabiting the seashore.

LOESS.—A marly deposit of recent (Post-Tertiary) date, which occupies a great part of the valley of

theRhine.

MALACOSTRACA.—The higher division of theCrustacea, including theordinary Crabs, Lobsters,Shrimps, &c., together with the Woodlice and Sand-hoppers.

MAMMALIA.—Thehighest class of animals, including theordinary hairyquadrupeds, theWhales, and

Man, and characterised by the production of living young which are nourished after birth by milk from the

teats(Mamnmæ, Mammary glands) of the mother. A striking difference in embryonic development has

led to the division of this class into two great groups; in one of these, when the embryo has attained a

certain stage, a vascular connection, called the placenta , is formed between the embryo and the mother;

in the other this is wanting, and the young are produced in a very incomplete state. The former, including

the greater part of the class, are called Placental mammals ; the latter, or Aplacental mammals , include

theMarsupials andMonotremes(Ornithorhynchus) .

MAMMIFEROUS. Having mammæ or teats (see MAMMALIA)

MANDIBLES, in Insects.—The first or uppermost pair of, jaws, which are generally solid, horny, biting

organs. In Birds the term is applied to both jaws with their horny coverings. In Quadrupeds the mandible

is properly the lower jaw.

MARSUPIALS.—An order of Mammalia in which the young are born in a very incomplete state of

development, and carried by the mother, while sucking, in a ventral pouch marsupium), such as the

Kangaroos, Opossums, &c. (see MAMMALIA).

MAXILLÆ, in Insects.—The second or lower pair of jaws, which are composed of several joints and

furnishedwith peculiar jointed appendages called palpi, or feelers.

MELANISM.—The opposite of albinism; an undue development of colouring material in the skin and its

appendages.

METAMORPHIC ROCKS.—Sedimentary rocks which have undergone alteration, generally by the

action of heat, subsequently to their deposition andconsolidation.

MOLLUSCA.—One of the great divisions of the Animal Kingdom, including those animalswhichhave

a soft body, usually furnished with a shell, and in which the nervous ganglia, or centres, present no definite

general arrangement. They aregenerallyknown under the denomination of "shell-fish;" the cuttle-fish, and

the common snails, whelks, oysters, mussels, and cockles, may serve as examples of them.





MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.—Plants in which the seed sends

up only a single seed-leaf (or cotyledon); characterised by the absence of consecutive layers of wood in

the stem (endogenous growth), by the veins of the leaves being generally straight, and by the parts of the

flowers being generally inmultiples of three. ( Examples, Grasses, Lilies, Orchids, Palms, &c.)

MORAINES.—The accumulations of fragments of rock brought down by glaciers.

MORPHOLOGY.—The law of form or structure independent of function.

MYSIS-STAGE.—A stage in the development of certain Crustaceans (Prawns), in which they closely

resemble the adults of a genus(Mysis) belonging to a slightly lower group.

NASCENT.—Commencing development.

NATATORY.—Adapted for the purpose of swimming.

NAUPLIUS-FORM.—The earliest stagein the development ofmany Crustacea, especially belonging to

the lower groups. In this stage the animal has a short body, with indistinct indications of a division into

segments, and three pairs of fringed limbs. This form of the common fresh-water Cyclops was described

as a distinct genus under the name of Nauplius .

NEURATION.—The arrangement of the veins or nervures in the wings of Insects.

NEUTERS.—Imperfectly developed females of certain social insects (such as Ants and Bees), which

perform all the labours of the community. Hence they are also calledworkers .

NICTITATING MEMBRANE.—A semi-transparent membrane, which can be drawn across the eye in

Birds and Reptiles, either to moderate the effects of a strong light or to sweep particles of dust, &c., from

the surface of the eye.

OCELLI.—The simple eyes or stemmata of Insects, usually situated on the crown of the head between

the great compound eyes.

ŒSOPHAGUS.—The gullet.

OOLITIC.—A great series of secondary rocks, so called from the texture of some of its members,

which appear to be made up of a mass of smallegg-like calcareous bodies.

OPERCULUM.—A calcareous plate employed by many Mollusca to close the aperture of their shell.

Theopercular valves of Cirripedes are those which close the aperture of the shell.

ORBIT.—The bony cavity for the reception of the eye.

ORGANISM.—An organised being, whether plant or animal.

ORTHOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which have the seed straight.

OSCULANT.—Forms or groups apparently intermediate between and connecting other groups are





said to be osculant.

OVA.—Eggs.

OVARIUM or OVARY (in plants).—The lower part of the pistil or female organ of the flower,

containing the ovules or incipient seeds; by growth after the other organs of the flower have fallen, it

usuallybecomes converted into the fruit.

OVARIUM or OVARY .—Egg-bearing.

OVULES (of plants).—The seeds in the earliest condition.

PACHYDERMS.—A group of Mammalia, so called from their thick skins, and including the Elephant,

Rhinoceros, Hippopotamus,&c.

PALÆOZOIC.—The oldest system of fossiliferous rocks.

PALPI.—Jointed appendages to some of the organs of the mouth in

Insects and Crustacea.

