IN criticising Peter Singer, Richard Bond (THES, May 29) sets up a straw man. Singer's argument was that, while we should not commit the naturalistic fallacy of arguing from "is" to "ought", we are lost if, in striving for what ought to be, we do not recognise the constraints of what is. Unless we know what makes us tick, we cannot hope to teach ourselves to tick to a better rhythm.

Bond states that "we are not slaves to our evolutionary inheritance any more than a bird is a slave to gravity". The analogy is unfortunate, for the anatomical and physiological peculiarities of birds and their greater uniformity of design than any other vertebrate are largely the result of the requirements of flight under the constraint of gravity. One hopes that human nature is less constrained by our evolutionary inheritance.

To equate Darwinism only with natural selection betrays a somewhat restricted view. While natural selection was a key idea in Darwin's intellectual revolution, so were the ideas that evolution had actually occurred (rather than the world being constant or cyclic), that living organisms were united by common descent (rather than being the result of separate creations), that the number of species had multiplied by the splitting of existing species, and that the process had been gradual (rather than through sudden saltations).

The concept of natural selection was the most innovative of Darwin's ideas. If it were, as Bond claims, merely a "simple and rather overrated observation", how is it that (apart from Weisman's early work on the predation of caterpillars) there was little direct evidence for natural selection for many decades? It was not until the "evolutionary synthesis" of the 1940s that selection was recognised as the main cause of evolutionary change, so revolutionary was the idea.

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The argument that the theory of evolution by natural selection is scientifically feeble does not deserve resurrection. Spencer's silly phrase "the survival of the fittest" (taken up by Wallace but not by Darwin) may well be a tautology (it is certainly ambiguous) but it no more describes Darwin's theory than the statement "what goes up must come down" describes Newton's theory of gravitation.

Karl Popper may once have doubted the scientific validity of the theory but in 1978 he wrote: "I am glad to have an opportunity to make a recantation", withdrawing his former view that it is untestable and tautological. It is true that detailed predictions about evolution are impossible. This results from the complexity of nature and perhaps because of the chaotic results of non-linear dynamics; we should no more reject Darwin on that basis than we reject Newton because we cannot predict exactly where Mir will fall to earth.

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Jeremy Greenwood. Director, British Trust for Ornithology, Thetford, Norfolk