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These National Geographic articlesprovide in-depth coverage of interesting topics in biology. They are referenced withinthe chapters at point-of-use to support or extend the chapter content. For newsabout recent scientific discoveries and research, go to .
Chapter 1 Scientific Theories . . . . . . . . . . . . . . . . . . . . .1060Chapter 3 Biomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1062Chapter 7 Microscopes . . . . . . . . . . . . . . . . . . . . . . . . . .1064Chapter 13 Selective Breeding of Cats . . . . . . . . . . . . . .1066Chapter 16 Primates . . . . . . . . . . . . . . . . . . . . . . . . . . . .1068Chapter 17 Kingdoms of Life . . . . . . . . . . . . . . . . . . . . .1070Chapter 18 Viruses . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1074Chapter 21 Plants for People . . . . . . . . . . . . . . . . . . . . .1076Chapter 28 Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1080Chapter 31 Dinosaurs . . . . . . . . . . . . . . . . . . . . . . . . . . .1084Chapter 32 Placental Mammals . . . . . . . . . . . . . . . . . . . .1086Chapter 36 Evolution of the Brain . . . . . . . . . . . . . . . . .1090
Science Skill Handbook 1092Active Reading and Study Skills 1092
Preparing to Read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1092Chapter Preview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1092Section Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1092Section Vocabulary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1092
Active Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1092Using Your Textbook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1092Concept Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1093Foldables™ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1094Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1094
Review for Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1094Get the Big Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1094Using an Analogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1094Section and Chapter Assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1094
Understanding Scientific Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1095
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Math and Problem-Solving Skills 1097Math Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1097
Measure in SI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1097Convert Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1097Calculate Magnification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1098Calculate Field of View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1098Make and Use Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1099Make and Use Graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1099
Problem-Solving Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1102Classify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1102Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1102Compare and Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1102Observe and Infer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1103Recognize Cause and Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1103Interpret Scientific Illustrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1103
Lab Skills and Techniques 1104Lab Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1104
Hypothesize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1104Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1104Use Variables, Constants, and Controls . . . . . . . . . . . . . . . . . . . . . . . . .1105
Microscope Care and Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1106
Reference Handbook 1107Safety Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1107Demonstrating Safe Lab Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1108
Preventing Accidents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1108Making Wise Choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1108Working in the Laboratory and the Field . . . . . . . . . . . . . . . . . . . . . . .1108Laboratory Cleanup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1109Emergencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1109
The Six-Kingdom Classification System . . . . . . . . . . . . . . . . . . . . . . . . . . .1110The Three-Domain Classification System . . . . . . . . . . . . . . . . . . . . . . . . .1111Periodic Table of Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1112
Glossary/Glosario 1113
Index 1153
Credits 1188
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ScientificTheories
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What is a scientific theory? Incasual usage, “theory” meansan unproven assumption about a set of facts. A scientific theory is an explanation of anatural phenomenon supportedby a large body of scientificevidence obtained from variousinvestigations and observations.The scientific process beginswith observations of the naturalworld. These observations leadto hypotheses, data collection,and experimentation. If weak-nesses are observed, hypothe-ses are rejected or modified andthen tested again and again.When little evidenceremains to cause ahypothesis to be rejected, itmay become a theory. Followthe scientific process describedhere that led to newtheories aboutdinoosaurs.
MAKE HYPOTHESESReptiles are ectotherms—animals with body temperatures influenced by theirexternal environments. Early in the study of dinosaur fossils, many scientistsassumed that because dinosaur skeletons resembled those of some modern rep-
tiles, dinosaurs, too, must have been ectotherms. This assumption led scien-tists to conclude that many dinosaurs, being both huge and ectothermic, were
slow-growing, slow-moving, and awkward on land. Because the most complete dinosaur skeletons occurred in rocks formed at the
bottom of bodies of water, scientists hypothesized that dinosaurs lived in waterand that water helped to support their great weight. When skeletons of duck-billed dinosaurs, called hadrosaurs, were discovered, this hypothesis gained sup-port. Hadrosaurs had broad, flat ducklike bills, which, scientists suggested,helped them collect and eat water plants.
FIELD MUSEUM OF NATURAL HISTORY, CHICAGO
IGUANODON
OBSERVEPeople have been unearthing fossils forhundreds of years. The first person toreconstruct a dinosaur named it Iguanodon,(ih GWAH nuh dahn) meaning “iguanatooth,” because its bones and teethresembled those of an iguana. By 1842these extinct animals were nameddinosaurs, meaning “terrible lizards.”
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EXPANDING Your View
APPLY CONCEPTS Robert Bakker’s research led to a different theoryregarding the physiology of dinosaurs. As new fossils are found andnew tools developed to study them, paleontologists will continue toreplace existing theories with newer ones. What kinds of scientific information or evidence can cause a scientific theory to be changed?
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FORM THEORIES Bakker’s hypotheses—supported by data gathered by otherpaleontologists and by dinosaur bones, growth patterns,and behavior—prompted scientists to reexamine theoriesabout dinosaurs. Were some dinosaurs endotherms andothers ectotherms? Did dinosaurs have their own uniquephysiology resembling neither reptiles nor mammals?Scientific theories about dinosaurs continue to evolve asnew fossils are discovered and new tools to study thosefossils are developed.
THINK CRITICALLYIn the 1960s, paleontologist (a scientist who stud-ies fossils) Robert Bakker (right) hypothesized thatdinosaurs were not sluggish ectotherms but fast-moving, land-dwelling endotherms—animals likebirds and mammals. Bakker observed that manydinosaurs had feet and legs built for life on land. Ifhadrosaurs had led a semiaquatic life, Bakker rea-soned, their feet would have been webbed with
long, thin, widely spaced toes. But hadrosaurs had short, stubbytoes and feet, obviously suited for land. In addition to Bakker’sobservations, studies of fossilized stomach contents revealed thathadrosaurs dined on the cones and leaves of cycads (above) andother land plants. After considering these data carefully, Bakkerproposed that many dinosaurs were quick, agile endotherms thatroamed Earth’s ancient landscape.
COLLECT DATA To test his hypotheses, Bakker intensified his research ondinosaur skeletons and bone structure. He found reportsfrom the 1950s comparing thousands of cross sections ofdinosaur bones with those of reptiles, birds, and mammals.These reports noted that many dinosaur bones were lessdense than those of modern reptiles and riddled with chan-nels for blood vessels. In short, many dinosaur bones resem-bled those of endotherms not ectotherms. Bakker confirmedhis observations by collecting supporting evidence fromother sources.