PAPILIONACEÆ.—An order of Plants (see LEGUMINOSÆ).—The flowers of these plants are

called papilionaceous , or butterfly-like, from the fancied resemblance of the expanded superior petals to

the wings of a butterfly.

PARASITE.—An animal or plant living upon or in, and at the expense of, another organism.

PARTHENOGENESIS.—The productionof living organisms from unimpregnated eggs or seeds.

PEDUNCULATED.—Supported upon a stem or stalk. The pedunculated oak has its acorns borne

upon a footstalk.

PELORIA or PELORISM.—The appearance of regularity of structure in the flowers of plants which

normally bear irregular flowers.

PELVIS.—The bony arch to which the hind limbs of vertebrate animals are articulated.

PETALS.—The leaves of the corolla, or second circle of organs in a flower. They are usually of delicate

texture and brightly coloured.

PHYLLODINEOUS.—Having flattened, leaf-like twigs or leafstalks instead of, true leaves.

PIGMENT.—The colouring material produced generally in the superficial partsof animals. The cells

secreting it are called pigment-cells .

PINNATE.—Bearing leaflets on each side of a central stalk.

PISTILS.—The female organs of a flower, which occupy a position in the centre of the other floral

organs. The pistil is generally divisible into the ovary or germen, the style and the stigma





PLACENTALIA, PLACENTATA, or Placental Mammals—See MAMMALIA.

PLANTIGRADES.—Quadrupeds which walk upon the whole sole of the foot, like the Bears.

PLASTIC.—Readily capable of change.

PLEISTOCENE PERIOD.—The latest portion of the Tertiary epoch.

PLUMULE (in plants).—The minute bud between the seed-leavesof newly-germinated plants.

PLUTONIC ROCKS.—Rocks supposed to have been produced by igneous action in the depths of the

earth.

POLLEN—The male element in flowering plants; usually a fine dust produced by the anthers, which, by

contact with the stigma effects the fecundation of the seeds. This impregnation is brought about by means

of tubes(pollen-tubes) which issue from the pollen-grains adhering to the stigma, and penetrate through

the tissues until they reach the ovary.

POLYANDROUS(flowers).—Flowers havingmany stamens.

POLOYAMOUS PLANTS.—Plants in which some flowers are unisexual and others hermaphrodite.

The unisexual (male and female) flowers, may be on the same or on different plants.

POLYMORPHIC.—Presenting many forms.

POLYZOARY.—The common structure formed by the cells of the Polyzoa, such as the well-known

Sea-mats.

PREHENSILE.—Capable of grasping.

PREPOTENT.—Having a superiority of power.

PRIMARIES.—The feathers forming the tip of the wing of a bird, and inserted upon that part which

represents the hand of man.

PROCESSES.—Projecting portions of bones, usually for the attachment of muscles, ligaments, &c.

PROPOLIS.—A resinous material collected by the Hive-Bees from the opening buds of various trees.

PROTEAN.—Exceedingly variable.

PROTOZOA.—The lowest great division of the Animal Kingdom. These animals are composed of a

gelatinousmaterial, and show scarcely any trace of distinct organs. The Infusoria, Foraminifera, and

Sponges, with some other forms, belong to this division.

PUPA (pl. PUPÆ).—The second stage in the development of an Insect, from which it emerges in the

perfect (winged) reproductive form. In most insects the pupal stage is passed in perfect repose.The

chrysalis is the pupal state of butterflies.

RADICLE.—The minute root of an embryo plant.





RAMUS.—One half of the lower jaw in the Mammalia. The portion which rises to articulate with the

skull is called theascending ramus .

RANGE.—The extent of country over which a plant or animal is naturally spread. Range in time

expresses the distribution of a species or group through the fossiliferous beds of the earth's crust.

RETINA.—The delicate inner coat of the eye, formed by nervous filaments spreading from the optic

nerve, and serving for the perception of the impressions produced by light.

RETROGESSION.—Backward development.When an animal, as it approaches maturity, becomes

less perfectly organised than might be expected from its early stages and known relationships, it is said to

undergo aretrograde development or metamorphosis .

RETROGESSION.—A class of lowly organised animals (Protozoa), havinga gelatinous body, the

surface of which can be protruded in the form of root-like processes or filaments, which serve for

locomotion and the prehension of food. The most important order is that of the Foraminifera.

RODENTS.—The gnawing Mammalia, such as the Rats, Rabbits, and Squirrels. They are especially

characterised by the possession of a single pair of chisel-like cutting teeth in each jaw, between which

and the grinding teeth there is a great gap.

RUBUS.—The Bramble Genus.

RUDIMENTARY.—Very imperfectly developed.

RUMINANTS.—The group of Quadrupeds which ruminate or chew the cud, such as oxen, sheep, and

deer. They have divided hoofs, and are destitute of front teeth in the upper jaw.

SACRAL.—Belonging to the sacrum, or the bone composed usually of two or more united vertebræ to

which the sides of the pelvis in vertebrate animals are attached.

SARCODE.—The gelatinous material of which the bodies of the lowest animals (Protozoa) are

composed.

SCUTELLÆ.—The horny plates with which the feet of birds are generally more or less covered,

especially in front.

