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DARWIN'S DANGEROUS IDEA by Daniel C Dennett, Allen Lane pounds 25

DARWIN'S dangerous idea is the theory of evolution by natural selection; Daniel Dennett calls it dangerous because that's how it's often perceived. He is a passionate supporter of Darwin's theory in its modern form, and he hits as hard as he can in its defence. His task is often easy, since the likelihood that the theory is false is about the same as the likelihood that the earth is flat.

There are, however, complications. There are potential objections to the theory that require careful treatment, many of which have been dealt with by Richard Dawkins in books like The Selfish Gene and The Blind Watchmaker. Dennett doesn't have the clarity or the concision of Dawkins, but he has his own gift for popular exposition of difficult ideas, and produces some very stylish parables and teaching stories in support of the view that "an impersonal, unreflective, robotic, mindless little scrap of molecular machinery [DNA] is the ultimate basis of all the agency, and hence meaning, and hence consciousness, in the universe."

Dennett's primary targets are not Creationists who deny Darwin's theory outright, but rather those who accept the basic theory while raising doubts about its details, power and scope. Some 47% of adult Americans believe that Homo sapiens is a species created by God less than 10,000 years ago, according to a 1993 Gallup poll, but Creationists are not worth serious attention. It's people like the palaeontologist and popular-science writer Stephen Jay Gould, or the great linguist Noam Chomsky, who recently claimed that "it is not easy even to imagine a course of [natural] selection that might have given rise to language", who get the criticism. It's the well- known entomologist and sociobiologist E O Wilson whom Dennett accuses of talking nonsense.

Gould has been a constant critic of "modern synthesis", ie Darwin's original theory coupled with subsequent refinements. In particular, he has challenged the "gradualist" view of evolution, proposing instead a theory of "punctuated equilibrium" according to which long periods without change are interspersed with what are, on the geological timescale, sudden periods of rapid evolution. With Richard Lewontin, Gould has also attacked "pervasive adaptationism" or "panadaptationism", the view that all features of organisms can be explained in terms of their "adaptive function" or survival value.

Neither of these challenges succeeds. Gould insists that his theory of punctuated equilibrium has been misunderstood, and that he wasn't proposing some "new or violent mechanism" of evolution. He did, however, claim to have identified a non-Darwinian creative force in evolution, and Dennett adds some very sharp arguments to Dawkins's demonstration, in The Blind Watchmaker, that Gould has in fact done no such thing.

As for adaptationism: Gould and Lewontin were right to warn against seeking adaptationist explanations of everything. But no competent Darwinian has ever done this, and the general strategy of assuming that a feature of an organism has a specific survival value, and then raising questions about what it might be - questions like "Why do certain cicadas have a 17-year reproductive cycle?" - remains valid and indispensable in evolutionary theory.

Gould's contrariness has often been overemotional, and Dennett responds in kind. He usually diagnoses some deep fear of Darwinism in his opponents, but this doesn't seem right in the case of Gould, for whom hostility to orthodoxy is a stronger passion. The fact remains that Gould has produced no valid objections to modern Darwinism; and if one looks at the long history of attempts to overthrow Darwin's theory, one finds, as Dennett says, that they have "managed only to invigorate their target, deepening our understanding of it while enhancing it with complexities undreamt of by Darwin".

The objections continue. The physicist Paul Davies claims that the reflective power of human minds can't be a "minor by-product of mindless purposeless forces". Dennett replies that there is every reason to think that that is exactly what it is - while insisting that its importance is in no way diminished by this origin. He shows how the development of language radically changes the environment in which natural selection takes place, argues plausibly that the existence of language raises no difficulty for the theory of evolution, and develops Dawkins's notion of "memes" - good ideas (like the idea of the wheel or the arch) that invade human brains like viruses, passing from one person to another, spreading through the population, reproducing, competing and evolving.

Dennett also stands up to the mathematician Roger Penrose, who does not see how natural selection can produce mathematical understanding of the sort of which human beings are capable. Dennett disposes effectively of his doubts, but his objections to philosophers like Jerry Fodor and Colin McGinn are less successful. He is too ready to believe that those who are pessimistic about explaining consciousness in physical terms are motivated by a secret love of mystery, or a desire to protect one last thing - the human mind - from the reductive crunch of science. This is a mistake. Many of those who insist that we have no remotely adequate scientific explanation of consciousness are no less intellectually tough-minded than Dennett. And in the case of consciousness, at least, they seem more clear- sighted.

Darwin's Dangerous Idea is a slow machine, weighty in its ingenuity. Dennett writes with an odd mixture of cajolery, condescension, and hostility. There are unnecessarily difficult passages; his initial account of Darwin's theory in the second chapter is disappointing, and his metaphors are not always helpful. But he is nearly always right. This is a rich, tenacious book, and many will find their understanding of Darwin's theory deepened by it.