BREAD PALM, CYCAD FAMILY
ROBERT BAKKER
PALEONTOLOGIST WORKING ON FOSSIL
ROBERT BAKKER WITHBRONTOSAUR BONE
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DINOSAUR BONE SHOWINGCHANNELS FOR BLOOD VESSELSMagnification: 25�
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A biome is a large group ofecosystems that share the same type of mature climaxcommunity. When you think of a biome, you may imagine lionson an African grassland or monkeys in the rain forest.However, ecologists look at climax communities of plantsrather than animals. Becauseplants don’t migrate, they are a better indicator of the long-term characteristics of a biome.The number of biomes in theworld is subject to change if climate changes. Some commonlyaccepted biomes are shown here.
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BiomesDESERT
GRASSLAND
TAIGA
WATER
WATER LILY
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THINK CRITICALLY Which biome do you think would recover mostslowly from destruction arising from natural events or human causes?Explain.
COMPARE AND CONTRAST Think about the general pattern of biometypes that exists from the equator to the poles. Do you think you wouldfind a similar pattern if you climbed from the foot of a mountain in thetropics to its peak? Explain.
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TUNDRA
TROPICAL RAIN FOREST
BIOMESEarth’s surface is marvelously diverse. Millions of species find a home here. But their distribution is not random.As the world map shows,Earth’s biomes exhibittremendous variety.
In general, three fac-tors—latitude, altitude,precipitation—determine
which biome dominates a terrestriallocation. A rainy, low-lying areanear the equator will have a tropicalrain forest as climax vegetation. Afew kilometers away on a moun-tainside, ecologists may find plantstypical of a biome thousands ofkilometers to the north or south.
Look at the world map. Noticethat Earth is more than two-thirdswater. This water is mostly oceans,which make up the saltwaterbiome. Freshwater from precipita-tion makes up the other majorwater biome on land.
TEMPERATE/DECIDUOUS FOREST
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The invention and developmentof the light microscope some300 years ago allowed scien-tists to see cells for the firsttime. Improvements havevastly increased the range of visibility of microscopes.Today researchers can usethese powerful tools to studycells at the molecular level.
MicroscopesTHIS HISTORIC MICROSCOPE —held by a modern researcher—was designed by Anton vanLeeuwenhoek (above). By 1700,
Dutch scientist, van Leeuwenhoek,had greatly improved the accuracy of microscopes.Grinding the lenses himself, van Leeuwenhoek built some240 single-lens versions. He dis-covered—and described for thefirst time—red blood cells and bacteria, taken from scrapings from his
teeth. By 1900, problems with lenses that
had once limited image quality had been overcome, and the com-pound microscope had evolvedessentially into its present form.
THIS EARLY COMPOUND MICROSCOPE, housed in a gold-embossedleather case, was designed by English scientist Robert Hookeabout 1665. Using it, he observed and made drawings of corkcells. Although the microscope has three lenses, they are ofpoor quality and Hooke could see little detail.
ANTON VAN LEEUWENHOEK
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HOOKE’S MICROSCOPE
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SCANNING ELECTRON MICROSCOPE IMAGE OF A MOSQUITO
RED BLOOD CELLSUNDER A COMPOUNDLIGHT MICROSCOPE
RED BLOOD CELLS UNDER A TRANSMISSIONELECTRON MICROSCOPEMagnification: 40 000�
SCANNING ELECTRON MICROSCOPEAn SEM sweeps a beam of electronsover the surface of a specimen, such as red blood cells (above),causing electrons to be emittedfrom the specimen. SEMs producea realistic, three-dimensional picture—but only the surface of an object can be observed. AnSEM can magnify only about20 000 times without losing clarity.
HOW IT WORKS The magnifyingpower of a microscope is deter-mined by multiplying the magnifi-cation of the eyepiece and theobjective lens.
A COMPOUND LIGHT MICROSCOPE(above) uses two or more glasslenses to magnify objects. Lightmicroscopes are used to look at living cells, such as red blood cells(top), small organisms, and pre-served cells. Compound lightmicroscopes can magnify up toabout 1500 times.
RED BLOOD CELLS UNDER ASCANNING ELECTRON MICROSCOPE
TRANSMISSION ELECTRON MICROSCOPE A TEM aims a beamof electrons through a specimen. Denser portions of anobject allow fewer electrons to pass through. Thesedenser areas appear darker in the image. Two-dimensionalTEM images are used to study details of cells such asthese red blood cells (above). A TEM can magnifyhundreds of thousands of times.
Magnification: 800�
Magnification: 10 000�
Magnification: 50�
SCANNING TUNNELINGMICROSCOPE IMAGEOF A DNA FRAGMENTMagnification: 2 000 000�
SCANNING TUNNEL-ING MICROSCOPEThe STM revolu-
tionized microscopyin the mid-1980s by
allowing scientists tosee atoms on an object’s sur-
face. A very fine metal probe isbrought near a specimen. Electronsflow between the tip of the probeand atoms on the specimen’s sur-face. As the probe follows surfacecontours, such as those on this DNAmolecule (above), a computer cre-ates a three-dimensional image. An STM can magnify up to onehundred million times.
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EXPANDING Your View
THINK CRITICALLY Which type of microscope should be used toexamine the contents of a small bacterial cell? Why?
COMPARE AND CONTRAST Compare and contrast the images seenwith an SEM to those seen with a TEM.
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RESEARCHER USING A TEM
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SelectiveGraceful, agile, and independent,cats are popular pets. In theUnited States alone, more than55 million cats are kept as pets.Although the origin of thedomestic cat is lost in antiquity,archeological evidence indicatesthat an association between catsand people existed as much as3500 years ago in ancient Egypt.Unlike dogs, cattle, and manyother domesticated animals,however, cats have only recentlybeen bred selectively to exhibitspecific traits. Currently, about 40 recognized breeds exist—developed by selectively mating cats having especially desirable or distinctive char-acteristics. Different breeds vary primarily in color, in length and texture of fur, and in temperament.
COLORFUL COATSCats come in many colors, butthe most common coats are tabby(a striped or blotchy pattern),black, and orange. Cats with“orange” coats range in colorfrom creamy yellow to dark gin-ger red. The genetic control ofcat fur color is complex and onlypartially understood. Solid whitefur is dominant to all other furcolors. Spots of white—especiallyon the face, throat, and paws—are also dominant to solid colorcoats. Some breeds such as theSiamese (below) have been bredfor a light-colored body with darklegs, tail, ears, and face—the per-fect frame for bright blue eyes.