SEDIMENTARY FORMATIONS.—Rocks deposited as sediments from water.

SEGMENTS.—The transverse rings of which the body of an articulate animal or Annelid is composed.

SEPALS.—The leaves or segments of the calyx, or outermost envelope of an ordinary flower. They are

usuallygreen, but sometimes brightly coloured.

SERRATURES.—Teeth like those of a saw.

SESSILE.—Not supported on a stem or footstalk.

SILURIAN SYSTEM.—A very ancient system of fossiliferous rocks belonging to the earlier part of the





Palæozoic series.

SPECIALISATION.—The setting apart of a particular organ for the performance of a particular

function.

SPINAL CHORD.—The central portion of the nervous system in the Vertebrata, which descends from

the brain through the arches of the vertebræ, and gives off nearly all the nerves to the various organs of the body.

STAMENS.—Themale organsof flowering plants, standing in a circle within the petals. Theyusually

consist of a filament and an anther, the anther being the essential part in which the pollen, or fecundating

dust, is formed.

STERNUM.—The breast-bone.

STIGMA.—The apical portion of the pistil in flowering plants.

STIPULES.—Small leafy organs placed at the base of the footstalks of the leaves in many plants.

STYLE.—The middle portion of the perfect pistil, which rises like a column from the ovary and supports

the stigmaat its summit.

SUBCUTANEOUS.—Situated beneath the skin.

SUCTORIAL.—Adapted for sucking.

SUTURES (in the skull).—The lines of junction of the bones of which the skull is composed.

TARSUS (pl. TARSI).—The jointed feet of articulate animals, such as Insects.

TELEOSTEAN FISHES.—Fishes of the kind familiar to us in the present day, having the skeleton

usuallycompletelyossified and the scales horny.

TENTACULA or TENTACLES.—Delicate fleshy organs of prehension or touch possessed by many

of the lower animals.

TERTIARY.—The latest geological epoch, immediatelypreceding the establishment of the presentorder of things.

TRACHEA.—The windpipe or passage for the admission of air to the lungs.

TRIDACTYLE.—Three-fingered, or composed of three movable parts attached to a common base.

TRILOBITES.—Apeculiar group of extinct Crustaceans, somewhat resembling the Woodlice in

external form, and, like some of them, capable of rolling themselves up into a ball. Their remains are

found only in the Palæozoic rocks, and most abundantly in those of Silurian age.

TRIMORPHIC.—Presenting three distinct forms.





UMBELLIFERÆ.—An order of plants in which the flowers, which contain five stamens and a pistil with

two styles, are supported upon footstalks which spring from the top of the flower stem and spread out

like the wires of an umbrella, so as to bring all the flowers in the same head(umbel) nearly to the same

level ( Examples, Parsley and Carrot).

UNGULATA.—Hoofed quadrupeds.

UNICELLULAR.—Consisting of a single cell.

VASCULAR.—Containing blood-vessels.

VERMIFORM.—Like a worm.

VERTEBBRATA orVERTEBRATEANIMALS.—Thehighest divisionof the animal kingdom, so

called from the presence in most cases of a backbone composed of numerous joints or vertebræ , which

constitutes the centre of the skeleton and at the same time supports and protects the central parts of thenervous system.

WHORLS.—The circles or spiral lines in which the parts of plants are arranged upon the axis of growth.

WORKERS.—See neuters.

ZOEA-STAGE.—The earliest stage in the development of many of the higher Crustacea, so called from

the name of Zoëa applied to these young animals when they were supposed to constitute a peculiar genus.

ZOOIDS.—In many of the lower animals (such as the Corals, Medusæ, &c.) reproduction takes place

in two ways, namely, by means of eggs and by a process of budding with or without separation from the

parent of the product of the latter, which is often very different from that of the egg. The individuality of

the species is represented by the whole of the form produced between two sexual reproductions; and

these forms,whichareapparently individual animals, have been called zooids

1I am indebted to the kindness of Mr. W. S. Dallas for this Glossary, which has been given because

several readers have complained to me that some of the terms used were unintelligible to them. Mr.

Dallas has endeavoured to give the explanations of the terms in as popular a form as possible.

|Go to Contents |

The Origin of Species Vol.1

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An Historical Sketch Of the Progress of Opinion on the Origin of

Species,

Previously to the Publication of the First Edition of this Work.

I will here give a brief sketch of the progress of opinion on the Origin of Species. Until recently the great

majority of naturalists believed that species were immutable productions, and hadbeen separatelycreated. This view has been ably maintained by many authors. Some few naturalists, on the other hand,

have believed that species undergo modification, and that the existing forms of life are the descendants by

true generation of pre-existing forms.Passing over allusions to the subject in the classical writers,1the first

author who in modern times has treated it in a scientific spirit was Buffon. But as his opinions fluctuated

greatly at different periods, and as he does not enter on the causes or means of the transformation of

species, I need not here enter on details.

