2 March 2013 | NewScientist | 33

almost entirely private. We must also assume that tribes wiped out earlier tribes, possibly beginning with the Neanderthals.

Nation states have also been involved, but they can protect as well as harm. The 18th-century British government tried to stop settlers going too far westwards, and Native Americans in what became Canada did not fare as badly as those farther south.Peterborough, Cambridgeshire, UK

Squirrelled awayFrom Constance Lever-TracyThank you for the cute photograph of a lonely red squirrel in Aperture (9 February, p 26). The text, however, raised some questions, saying the grey squirrel has “outcompeted the native reds for food and habitat” over most areas of the UK during the last century.

But how much is grey competition to blame for the red’s decline? Have the red’s poor adaptability and resistance to anthropogenic stresses and to disease been the main causes? If no greys had been introduced a century ago, what are the chances that there would in fact be few squirrels of any colour in most parts of Britain?

In an era of climate change, species everywhere are impelled to move to new habitats, where new ecological balances will need to be struck. The presumption of guilt against “invasive species” needs to be replaced by case-by-case evaluation, and where applicable by facilitating new adaptations and introducing wildlife bridges and corridors.Adelaide, South Australia

Right is wrongFrom Isaac MayerThere are indeed liberals and progressives who have unscientific beliefs, as Alex Berezow and Hank Campbell

To join the debate, visit newscientist.com/letters

Letters should be sent to: Letters to the Editor, New Scientist, 84 Theobald’s Road, London WC1X 8NS Fax: +44 (0) 20 7611 1280 Email: letters@newscientist.com

Include your full postal address and telephone number, and a reference (issue, page number, title) to articles. We reserve the right to edit letters. Reed Business Information reserves the right to use any submissions sent to the letters column of New Scientist magazine, in any other format.

For the recordn We are irate at having said that an Aristotle quote was from Rhetoric, rather than Nicomachean Ethics, in our look at the possible upsides of anger (9 February, p 48)

point out (2 February, p 24). But in political parties with left leanings, these unscientific beliefs are relegated to the fringe, whereas, they are in the mainstream of the US Republican party.

Maybe the fact, as reported in your editorial in the same issue (p 3), that only 9 per cent of scientists identify themselves as conservative, with even fewer seeing science as a conservative field, should be considered further. Why is it that the majority of scientists are liberal? Maybe because a majority of conservatives, at least in the US, are anti-science.San Diego, California, US

Sleep on itFrom Ken GreenI’d like to shed more light on the possible purposes of sleep and dreaming (2 February, p 30). Throughout my active life I have always solved my most intractable problems, mostly technical, while asleep – provided that I first prime myself by going through the history of the problem in a logical sequence. My most creative thinking has come about this way.Tintagel, Cornwall, UK

From Jean MacnaughtonThe number of people who claim they dream in colour rose once films and TV stopped being in black and white. David Robson also mentions visual elements predominating over sounds and smells. I was born in 1935, and so I grew up with radio, not TV, and

my dreams are still full of the spoken word.

For example, very early in the morning on the UK’s election day in 1992, I heard the BBC announce the result. It was some time before I was awake enough to realise this was a dream, since the polling stations hadn’t opened.

The next morning, the BBC announced the result just as I’d dreamed it: a nightmare come true, heralding five more years of Conservative rule.Wooler, Northumberland, UK

Font of some wisdomFrom Arthur PrentYour look at how fonts can mess with our minds highlighted the fuss over Comic Sans being used in a presentation announcing the possible discovery of the Higgs boson at CERN (22/29 December, p 68). In a technical context, lack of precision can easily lead to misunderstanding or even danger. Comic Sans is one of the few fonts that clearly distinguish between capital “I”, lower case “l”, and the number “1”. Maybe Comic Sans is not such a bad choice for scientists after all.Brentwood, Essex, UK

Where there’s muckFrom Guy CoxYour article on fertilising crops with human sewage omitted one key point (16 February, p 48). Plants have a highly efficient water-treatment system of their own. In the outer layer of the roots – the cortex – water travels largely through and between the cell walls, which form a very fine physical filter. At the inner boundary of the cortex – the endodermis – the cell walls are impermeable and all water and nutrients must pass through the cells themselves, where any foreign material that has penetrated that far can be destroyed in enzyme-filled

compartments called lysosomes. It is most unlikely that infectious agents will get through to the water-conducting tissues.

Plants evolved these defences for their own protection, but they also protect any animal that eats the leaves and fruit.

The key to safe use of sewage, therefore, is to ensure that it only contacts the roots. Simple, low-tech solutions can achieve this. The converse is also true – using contaminated water in spray irrigation is a recipe for trouble.St Albans, New South Wales, Australia

From Christopher JessopFurther to Fred Pearce’s excellent piece on sewage as a resource, one wonders how much potential soil conditioner the world’s vacuum cleaners “harvest” annually, only for it to go to landfill.Marloes, Pembrokeshire, UK

130302_Op_Letters.indd 33 22/2/13 17:02:29