Breeding of Cats
SOMALI—ALONGHAIREDABYSSINIANBREED—ATCAT SHOW
KITTENS (ABOVE AND BELOW) WITH DOMINANT WHITE MARKINGSON FACE, PAWS, AND THROAT
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SIAMESE
(b)Walter Chandoha, (crossover)Suzanne Dunn/The Image Works
SILVER MANX TABBY(ABOVE), AND WHITE
PERSIAN GETTINGGROOMED
TRAITS AND TEMPERAMENTSSome cats have been bred for special traits. The Manx (right), for example, is tailless. Manx cats trace their roots to the Isle of Man off the coast of England. With hind legs longer than front legs, Manx cats run with a rabbitlike, hopping gait. The breed known as Ragdoll gets its name from the fact that it relaxes its muscles and goes completely limp when picked up. Fearless and calm, the Ragdoll is a fairly
new breed of cat, which originated in the United States in the 1960s. Different breeds of cats have different temperaments, or personalities. Siamese tend to be vocal and demanding. TheJapanese bobtail—thought to bring goodluck—is playful andadaptable. The elegantAbyssinian is known forbeing quiet and veryaffectionate.
SHORT VERSUS LONG Cat breeds can be divided into two major groups: those with short hairand those with long. The Abyssinian—slender and regal-looking withlarge ears and almond-shaped eyes—is a popular short-haired breed. Theancestry of Abyssinians is unclear, but they may be descended from thesacred cats of ancient Egypt. Certainly, their similarity to Egyptian catsculptures, such as the one at left, is striking. The American shorthair, on the other hand, is a sturdy muscular breed developed from cats thataccompanied European settlers to the American colonies.
There are about a dozen breeds of longhaired cats, ranging from thelarge (up to 13.5 kg, or 30 pounds), shaggy Maine coon cat to the popularPersian. Persian cats (below) are prized for their extremely long fur that stands out from their bodies, especially on the neck, face, and tail.Hundreds of years of careful breeding have refined the distinct “powder puff” appearance of the modern Persian.
SCULPTURE OF EGYPTIAN GODDESS BASTET AS A CAT
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EXPANDING Your View
THINK CRITICALLY The ancient Egyptians stored large amounts ofgrain to ensure there would be enough to eat if crops failed. Infer whyancient Egyptians may have been motivated to domesticate cats.
JOURNAL WRITING Research a domestic cat breed. In your journal,record the breed’s history and specific traits for which it was bred.
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STREPSIRRHINESThese small, tree-dwelling animalslook least like other primates. Theirtriangular faces, set off by largeround eyes, lack musclesneeded to make facialexpressions that other pri-mates use for communica-tion. They can be as smallas a mouse or as big as alarge house cat. Perhapsthe best known members of this group are the lemurs (right),which live only in Madagascar andneighboring islands off the coast ofeastern Africa.
OLD WORLD MONKEYSMonkeys foundin Europe, Asia,and Africa arecalled Old Worldmonkeys. Theygrow larger thanNew Worldmonkeys andhave no prehen-sile tail forgrasping. Pads of tough skin on
their rumps cushion them while theyare seated. Among Old World monkeys, the mandrill (above) has the most colorful face. The Japanese macaque (right), or snow monkey, lives farther north than any other species of monkey.
Catch the gaze of anorangutan and you’ll bestaring into a face very muchlike your own. Similaritiesbetween apes and humans are striking—expressive eyes, fingers that can grasp,keen intelligence, and com-plex social systems. Theresemblance is no coinci-dence. Apes and other pri-mates are humans’ closestrelatives. The Primate order is made up of 13 families,including Hominidae, to which Homo sapiens, ourspecies, belongs.
PrimatesRING-TAILED LEMUR
JAPANESE MACAQUES
MANDRILL
SQUIRREL MONKEY
AYE-AYE
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NEW WORLD MONKEYSUnlike their Old World counter-parts, New World monkeys haveprehensile tails. Sometimes com-pared to an extra hand, the tail canwrap around a tree limb and sup-port the monkey’s weight. Thus,the animal can dangle upsidedown to eat. The capuchins(above) of Central and SouthAmerica have thumbs that can move to touchother fingers and helpthem pick up food.
APESUnlike monkeys, apes have no tails, andthey are usually larger than monkeys.While monkeys run on all fours, apes walkon two legs with support from their hands.Chimpanzees (left), gibbons, gorillas, andorangutans (above) are all apes. Living inAfrica and Asia, these primates have largebrains and are considered to be more likehumans than any other animal. They aresubject to many of the same diseases ashumans, can use simple tools, and somehave been taught to communicate withhumans using sign language.
ORANGUTAN
CAPUCHINS
CHIMPANZEE
EXPANDING Your View
THINK CRITICALLY Examine the photo of New World monkeys. How is having a tail an adaptive advantage for these primates?
COMPARE AND CONTRAST Which of the species in this feature probably live in trees? Explain your answer.
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ARISTOTLE RECOGNIZESPLANTS AND ANIMALSTaxonomy, as the science of bio-
logical classification is called,began with the Greek
philosopher Aristotle(384–322 B.C.). A keenobserver of nature,Aristotle separated allliving things into twomajor groups: plants andanimals. He grouped
plants into herbs, shrubs,and trees, and classified
animals on the basis of size,where they lived—on the land
or in the water, and how theymoved. Although Aristotle’s sys-tem of classification did little toreveal natural relationships amongliving things, it was widelyaccepted and used, with few mod-ifications, into the Middle Ages.
PUFFIN
OAK TREE
CONEFLOWER
DROMEDARY
The great diversity of life onEarth—estimated at 3 to 10 mil-lion species and counting—can beoverwhelming. To make sense ofthis bewildering array of livingthings, biologists use classifica-tion systems to grouporganisms in ways thathighlight their similari-ties, differences, andrelationships. The sys-tematic grouping of liv-ing things originated inthe 4th century B.C. Butbiological classificationhas changed a great deal over the years,as new tools andtechnologies have made it possi-ble to examine organisms inincreasing detail and trace theircomplex evolutionary pathwaysthrough time.
MOTH COLLECTION
Kingdomsof
RHODODENDRON
Life
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LINNAEUS IDENTIFIES TWO KINGDOMSModern classification began with the work of John Ray (1627–1705), an English naturalist who outlined the idea of species. In the mid-1700s, Swedish botanist Carolus Linnaeus (1707–1778) picked up on this idea and developed a clas-sification scheme that formed the basis of the system we use today. Linnaeus divided all living things between two kingdoms—plants and animals. But he subdivided these kingdoms into a hierarchy of smaller and more specific groups: classes, orders, genera, and species. Linnaeus placed organ-isms in these groups primarily on the basis of their physical similarities and differences.