Lamarck was the first man whose conclusions on the subject excited much attention. This

justly-celebrated naturalist first published his views in 1801; he much enlarged them in 1809 in his

'PhilosophicZoologique,' and subsequently, in 1815, in the Introduction to his 'Hist. Nat. desAnimauxsans Vertébres.' In these works he upholds the doctrine that all species, including man, are descended

from other species. He first did the eminent service of arousing attention to the probability of all change in

the organic, as well as in the inorganic world, being the result of law, and not of miraculous interposition.

Lamarck seems to have been chiefly led to his conclusion on the gradual change of species, by the

difficulty of distinguishing species and varieties, by the almost perfect gradation of forms in certain groups,

and by the analogy of domestic productions. With respect to the means of modification, he attributed

something to the direct action of the physical conditions of life, something to the crossing of already

existing forms, and much to use and disuse, that is, to the effects of habit. To this latter agency he seems

to attribute all the beautiful adaptations in nature;—such as the long neck of the giraffe for browsing on

the branches of trees.But he likewise believed in a law of progressive development; and as all the forms

of life thus tend to progress, in order to account for the existence at the present day of simple

productions, he maintains that such forms are now spontaneouslygenerated2

Geoffroy Saint-Hilaire, as is stated in his 'Life,' written by his son, suspected, as early as 1795, that what

we call species are various degenerations of the same type. It was not until 1828 that he published his

conviction that the same forms have not been perpetuated since the origin of all things. Geoffroy seems to

have relied chiefly on the conditions of life, or the"monde ambiant" as the cause of change. He was

cautious in drawingconclusions, and did not believe that existing species are now undergoing

modification; and, as his son adds, "C'est done un problème à réserver entièrement à l'avenir, supposé

même que l'avenir doive avoir prise sur lui."

In 1813, Dr. W. C. Wells read before the Royal Society 'An Account of a White female, part of whose

skin resembles that of a Negro'; but his paper was not published until his famous 'Two Essays upon Dew





and Single Vision' appeared in 1818. In this paper he distinctly recognizes the principle of natural

selection, and this is the first recognition which has been indicated; but he applies it only to the races of

man, and to certain characters alone. After remarking that negroes and mulattoes enjoy an immunity from

certain tropical diseases, he observes, firstly, that all animals tend to vary in some degree, and, secondly,

that agriculturists improve their domesticated animals by selection; and then, he adds, but what is done in

this latter case "by art, seems to be done with equal efficacy, though more slowly, by nature, in the

formation of varieties ofmankind, fitted for the countrywhich they inhabit. Of the accidental varieties of man, which would occur among the first few and scattered inhabitants of the middle regions of Africa,

some one would be better fitted than the others to bear the diseases of the country. This race would

consequentlymultiply, while the otherswould decrease; not only from their inability to sustain the attacks

of disease, but from their incapacity of contending with their more vigorous neighbours. The colour of this

vigorous race I take for granted, from what has been already said, would be dark. But the same

disposition to form varieties still existing, a darker and a darker race would in the course of time occur:

and as the darkest would be the best fitted for the climate, this would at length become the most

prevalent, if not the only race, in the particular country in which it had originated." He then extends these

same views to the white inhabitants of colder climates. I am indebted to Mr. Rowley, of the United

States, for having called my attention, through Mr. Brace, to the above passage in Dr. Well's work.

The Hon. and Rev. W. Herbert, afterwards Dean of Manchester, in the fourth volume of the

'Horticultural Transactions,' 1822, and in his work on the 'Amaryllidaceæ' (1837, pp. 19, 339), declares

that "horticultural experiments have established, beyond the possibility of refutation, that botanical species

are only a higher and more permanent class of varieties." He extends the same view to animals. The Dean

believes that single species of each genus were created in an originally highly plastic condition, and that

thesehave produced, chiefly by intercrossing, but likewise by variation, all our existing species.

In 1826 Professor Grant, in the concluding paragraph in hiswell-known paper ('Edinburgh Philosophical

Journal,' vol. xiv. p. 283) on the Spongilla, clearly declares his belief that species are descended from

other species, and that they become improved in the course of modification. This same view was given inhis 55th Lecture, published in the 'Lancet' in 1834.

In 1831 Mr. Patrick Matthew published his work on 'Naval Timber and Arboriculture,' in which he

gives precisely the same view on the origin of species as that (presently to be alluded to) propounded by

Mr. Wallace and myself in the 'Linnean Journal,' and as that enlarged in the present volume.

Unfortunately the view was given by Mr. Matthew very briefly in scattered passages in an Appendix to a

work on a different subject, so that it remained unnoticed until Mr. Matthew himself drew attention to it in

the 'Gardener's Chronicle,' on April 7th, 1860. The differences of Mr. Matthew's view from mine are not

of much importance: he seems to consider that the world was nearly depopulated at successive periods,

and then re-stocked; and he gives as an alternative, that new forms may be generated "without the presence of any mould or germ of former aggregates." I am not sure that I understand some passages;

but it seems that he attributes much influence to the direct action of the conditions of life. He clearly saw,

however, the full force of the principle of natural selection.

The celebrated geologist and naturalist, Von Buch, in his excellent 'Description Physique des Isles

Canaries' (1836, p. 147), clearly expresses his belief that varieties slowly become changed into

permanent species,which are no longer capable of intercrossing.