PROTISTS: THE THIRD KINGDOMLinnaeus’ classification system revolutionized taxonomy, but from the startthere were problems. Organisms such as mushrooms and sponges resembleplants but do not make their own food. To whichkingdom did they belong? As light microscopesimproved, the situation became much more com-plex as biologists discovered a vast assortment ofminute, primarily one-celled organisms. In 1866,German zoologist Ernst Haeckel (1834–1919) pro-posed giving these unicellular organisms—namedprotists—a kingdom of their own.
PROKARYOTES AND FOUR KINGDOMSThe three-kingdom classification system persisted, however,until the middle of the 20th century when the electron micro-scope and advances in biochemistry made it possible to studyliving things at the subcellular level. These new tools revealedthat there are two fundamentally different kinds of cells in theliving world—prokaryotes and eukaryotes. Prokaryotes, such asthe bacterium Salmonella (below left), lack the membrane-bound nuclei and mostof the organelles characteristic of eukaryotic cells. All prokaryotes were then rec-ognized as a separate kingdom that contained all the bacteria.
SALMONELLA (PROKARYOTE) Magnification: 34 300�
KELP (EUKARYOTE)
VOLVOX (EUKARYOTE) Magnification: 15�
QUEEN ANGELFISH
FLY AGARIC MUSHROOMS
BARREL SPONGE WITH CRINOIDS
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PLANTSANIMALSFUNGI
PROKARYOTES
MONERANS
EVOLUTIONARY RELATIONSHIPSWith the five-kingdom system in place,many taxonomists focused their research onreclassifying living things in terms of theirevolutionary relationships rather than ontheir structural similarities. Present-dayorganisms, such as the millipede (below),were compared with extinct forms pre-served in the fossil record, such as thetrilobite (below right).
New biochemical techniquesmade it possible to comparenucleotide sequences in genesand amino acid sequences inproteins from different organ-isms to determine how closelythose organisms were related.
THE FIVE-KINGDOM SYSTEMA flurry of ideas for new classification sys-tems followed close on the heels of the dis-covery of prokaryotes. In 1969, Americanbiologist R. H. Whittaker (1924–1980) pro-posed a five-kingdom system (right) thatsoon became universally accepted. The fivekingdoms were Monera (bacteria), Protista(algae and other protists), Fungi (mushrooms,molds, and lichens), Plantae (mosses, ferns,and cone-bearing and flowering plants), andAnimalia (invertebrate and vertebrate ani-mals). The kingdom Monera included all theprokaryotes; the other four kingdoms con-sisted of eukaryotes. Fungi, plants, and ani-mals were easily distinguished by theirmodes of nutrition. But the kingdom Protistawas a grab bag, a diverse assortment of livingthings—some plantlike, some animal-like,some funguslike—that did not fit clearly intoany of the other eukaryotic kingdoms.
MILLIPEDE
PROTISTS
WHITTAKER’S SYSTEM
EUKARYOTES
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THINK CRITICALLY How have technological advances, such asimproved microscopes and new biochemical tests, changed biological classification?
JOURNAL WRITING The kingdom Protista contains very diverseorganisms—from unicellular “animal-like” amoebas to multicellular“plantlike” giant kelp. In your journal, predict what might happento the protist kingdom in the next few years as biologists study itsmembers in more detail at biochemical and genetic levels.
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THE SIXTH KINGDOMIn the 1970s, genetic tests showedthat members of the kingdomMonera were far more diversethan anyone had suspected. Onegroup of bacteria, originally calledarchaebacteria (ancient bacteria),seemed especially unusual.Archaebacteria, or archaeans, asmost biologists now refer to them,often live in extreme environ-ments—very hot or salty places—such as the Grand Prismatic Spring(left) in Yellowstone National Park. In 1996,researchers sequenced the archaean genomeand discovered that these tiny cells are as dif-ferent from bacteria as you are. A sixth king-dom was formed.
ARCHAEA Magnification: 29 000�
GRAND PRISMATIC SPRING, YELLOWSTONE NATIONAL PARK
WOESE’S SYSTEM
EXPANDING Your View
DOMAINSThe discovery of the nature of Archaea led C. R. Woese and his colleagues at theUniversity of Illinois to propose a new classifi-cation scheme (left) made up of threedomains. The domain Bacteria has one king-dom, Eubacteria (true bacteria). The domainArchaea contains the kingdom Archaebacteria.The domain Eukarya consists of the king-doms Protista, Fungi, Plantae, and Animalia.
The domain classification system takes intoaccount more of the evolutionary history oforganisms. Traditionally, the classification sys-tems were weighted more toward physicalcharacteristics of organisms, but the domainsystem is designed to show that there are vastdifferences between archaebacteria, eubacte-ria, and the other forms of life—fungi, ani-mals, protists, and plants. These differencesare not easy to see but are significant in termsof genetic makeup.
DOMAIN DOMAIN DOMAIN
Bacteria Archaea Eukarya
KINGDOM KINGDOM KINGDOMSEubacteria Animalia
Archaebacteria Plantae
Fungi
Protista
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FOCUSONSTRUCTUREA single drop of blood can containbillions of viruses. Despite theirincredibly small size, many viruses,such as this tobacco mosaic virus(below), have complex structures.All viruses consist of a core of
nucleic acid—eitherDNA or RNA—enclosed in a proteincoat called a capsid.Both the type andarrangement of pro-teins in the capsidgive different virusescharacteristic shapes.
Viruses lurk everywhere—oncomputer keyboards, in birddroppings, under your fin-gernails—just waiting toget inside your body orsome other living thing.Smaller than the small-est bacteria, virusesare not alive. By them-selves, they cannotmove, grow, or repro-duce. But give viruses thechance to invade a living cell,and they will take over itsmetabolic machinery, repro-gramming it to churn out moreviruses to attack other cells.
INVISIBLE INVADERSScientists have identified thou-sands of viruses. Some invadeplants, others attack animals, andstill others target bacteria.
In humans, viruses are responsi-ble for chicken pox, warts, coldsores, and the common cold, aswell as dreaded diseases suchas rabies, influenza, hepatitis,and AIDS.
ICOSAHEDRAL VIRUSESMany animal viruses—such as polio (above)and adenovirus—have20-sided, or icosahedral,capsids. Viewed underan electron microscope,icosahedral viruses looklike perfectly symmetri-cal crystals.
POLIO VIRUSMagnification:85 000�
MODEL OF TOBACCOMOSAIC VIRUS
Viruses
WOMAN EXPERIENCING SYMPTOMS OFTHE COMMON COLD
MODEL OF AIDS VIRUS
DNACAPSID
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EXPANDING Your View
THINK CRITICALLY Describe why viruses like those that cause commoncolds are considered “more successful” than viruses such as Ebola.
JOURNAL WRITING Read The Andromeda Strain (1969) by MichaelCrichton, a science fiction story about an alien virus that comes to Earth.In your journal, record your reactions to the book. What similarities didyou notice between how scientists in the story handled the alien virus and how present-day scientists study viruses such as Ebola?
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ENVELOPED VIRUSESSome viruses, such as influenzaand HIV (the virus that causesAIDS), are enclosed in an enve-lope composed of lipids, carbohy-drates, and proteins. Envelopeproteins (right) form spiky projec-tions that help the virus gain entryto a host cell, much like keysfitting into a lock.
HELICAL VIRUSESHelical viruses are shaped like tinycylinders, with the viral geneticmaterial spiraling down the centerof a hollow protein tube. Tobaccomosaic virus (below), which infectsplants (right), is a long helicalvirus.
PHAGESBacteriophages, or phages for short, are viruses that infect bacteria. ThisT4-phage (top left), looks like a miniature lunar-landing module. It has aDNA-containing head, a protein tail, and protein tail fibers that attach tothe surface of a bacterium. Once viruses are attached (left), the tail sec-tion contracts and pierces the cell wall, and viral DNA is injected intothe host cell.
T-PHAGE E-COLI
T-PHAGE
TOBACCO MOSAIC VIRUS Magnification: 30 000� PLANT INFECTED BY TOBACCO MOSAIC VIRUS
INFLUENZA VIRUSMagnification: 17 150�
T-PHAGES, IN BLUE, INFECTING E. COLI BACTERIUMMagnification: 90 000�
EBOLA VIRUSMagnification:19 000�
DEADLY BEAUTYSome viruses have irregularshapes. The Ebola virus (below),which causes massive internalbleeding in humans, has a twisted,worm-like form. A strain of Ebolavirus from Zaire, Africa, is one ofthe most deadly viruses researchershave ever studied.
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FOCUSON
Agriculture was perhaps thesingle most important develop-ment in human history. It is noaccident that the beginnings ofcivilization occurred in produc-tive farming areas. Today, manyfarmers grow just one cropcalled a monoculture.Monocultures enablefarmers to use mach-inery for planting,cultivating, and har-vesting. However,monocultures are veryvulnerable to disease.Pest infestations canspread rapidly, wipingout an entire season’scrop. To combat this problem,farmers have begun to usenative species that are less sus-ceptible to disease. Scientistsare also using genetic engineer-ing techniques to make cotton,corn, and other crop plantsmore insect resistant.
WHEAT — Nearly one-third of all land in the world used for cropproduction is planted in wheat.Wheat probably originated inthe Middle East and was an impor-tant food for the ancientMesopotamian, Egyptian, andIndus civilizations.
RICE — Most humans equate ricewith survival. In Asia, rice is thebasis of almost all diets. More than95 percent of the world’s rice cropis used to feed humans. Rice is the only grain that can grow sub-merged in water.
Plants for PeopleAGRICULTURE
RICE FIELDS
GATHERING WHEAT
CORN — Farmers in the UnitedStates use more acreage to growcorn than any other crop. Livestockconsume most of the corn crop, buta significant portion also goes tomanufacture starch, oil, sugar, meal,breakfast cereals, and alcohol.
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POTATOES — A South American native, the potato arrived inthe United States via Europe. This nutritious root vegetablecontains many essential amino acids. Potatoes also containvitamins B and C and the minerals calcium and iron.
OATS — Oats make excellent food for both animals andhumans. Containing from 10 to 16 percent protein, oats arelow in fat and high in carbohydrates, proteins, B vitamins,fiber, and minerals. Native to northern Europe, oats growwell in poor soils as well as in cool, wet climates.
BARLEY — Barley was probably oneof the first grain crops grown byhumans. It was grown in theMiddle East about 10 000 yearsago. The world’s fourth largestcereal crop, barley grows fast andis able to withstand harsh growingconditions in rugged climates suchas Lapland and the Himalaya.
SORGHUM — Since prehistorictimes, sorghum has been a majorfood crop in Africa. Because of itsextensive root system, sorghum isespecially drought resistant, pro-viding food for people and hayfor cattle.
POTATO HARVEST
SORGHUM CROP
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SILVICULTUREOften confused with forestry, silvi-culture includes the growing oftrees for lumber and paper and for food crops, such as apples andpecans. Oranges, walnuts, andolives are some of the foods grownon trees. Trees are also sources ofmedicines, such as aspirin, origi-nally derived from the bark of thewillow tree, and quinine, from theCinchona tree, used to treat malaria.Trees help reduce air pollution andreplenish the oxygen we breathe.
CHEMICALS AND SPICES — Wood is the source ofmany chemicals including wood alcohol, latex forrubber, and cellulose used to make paper.Charcoal and rayon as well as spices, such as cin-namon and cloves, also are tree products. Taxol,an extract from the bark of a Pacific Coast yewtree, is currently being used to fight cancer.
LUMBER AND FUEL — When people think of forest products, they often think of lumber. A common building material, wood is easy to
work with, durable, relatively abundant, andlightweight—making it ideal for home construc-
tion. Easy to transport and to store, wood is also asource of fuel throughout much of the world.
RESEARCH ON FRUIT TREES
STRIPPING BARK FOR MEDICINAL USE
HARVESTED LOGS
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EXPANDING Your View
APPLY CONCEPTS Take a piece of paper and make two columns. Inthe left column, list all of the products essential to your life that comefrom plants and/or agriculture. In the right column, list all of the essen-tial products that do not come from plants and/or agriculture. Nowwrite a paragraph summarizing the role of plants in your life.
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HORTICULTUREMany of the plants you are familiar with were originally brought from distant lands by naturalists and explorers. By preserving and cultivatingthese species, horticulturists gave us a legacy of beautiful and useful flowers and foods. Selective breeding, grafting, and more recently, geneticengineering are some methods used by plant scientists.
FLOWERS AND MEDICINAL PLANTS —Tulips, roses, scented herbs, andornamentals of all kinds have been asource of pleasure for generations.Plants have been used for medicinalpurposes for thousands of years.Today plants continue to be a majorsource of pharmaceuticals andherbal remedies.
FRUITS — The two main types offruits are dry fruits and fleshyfruits. Acorns, walnuts, and pecansare among the dry fruits used asfood by wildlife and humans.Fleshy fruits include pears, rasp-berries, apricots, and cherries.