Rafinesque, in his 'New Flora of North America,' published in 1836, wrote (p. 6) as follows:—"All

speciesmight have been varieties once, andmany varieties aregradually becoming species by assuming

constant and peculiar characters;" but farther on (p. 18) he adds, "except the original types or ancestors

of the genus."





In 1843-44 Professor Haldeman ('Boston Journal of Nat. Hist. U. States,' vol. iv. p. 468) has ably given

the arguments for and against the hypothesis of the development and modification of species: he seems to

lean towards the side of change.

The 'Vestiges of Creation' appeared in 1844. In the tenth and much improved edition (1853) the

anonymous author says (p. 155):—"The proposition determined on after much consideration is, that the

several series of animated beings, from the simplest and oldest up to the highest and most recent, are,under the providence of God, the results, first , of an impulse which has been imparted to the forms of

life, advancing them, in definite times, bygeneration, through grades of organisation terminating in the

highest dicotyledons andvertebrate, thesegradesbeing few in number, and generallymarkedby intervals

of organic character,which we find to be a practical difficulty in ascertaining affinities; second , of another

impulse connectedwith the vital forces, tending, in the course of generations, tomodify organic structures

in accordance with external circumstances, as food, the nature of the habitat, and the meteoric agencies,

these being the 'adaptations' of thenatural theologian." The author apparently believes that organisation

progresses by sudden leaps, but that the effects produced by the conditions of life are gradual. He argues

with much force on general grounds that species are not immutable productions. But I cannot see how

the two supposed "impulses" account in a scientific sense for the numerous and beautiful coadaptationswhich we see throughout nature; I cannot see that we thus gain any insight how, for instance, a

woodpecker has become adapted to its peculiar habits of life. The work, from its powerful and brilliant

style, thoughdisplaying in the earlier editions little accurateknowledge and a greatwant of scientific

caution, immediately had a very wide circulation. In my opinion it has done excellent service in this

country in calling attention to the subject, in removing prejudice, and in thus preparing the ground for the

reception of analogous views.

In 1846 the veteran geologist M. J. d'Omalius d'Halloy published in an excellent though short paper ('

Bulletins de l'Acad. Roy. Bruxelles,' tom. xiii. p. 581) his opinion that it is more probable that new

species have been produced by descent with modification than that they have been separately created:

the author first promulgated this opinion in 1831.

Professor Owen, in 1849 (' Nature of Limbs,' p. 86), wrote as follows:—"The archetypal idea was

manifested in the flesh under divers such modifications, upon this planet, long prior to the existence of

those animal species that actually exemplify it. To what natural laws or secondary causes the orderly

succession and progression of such organic phenomena may have been committed, we, as yet, are

ignorant." In his Address to the British Association, in 1858, he speaks (p. ii.) of "the axiom of the

continuous operation of creative power, or of the ordained becoming of living things." Farther on (p. xc.),

after referring to geographical distribution, he adds, "Thesephenomena shakeour confidence in the

conclusion that the Apteryx of New Zealand and the Red Grouse of England were distinct creations in

and for those islands respectively. Always, also, it may be well to bear in mind that by the word 'creation'the zoologist means 'a process he knows not what.'" He amplifies this idea by adding that when such

cases as that of the Red Grouse are "enumerated by the zoologist as evidence of distinct creation of the

bird in and for such islands, he chiefly expresses that he knows not how the Red Grouse came to be

there, and thereexclusively; signifying also, by thismodeof expressing such ignorance, his belief that both

the bird and the islands owed their origin to a great first Creative Cause." If we interpret these sentences

given in the same Address, one by the other, it appears that this eminent philosopher felt in 1858 his

confidence shaken that the Apteryx and the Red Grouse first appeared in their respective homes, "he

knew not how," or by some process "he knew not what."

This Address was delivered after the papers by Mr. Wallace and myself on the Origin of Species,

presently to be referred to, had been read before the Linnean Society. When the first edition of this work

was published, I was so completely deceived, as were many others, by such expressions as "the

continuous operation of creative power," that I included Professor Owen with other palæntologists as





being firmly convinced of the immutability of species; but it appears (' Anat. of Vertebrates,' vol. iii. p.

796) that this was on my part a preposterous error. In the last edition of this work I inferred, and the

inference still seems to me perfectly just, from a passage beginning with the words "no doubt the

type-form," &c. (Ibid. vol. i. p. xxxv.), that Professor Owen admitted that natural selection may have

done something in the formation of a new species; but this it appears (Ibid. vol. iii. p. 798) is inaccurate

and without evidence. I also gave some extracts from a correspondence between Professor Owen and

the Editor of the 'London Review,' from which it appeared manifest to the Editor as well as to myself,that Professor Owen claimed to have promulgated the theory of natural selection before I had done so;

and I expressed my surprise and satisfaction at this announcement; but as far as it is possible to

understand certain recently published passages (Ibid. vol. iii. p. 798) I have either partially or wholly

again fallen into error. It is consolatory to me that others find Professor Owen's controversial writings as

difficult to understand and to reconcile with each other, as I do. As far as the mere enunciation of the

principle of natural selection is concerned, it is quite immaterial whether or not Professor Owen preceded

me, for both of us, as shown in this historical sketch, were long ago preceded by Dr. Wells and Mr.

Matthews.

M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of which a Résumé appeared in the'Revue et Mag. de Zoolog.,' Jan. 1851), briefly gives his reason for believing that specific characters

"sont fixés, pour chaque espèce, tant qu'elle se perpétue au milieu des mêmes circonstances: ils se

modifient, si les circonstances ambiantes viennent à changer." "Enrésumé,l'observation des animaux

sauvages démontre déjà la variabilitélimitée des espèces. Lesexpériences sur les animaux sauvages,

devenus domestiques, et sur les animaux domestiques redevenus sauvages, la démontrent plus clairement

encore. Ces mêmes expériences prouvent, de plus, que les différences produites peuvent être devaleur

générique." In his 'Hist. Nat. Générale' (tom. ii. p. 430, 1859) he amplifies analogous conclusions.

From a circular lately issued it appears that Dr. Freke, in 1851 ('Dublin Medical Press,' p. 322),

propounded the doctrine that all organic beings have descended from one primordial form. His grounds

of belief and treatment of the subject are wholly different from mine; but as Dr. Freke has now (1861) published his Essay on the 'Origin of Species by means of Organic Affinity,' the difficult attempt to give

any idea of his views would be superfluous on my part.

Mr. Herbert Spencer, in an Essay (originally published in the 'Leader,' March, 1852, and republished in

his 'Essays,' in 1858), has contrasted the theories of the Creation and the Development of organic beings

with remarkable skill and force. He argues from the analogy of domestic productions, from the changes

which the embryos ofmany speciesundergo, from the difficulty of distinguishing species and varieties,

and from the principle of general gradation, that species have been modified; and he attributes the

modification to the change of circumstances. The author (1855) has also treated Psychology on the

principle of the necessary acquirement of each mental power and capacity by gradation.

In 1852 M. Naudin, a distinguished botanist, expressly stated, in an admirable paper on the Origin of

Species (' Revue Horticole,' p. 102; since partly republished in the 'Nouvelles Archives du Muséum,'

tom. i. p. 171), his belief that species are formed in an analogous manner as varieties are under

cultivation; and the latter process he attributes to man's power of selection. But he does not show how

selection acts under nature. He believes, like Dean Herbert, that species, when nascent, were more

plastic than at present. He lays weight on what he calls the principle of finality, "puissance mystérieuse,

indéterminée; fatalité pour les uns; pour les autres, volonté providentielle, dont l'action incessante sur les

êtres vivants détermine, à toutes les époques de l'existence du monde, la forme, le volume, et la durée de

chacun d'eux, en raison de sa destinée dans l'ordre de choses dont il fait partie.C'est cette puissance qui

harmonise chaque membre à l'ensemble, en l'appropriant à la fonctionqu'il doit remplir dans l'organisme

général de la nature, function qui est pour lui sa raison d'être."3





In 1853 a celebrated geologist, Count Keyserling ('Bulletin de la Soc. Géolog.,' 2nd Ser., tom. x. p.

357), suggested that as new diseases, supposed to have been caused by some miasma, have arisen and

spread over the world, so at certain periods the germs of existing species may have been chemically

affected by circumambient molecules of a particular nature, and thus have given rise to new forms.

In this same year, 1853, Dr. Schaaffhausen published an excellent pamphlet (' Verhand. des Naturhist.

Vereins der Preuss. Rheinlands,' &c.), in which he maintains the development of organic forms on theearth. He infers that many species have kept true for long periods, whereas a few have become modified.

The distinction of species he explains by the destruction of intermediate graduated forms. "Thus living

plants and animals are not separated from the extinct by new creations, but are to be regarded as their

descendants through continued reproduction."

A well-known French botanist, M. Lecoq, writes in 1854 ('Etudes sur Géograph. Bot.,' tom. i. p. 250),

"On voit que nos recherches sur la fixité ou la variation de l'espèce, nous conduisent directement aux

idées émises, par deux hommes justement célèbres, Geoffroy Saint-Hilaire et Gæthe." Some other

passages scattered through M. Lecoq's large work, make it a little doubtful how far he extends his views

on themodification of species.

The 'Philosophy of Creation' has been treated in a masterly manner by the Rev. Baden Powell, in his

'Essays on the Unity of Worlds,' 1855. Nothing can be more striking than the manner in which he shows

that the introduction of new species is "a regular, not a casual phenomenon," or, as Sir John Herschel

expresses it, "a natural in contradistinction to a miraculous process."

The third volume of the 'Journal of the Linnean Society' contains papers, read July 1st, 1858, by Mr.

Wallace and myself, in which, as stated in the introductory remarks to this volume, the theory of Natural

Selection is promulgated by Mr. Wallace with admirable force and clearness.

Von Baer, towards whom all zoologists feel so profound a respect, expressed about the year 1859 (seeProf. RudolphWagner, 'Zoologisch-AnthropologischeUntersuchungen,' 1861, s. 51) his conviction,

chiefly grounded on the laws of geographical distribution, that forms now perfectlydistinct have

descended from a single parent-form.