VEGETABLES — Vegetables wereamong the first plants cultivated byhumans. Green cabbage, water-cress, and radishes—members ofthe mustard family—were knownto Egyptians and Romans in theBronze Age. Root crops such asbeets, carrots, sweet potatoes, andturnips are highly prized for fiberand nutrients and because they canbe stored easily over winter.
FRUIT MARKET
FLOWERING BULBS
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Without insects, life as weknow it would be impossible.Two-thirds of all floweringplants depend on insects topollinate them. Insects alsodigest and degrade carrion,animal wastes, and plantmatter. Their actions helpfungi, bacteria, and otherdecomposers recycle nutri-ents and enrich the soil onwhich plants and all terres-trial organisms depend.
SIZE AND DIVERSITYInsects are members of the phylumArthropoda and the class Insecta.The most diverse class in the animal kingdom, Insecta is also the largest—it contains more species than all other animal groups combined.
CHARACTERISTICSInsects have three body divisions—head, thorax, abdomen—and six legs attached to thethorax. The abdomen has multiple segments,
the last ones often possessing external reproductive organs.
Most adult insects have wings, usually one or two pairs. An insect’s skin, or integument, is hard yet flexible, and waterproof. Many
insects must molt in order to grow larger before metamorphosing into
adult forms. Because of their ability to fly, a rapid repro-
ductive cycle, and a tough, external skeleton,insects are both resilient and successful.
Insects
COW KILLER ANT
FLAME SKIMMER
DRAGONFLIESHONEYBEE
MONARCH BUTTERFLIES
FOCUSON
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MOUTHPARTSInsects get food by biting, lapping, and suck-ing. Some insects, such as grasshoppers andants, have mouthparts for biting and chewing,with large mandibles for tearing into plant tis-sue or seizing prey. The powerful mandibles
of bulldog ants, for example, are hinged at the sides of the head and bite inward—with great force—from side to side. Butterflies and honeybees have mouthparts shaped for lapping up nectar. Aphids and cicadas can pierce plant stems and then plant juices can be sucked like soda through a straw.
VERSATILITYSome insects, such as the Arcticwoolly bear caterpillar, can survivemany months each year in subzerotemperatures. Others, such as themonarch butterfly, migrate thou-sands of miles to warmer regions.Honeybees conserve heat in freez-ing temperatures by clumping intoa ball that hums and churns allwinter. Although some insects areplant pests, many others prey ontheir plant-munching relatives,and in so doing aid humans in thefight to control crop damage.
SENSE ORGANSInsects gather information about their environment using avariety of sense organs that detect light, odors, sound, vibra-tions, temperature, and even humidity. Most adult insects havecompound eyes, as well as two or three simple eyes on top oftheir heads. The compound eye of a large dragonfly contains ahoneycomb of 30 000 lenses. The image from each lens is sentto the brain and somehowcombined into a compos-ite image, but we don’tknow exactly what suchinsects see. Some insectsnavigate by using sound
waves or following odor trails.Katydids and crickets have “ears” ontheir front legs; houseflies have tastereceptors on their feet.
BULLDOG ANT
ARCTIC WOOLLY BEAR CATERPILLAR
BLUE MORPHO BUTTERFLY
KATYDID
COMPOUND EYES OF GIANT RUDDER FLY
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A SUPERLATIVE CRITTERSome beetles can chew through metals, such aslead or zinc, or timber—not to mention wholefields of cotton. A leaf beetle in the KalahariDesert produces a toxin powerful enough to fellan antelope. The American burying beetle can
lift 200 times its weight. Among Earth’s most rec-ognizable beetles, fireflies light up summer
evenings, and ladybugs control garden pests.
HARMFUL VERSUS HELPFULSome beetles damage crops andspread disease. Spotted cucumberbeetles, for example, devour leavesand flowers of cucumbers, melons,and squashes. They can alsospread bacterial diseases to theplants they attack.
Many other beetles, such asladybugs (also known as lady-birds), should be welcome visitorsanywhere. Gardeners, farmers, and fruit-growers release thou-sands of ladybugs into gardens,fields, and orchards as a first lineof defense against insect pests, especially aphids. The bright red-orange of ladybug beetles is anunmistakable warning to potentialpredators that the beetles areextremely distasteful.
LEAF BEETLE
LADYBUG BEETLES
SOLDIER BEETLE
SPOTTED CUCUMBER BEETLE
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BODY ARMORMany scientists consider beetles to be evolution’s biggest success story and think that thou-sands of additional species remainundiscovered. Beetles—all 350 000described species—presentlyaccount for approximately 1 in 4known animal species. Beetlesthrive in deserts, under tropicalforest canopies, and in water. Onekey to beetles’ adaptability is their“shell”—actually a pair of hard-ened wings called elytra. Elytrapermit some beetles to live indeserts by sealing in moisture andother species to breathe underwaterby trapping air. Many beetles areremarkably resistant to pesticides.
A SPECIAL NICHEThe Mesozoic Era is often identi-fied as the age of dinosaurs. Butthe truly colossal event during thisperiod in Earth’s history was theproliferation of flowering plants.Primary pollinators of the era,beetles most likely fueled thisexplosion of color and fragrance.Beetles fill critical ecological niches as scavengers and as harvesters of caterpillars and otherpests, which, left untended, woulddevour thousands of acres of cropsand forest trees each year. When abeetle species faces extinction—as12 species in the United Statescurrently do—scientists see it as an
early warning system alerting us to significant environ-
mental change.
WEEVIL BEETLE
COLORADO POTATO BEETLE
LONG-HORNEDBEETLE
EXPANDING Your View
THINK CRITICALLY What are the advantages and disadvantages ofan exoskeleton?
JOURNAL WRITING Research social behavior in insects and write ashort essay to present to the class.
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A FLEET-FOOTED HERBIVORESlender, graceful Hypsilophodon (above) was one ofthe fastest-moving ornithischians. With long hindlegs adapted for running, this 1.5-meter herbivoreprobably was able to outdistance most predatorswith ease. Hypsilophodon had a sharp beak and smalloverlapping teeth suited for grinding leaves andother plant material.
THE DUCKBILLSThere were many species of duck-billed dinosaurs. All had long tails,oddly shaped “bills,” webbed fingers, and hooflike, three-toed hindfeet. Despite the duckbills’ webbed fingers, most paleontologists nowthink that duckbills lived on land. Some species, such asParasaurolophus (left), had large, hollow crests on their heads that mayhave amplified whatever sounds these dinosaurs made. Fossil evidenceindicates that duckbills were social animals that moved in herds andcared for their young.