In June, 1859, Professor Huxley gave a lecture before the Royal Institution on the 'Persistent Types of

Animal Life.' Referring to such cases, he remarks, "It is difficult to comprehend the meaning of such facts

as these, if we suppose that each species of animal and plant, or each great type of organisation, was

formed and placed upon the surface of the globe at long intervals by a distinct act of creative power; and

it is well to recollect that such an assumption is as unsupported by tradition or revelation as it is opposed

to the general analogy of nature. If, on the other hand, we view 'Persistent Types' in relation to thathypothesis which supposes the species living at any time to be the result of the gradual modification of

pre-existing species a hypothesis which, though unproven, and sadly damaged by some of its supporters,

is yet the only one to which physiology lends any countenance; their existence would seem to show that

the amountofmodificationwhich livingbeingshaveundergone duringgeological time is but very small in

relation to the whole series of changes which they have suffered."

In December, 1859, Dr. Hooker published his 'Introduction to the Australian Flora.' In the first part of

this great work he admits the truth of the descent and modification of species, and supports this doctrine

by many original observations.

The first edition of this work was published on November 24th, 1859, and the second edition on

January 7th, 1860.





1Aristotle, in his 'Physicæ Auscultationes' (lib. 2, cap. 8, s. 2), after remarking that rain does not fall in

order to make the corn grow, any more than it falls to spoil the farmer's corn when threshed out of doors,

applies the same argument to organisation; and adds (as translated by Mr. Clair Grece, who first pointed

out the passage to me), "So what hinders the different parts [of the body] from having this merely

accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted

for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there

appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that is all the parts

of one whole) happened like as if they were made for the sake of something, these were preserved,

havingbeen appropriately constituted by an internal spontaneity; and whatsoever things were not thus

constituted, perished, and still perish." We here see the principle of natural selection shadowed forth, but

how little Aristotle fully comprehended the principle, is shown by his remarks on the formation of the

teeth.

2I have taken the date of the first publication of Lamarck from Isid. Geoffroy Saint-Hilaire's ('Hist. Nat.

Générale,' tom. ii. p. 405, 1859) excellent history of opinion on this subject. In this work a full account isgiven of Buffon's conclusions on the same subject. It is curious how largely my grandfather, Dr. Erasmus

Darwin, anticipated the views and erroneous grounds of opinion of Lamarck in his 'Zoonomia' (vol. i. pp.

500-510), published in 1794. According to Isid. Geoffroy there is no doubt that Goethe was an extreme

partisan of similar views, as shown in the Introduction to a work written in 1794 and 1795, but not

published till long afterwards: he has pointedly remarked ('Goethe als Naturforscher,' von Dr. Karl

Meding, s. 34) that the future question for naturalists will be how, for instance, cattle got their horns, and

not for what they are used. It is rather a singular instance of the manner in which similar views arise at

about the same time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint-Hilaire (as we

shall immediately see) in France, came to the same conclusion on the origin of species, in the years

1794-5.

3From references in Bronn's 'Untersuchungen über dieEntwickelungs-Gesetze,' it appears that the

celebrated botanist andpalæontologistUnger published, in 1852, his belief that species undergo

development andmodification. Dalton, likewise, in Pander andDalton'swork on Fossil Sloths,

expressed, in 1821, a similar belief. Similar views have, as is well known, been maintained by Oken in his

mystical 'Natur-Philosophie.' From other references in Godron's work 'Sur l'Espèce,' it seems that Bory

St. Vincent, Burdach, Poiret, and Fries, have all admitted that new species are continually being

produced.

I may add, that of the thirty-four authors named in this Historical Sketch, who believe in the modification

of species, or at least disbelieve in separate acts of creation, twenty-seven have written on special branches of natural history or geology.

|Go to Contents |

Introduction





WHEN on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution

of the organic beings inhabiting South America, and in the geological relations of the present to the past

inhabitants of that continent. These facts, as will be seen in the latter chapters of this volume, seemed to

throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our

greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be

made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years' work I allowed myself to speculate, on the subject and

drew up some short notes; these I enlarged in 1844 into a sketch of the conclusions, which then seemed

to me probable: from that period to the present day I have steadily pursued the same object. I hope that I

may be excused for entering on these personal details, as I give them to show that I have not been hasty

in coming to a decision.

My work is now (1859) nearly finished; but as it will take me many more years to complete it, and as

my health is far from strong, I have been urged to publish this Abstract. I have more especially been

induced to do this, as Mr. Wallace, who is now studying the natural history of the Malay archipelago, has

arrived at almost exactly the same general conclusions that I have on the origin of species. In 1858 hesent me a memoir on this subject, with a request that I would forward it to Sir Charles Lyell, who sent it

to the Linnean Society, and it is published in the third volume of the Journal of that society. Sir C. Lyell

and Dr. Hooker, who both knew of my work—the latter having read my sketch of 1844—honoured me

by thinking it advisable to publish,withMr. Wallace's excellentmemoir, somebrief extracts from my

manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I cannot here give references and

authorities for my several statements; and I must trust to the reader reposing some confidence in my

accuracy. No doubt errors will have crept in, though I hope I have always been cautious in trusting to

good authorities alone. I can here give only the general conclusions at which I have arrived, with a few

facts in illustration, but which, I hope, in most cases will suffice. No one can feel more sensible than I doof the necessityof hereafter publishing in detail all the facts, with references, onwhichmyconclusions

have been grounded; and I hope in a future work to do this. For I am well aware that scarcely a single

point is discussed in this volume on which facts cannot be adduced, often apparently leading to

conclusions directly opposite to those at which I have arrived. A fair result can be obtained only by fully

stating and balancing the facts and arguments on both sides of each question; and this is here impossible.