Dinosaurs ruled the world for130 million years, throughout theMesozoic Era. Paleontologistshave identified several hundredspecies of dinosaurs, and about a dozen new types are unearthedeach year. Descended fromancient reptiles, dinosaurs aregrouped into two generalcategories—ornithischians and saurischians—based on the structure of theirhip bones.
HYPSILOPHODON
BODY ARMORMany slow-moving ornithischians had elaborate body
armor. Seven meters long and built like anarmored tank, Euoplocephalus was a peaceful
grazer that must have frustrated many ahungry carnivore. Its body wascompletely encased in bony
plates—even the eyelids werebone-reinforced. It had a tail tippedwith a massive bone “club.” Whenthreatened, Euoplocephalus could havehugged the ground and swung its
club-studded tail from side to sideto protect itself.
EUOPLOCEPHALUS
CORYTHOSAURUS
EDMONTOSAURUS
ORNITHISCHIANS The ornithischians were the grazers of the Mesozoic Era. They are called “bird-hipped” becausetheir hip bones (shown at left) angled backwards like those of modernbirds.The ornithischians had diverse adaptations for eating plants andfor defending themselves against their predatory relatives.
Dinosaurs
PARASAUROLOPHUS
FOCUSON
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BIG BROWSERS The largest dinosaurs ever to roam Earth’s surface were the
long-tailed, long-necked, barrel-bodied sauropods such asSeismosaurus, Diplodocus, and Apatosaurus. These enormous plant-
eaters—Seismosaurus was 36 meters long and weighed
between 80 and 100 tons—could have browsed on leaves
high in the treetops. Sauropodshad small jaws and teeth. Theirleafy meals were ground up in theirstomachs with the help of sharp-
edged pebbles, called gastroliths,that were probably swallowedalong with their food.
THE GIANT MEAT-EATERSBig predatory theropods came in every imaginable shape and size. Fearsome Allosaurus and infamous Tyrannosaurus belong to this group, as does a new 12-meter-long dinosaur from the Sahara—Suchomimustenerensis. Discovered in Niger in 1997 by a team of paleontologists led by Paul Sereno (shown in lower right of large photo), Suchomimus was a fish-eating predator with huge but narrow croc-odilelike jaws and powerful forelimbs with long thumb claws. Although Suchomimus, which means “crocodile mimic,” was adapted to eating large fish, it probably stalked terrestrial prey as well.
THE SMALLEST DINOSAURLess than a meter long, Compsognathus (above), which means “pretty-jawed,” was the smallest of all dinosaurs. A delicate predator, this diminutive theropod probably hunted lizards and small mammals. A double-hinged jaw made it easy for Compsognathus to swallow its prey whole.
SAURISCHIANSThe “lizard-hipped” dinosaurs, or saurischians, had hip bones (shownbelow) like those of present-day lizards, with the pubic bone projecting forward. Two major groups of saurischians are the theropods, or three-toedcarnivores, and the sauropods, or long-necked herbivores.
FOOT OF LARGE THEROPOD
PALEONTOLOGIST SERENO AND TEAM IN NIGER
COMPSOGNATHUS
SERENO AT THE NATIONAL GEOGRAPHIC SOCIETY WITH CASTSKULL OF SUCHOMIMUS TENERENSIS
ALLOSAURUS
EXPANDING Your View
THINK CRITICALLY Compare and contrast the feeding adaptations ofa plant-eating ornithischian and a meat-eating saurischian.
JOURNAL WRITING Using library resources and Web links at, research the hypothesis that dinosaurs disap-
peared as the result of a mass extinction caused by a giant meteor orasteroid colliding with Earth. Analyze, review, and critique the strengthsand weaknesses of this hypothesis.
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APATOSAURUS
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ORDER LAGOMORPHAFrom a standstill, a jackrab-bit can leap straight into theair. Rabbits, pikas, and haresbelong to the lagomorphorder. Most lagomorphs havehind legs suited for leaping.They also have two pairs ofchisel-like incisors that growthroughout their lives.
Most of the more than 4300species of mammals are placentalmammals—whose young arenourished by a placenta whilethey complete development with-in the mother’s uterus. At birth,however, newborn placentalmammals vary widely. Somenewborn gazelles can run fastenough to keep up with the herdwithin days of birth. Young kit-tens are blind and helpless. Ahuman baby spends many yearsdependent on its parents beforeit can take care of itself. Manymammalogists recognize 18orders of placental mammals, ofwhich 12 are shown here.
ORDER PRIMATESChimpanzees communicate, walkupright, and make and use tools. Someunique characteristics of primatesinclude opposable thumbs, flattenednails, excellent vision, and keen intelli-gence. Most primates also have complexsocial lives. Chimps—like orangutans,gorillas, and gibbons—are apes. Alongwith apes, lemurs, Old and New Worldmonkeys, and humans are primates.
PlacentalMammals
ORDER CARNIVORAPowerful and golden-eyed, the jaguar is the largest cat in North andSouth America. Like all carnivores, the jaguar has long, pointed caninesto puncture and tear, and molars to cut and crush the flesh of prey.Some carnivores have claws that help them seize their prey. Most carni-vores are meat eaters; some, such as bears and raccoons, consume plantmaterial as well, and the panda mainly eats bamboo.
JAGUAR
CHIMPANZEE
JACKRABBIT
FOCUSON
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ORDER SIRENIA Slow-moving manatees cruise near the surface of warm, tropical waters and are often injured by speedboats. Theycan nap underwater for up to 15 minutes, but they must come to the surface to breathe. They have tails and frontflippers like whales, distinct heads with a snout that points downward, and short necks. Manatees and dugongs, bothof which are nicknamed “sea cows,” belong to the order Sirenia, which includes only four species.
ORDER RODENTIANeedle-sharp quills protect porcupines from enemies. The Africancrested porcupine—shown here eating a desert melon—is larger andheavier than its distant relatives in North and South America and hasmuch longer quills. Porcupines, beavers, and chipmunks are rodents,along with rats and mice. Rodents, the largest order of mammals, live inall environments. Rodents have continuously growing, razor-sharp inci-sors, which they use to gnaw on hard seeds, bark, twigs, and roots.
ORDER CETACEAThe bottle-nosed dolphin, a kind of toothed whale, uses squeaks,growls, whistles, and other sounds to communicate. Dolphins,porpoises, and whales—all members of the order Cetacea—havelarge complex brains. Little or no hair, streamlined bodies, dorsalfins, specialized forelimbs (flippers), extremely small or absenthind limbs, and the ability to breathe through blowholes on thetops of their heads are other characteristics they share.
MANATEE
AFRICAN CRESTED PORCUPINE
BOTTLE-NOSED DOLPHINS
(t)Anthony Bannister/Animals Animals, (c)Flip Nicklin/Minden Pictures, (b)Brian Parker/Tom Stack & Associates
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ORDER INSECTIVORAThe hairy-tailed mole of North America seldom comes out of its tun-nel. Designed for digging, the mole’s front feet are powerful earthmovers. Its eyes are nearly covered by thick, soft fur, and its eyesight ispoor. Moles, shrews, and hedgehogs use their senses of smell, hearing,
and touch to find food. Although they all mainly eat insects and most have pointed snouts and sharp claws for dig-ging, insectivores have few other shared characteristics.
ORDER PERISSODACTYLAThe wild Przewalski (Pruz WOL skee) horses from Mongolia look similar to theancestor of all modern horses. These hairy horses have thick legs and sturdy bodies. Zoos throughout the world have breeding programs to save this species.Hooved mammals with an odd number of toes, including horses, tapirs, and rhinos, belong to the order Perissodactyla. Most hoofed mammals are herbivoreswith molars for grinding.
ORDER XENARTHRADon’t let their armor fool you—these are mammals. Plates ofskin-covered bone protect most of the armadillo’s body. Butlike anteaters and sloths—the other members of this order—armadillos are well equipped for digging. They lack incisorsand canines and their molars are small cylinders withoutenamel. Xenarthras are found in Central and South Americaand in southern regions of North America.
ARMADILLO
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ORDER PROBOSCIDEAProboscideans use their flexible trunks mostly to gatherplants for eating and to suck in water for drinking. One pair ofincisors is modified into large tusks for digging up roots, strip-ping bark from trees, in social interactions, and as weapons.The largest living land animals, ele-phants spend most of their time eat-ing. They have complex socialsystems, and can live 60–70 years.Their sight and hearing are poor, butthey have an excellent sense of smell.They communicate vocally and bystomping on the ground. African ele-phants are larger than Asian elephants,their ears are larger, they have two“fingers” on the end of their trunks,and their skin is more wrinkled.
ORDER ARTIODACTYLAFound along African rivers, the hippo is one of the largest land mammals.Hippos, cows, giraffes, deer, and the other artiodactyls eat grass and vege-tation. Artiodactyls are hoofed animals with an even number of toes oneach foot. They also have modified stomachs that enable cellulose to bebroken down into nutrients that can be absorbed and used.
ORDER CHIROPTERAThe short-tailed, fruit bat—the nocturnalequivalent of the hummingbird—feedson fruit, pollen, and nectar. Many plantsdepend on bats for pollination and wouldbecome extinct without them. Fruit batsusually use visual navigation rather thanecholocation, a technique involving high-frequency sounds and their echoes. Mostinsect-eating bats, however, use echoloca-tion to navigate. In flight, these bats emitshort, high-pitched cries. When thesounds hit an object, echoes bounce backto the bat, allowing it to locate the object.A bat has skin that stretches from body,legs, and tail to arms and fingers to formthin, membranous wings. Chiropteransare the only mammals with true wingsand the ability to fly.
HIPPO
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SHORT-TAILED FRUIT BAT
EXPANDING Your View
UNDERSTAND CONCEPTS Explain, by using examples, how mammalshave become so successful.
JOURNAL WRITING Research one of the orders not described here.Write about its unique characteristics.
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(l)Frans Lanting, (r)Stephen Dalton/Animals Animals
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THE EVOLVING BRAIN Jumping ahead millions of years to whenthe vertebrates emerged, the five brainsshown here illustrate how evolution hastransformed a simple ganglion to a com-plex brain. As the brain evolved, areasthat control senses, behavior, and coordi-nation became predominant.
Notice that in humans the brain is pro-portionally much larger than it is in manyother vertebrates and that the area dedi-cated to thinking, the cerebrum, coversand dominates everything else.
As animals have evolved overhundreds of millions of years,there has been a tendencytoward ever-increasing com-plexity in the nervous system,and especially, in the brain.Brains had their beginnings asrelatively simple bundles ofnerve cells. But over time, thebrains of vertebrate animalshave become more complex andspecialized. Humans possess themost complex brain in the animal kingdom, a remarkableorgan that enables us to reason, wonder, and dream.
of the
THE SIMPLEST BRAINFlatworms are the simplestanimals that have an identifi-able brain. A planarian, forexample, has a mass of nervetissue called a ganglion thatlies beneath each eyespot.Extending back from theseganglia are long nerve cordsthat run the length of thebody. Between the cords arecross connections that makethe planarian nervous systemlook like a ladder.
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EvolutionBrain
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bdol.glencoe.com/news(t c b)Marie T. Dauenheimer/CMI/National Geographic Society Image Collection, (crossover)Richard Martin/Allsport
EXPANDING Your View
THINK CRITICALLY What are the advantages of having nervous tissueand sense organs concentrated in the head region?
JOURNAL WRITING In your journal, record your predictions about how a person’s behavior might be affected by an injury to the cerebellum.
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HUMAN
THE HUMAN BRAINThe largest part of the human brain is the cerebrum. The cerebrum is dividedinto two hemispheres, left and right. But the feature that makes the cerebrumunique is an outer, folded layer less than 5 mm thick—the cerebral cortex.Because of the cortex, you can remember, reason, organize, communicate,understand, and create.
When you watch an Olympic gymnast perform on the balance beam, youare witnessing the work of a well-trained cerebellum. It is here that musclesare coordinated and the memories of physical skills are stored.
The brain stem consists of the medulla, pons, and midbrain. The brain stemregulates breathing, heart rate, circulation, and other vital body processes.
COMPLEX COORDINATIONThe cerebral cortex may look like a uniform mass of nerve tissue. But differ-ent areas of the cortex receive and process different types of sensory, motor,and integrative nerve impulses. Using a technological tool known as a PET(positron emission tomography) scan (above left), scientists can pinpointareas of increased metabolic activity in the brain. In so doing they can iden-tify specific regions of the cortex that are involved in complex behaviorssuch as playing—from memory—a musical composition on the violin.
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GIRL PLAYING A VIOLIN
PET SCAN
(tr) (tl tr cl)Marie T. Dauenheimer/CMI/National Geographic Society Image Collection, (c)Frank Siteman/Stock Boston/PNI, (b)Marcus E. Raichle, M.D./Washington University School of Medicine