I much regret that want of space prevents my having the satisfaction of acknowledging the generous

assistance which I have received from very many naturalists, some of them personally unknown to me. I

cannot, however, let this opportunity pass without expressing my deep obligations to Dr. Hooker, who,

for the last fifteen years, has aided me in every possible way by his large stores of knowledge and hisexcellent judgment.

In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual

affinities of organic beings, on their embryological relations, their geographicaldistribution, geological

succession, and other such facts, might come to the conclusion that species had not been independently

created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even if

well founded,would be unsatisfactory, until it could be shown how the innumerable species inhabiting this

world have been modified, so as to acquire that perfection of structure and coadaptation which justly

excites our admiration.Naturalists continually refer to external conditions, such as climate, food,&c., as

the only possible cause of variation. In one limited sense, as we shall hereafter see, this may be true; but it

is preposterous to attribute to mere external conditions, the structure, for instance, of the woodpecker,

with its feet, tail, beak, and tongue, so admirably adapted to catch insects under the bark of trees. In the

case of the mistletoe, which draws its nourishment from certain trees, which has seeds that must be





transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency

of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the

structure of this parasite, with its relations to several distinct organic beings, by the effects of external

conditions, or of habit, or of the volition of the plant itself.

It is, therefore, of the highest importance to gain a clear insight into the means of modification and

coadaptation. At the commencement of my observations it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer the best chance of making out this obscure

problem. Nor have I been disappointed; in this and in all other perplexing cases I have invariably found

that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest

clue. I may venture to express my conviction of the high value of such studies, although they have been

very commonly neglected by naturalists.

From these considerations, I shall devote the first chapter of this Abstract to Variation under

Domestication. We shall thus see that a large amount of hereditary modification is at least possible; and,

what is equally or more important, we shall see how great is the power of man in accumulating by his

Selection successive slight variations. I will then pass on to the variability of species in a state of nature; but I shall, unfortunately, be compelled to treat this subject far too briefly, as it can be treated properly

only by giving long catalogues of facts. We shall, however, be enabled to discuss what circumstances are

most favourable to variation. In the next chapter the Struggle for Existence amongst all organic beings

throughout the world, which inevitably follows from the highgeometrical ratio of their increase,will be

considered. This is the doctrine of Malthus, applied to the whole animal and vegetable kingdoms. As

many more individuals of each species are born than can possibly survive; and as, consequently, there is

a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any

manner profitable to itself, under the complexand sometimes varying conditions of life, will have a better

chance of surviving, and thus benaturally selected . From the strong principle of inheritance, any

selected variety will tend to propagate its new and modified form.

This fundamental subject of Natural Selection will be treated at some length in the fourth chapter; and we

shall then see how NaturalSelection almost inevitably causes muchExtinction of the less improved forms

of life, and leads to what I have called Divergence of Character. In the next chapter I shall discuss the

complex and little known laws of variation. In the five succeeding chapters, the most apparent and

gravest difficulties in accepting the theorywill begiven:namely, first, the difficulties of transitions, orhow

a simple being or a simple organ can be changed and perfected into a highly developed being or into an

elaborately constructed organ; secondly, the subject of Instinct, or themental powers of animals; thirdly,

Hybridism, or the infertility of species and the fertilityof varieties when intercrossed; and fourthly, the

imperfection of the Geological Record. In the next chapter I shall consider the geological succession of

organic beings throughout time; in the twelfth and thirteenth, their geographical distribution throughoutspace; in the fourteenth, their classification ormutual affinities, bothwhenmature and in an embryonic

condition. In the last chapter I shall give a brief recapitulation of the whole work, and a few concluding

remarks.

No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and

varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the

many beings which live around us. Who can explain why one species ranges widely and is very

numerous, and why another allied species has a narrow range and is rare? Yet these relations are of the

highest importance, for they determine the present welfare and, as I believe, the future success and

modification of every inhabitant of this world. Still less do we know of the mutual relations of the

innumerable inhabitants of the world during the manypast geological epochs in its history.Althoughmuch

remains obscure, and will long remain obscure, I can entertain no doubt, after the most deliberate study

and dispassionate judgment of which I am capable, that the view which most naturalists until recently





entertained, andwhich I formerly entertained—namely, that each species has been independently

created—is erroneous. I am fully convinced that species are not immutable; but that those belonging to

what are called the same genera are lineal descendants of some other and generally extinct species, in the

same manner as the acknowledged varieties of any one species are the descendants of that species.

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Darwin, Charles - On the Origin of Species