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Charles Darwin was not just the venerable old boffin who peers from portraits in college dining halls. His care-worn countenance, haloed in white hair and beard, cuts the image of the Ancient of Days, but in the flesh he was a Kentish "farmer" (according to postal directories) with a flair for natural history. No narrow specialist was he either, a butterfly or bird eggs man. He ranged across all fields, viewing nature with a virtuosity possible only for a gent of independent means - and mind.

A crack geologist before he was 30, with an F.R.S. for his work on the origin of continents, Darwin explored the Tree of Life from its shoots to its roots - the whole great rambling, randomly branching genealogy of organisms. All creatures great and small intrigued him. He bred plants, pigeons and ten babies. He dissected every known barnacle, fossil and living. He theorised about the origin of species, the mechanism of inheritance and the nature of human emotions. He ended his career as he began, studying the formation of the earth's crust, and was laid to rest in Westminster Abbey, an expert on earthworm ethology.

Darwin's broad vision was born of an age when a scientist's home was his lab. Even before the great man died, that vision began to disintegrate. Biology was professionalised, the Tree of Life parceled out to academic specialists, root and branch. Some studied extinct species, others living ones; some traced the path of evolution, others explained its mechanisms, others described the systematic resemblances among its products. One group, then another, bid to seize the commanding heights of life's history. Arguments raged. For decades Darwin himself was eclipsed, his saintly figure obscured by clouds of controversy.

Only since 1959 and the centenary of the On the Origin of Species has he reappeared in all his glory, dusted down and touched up; the vaunted discoverer of the key theory in modern biology, evolution by natural selection. Population geneticists now proclaim its hegemony in all fields, the one supreme mechanism to which every specialist must finally defer. To these "ultra-Darwinians", natural selection is the driving force behind organic change - the Tree of Life is the historical work of competition at the species level, among organisms or even genes, for reproductive success. Full stop.

Or so says Niles Eldredge, curator of invertebrates at the American Museum of Natural History. In Reinventing Darwin he likens the ultra-Darwinians to an Oxbridge elite at high table, setting the terms on which other specialists may join their lofty forum. Here in 1984, surrounded by portraits of the evolutionary great and good, seated squarely beneath Darwin's beatific face, the doyen of the British ultras, John Maynard Smith, first offered places to paleontologists like Eldredge, Stephen Jay Gould, and Elisabeth S. Vrba, as well as ecologists, systematists and others who reject the ultras' "gene-centred and essentially reductionist approach to evolutionary explanation".

To these "naturalists" (as Eldredge quaintly calls them), evolution takes place at a real hierarchy of levels - social systems, ecosystems, species - which are not mere epiphenomena of genetic competition. Natural selection is "the sole deterministic moulder of adaptive evolutionary change" (or indeed stability) but it follows "rules and circumstances" at each level, most notably those set by the physical, nonbiotic environment (global cooling, meteor impacts, etc). Eldredge himself sees selection as a "filter", not an active force; it simply "biases the distribution of genes between generations".

On one side of the high table, then, sit the newcomer naturalists; on the other, Maynard Smith and friends, notably Richard Dawkins with his selfish genes and the Yankee "prime mover" of ultra-Darwinism, George C. Williams. Each group differs within itself but the table's sides are two.

Now enter Daniel C. Dennett, another Yank, professor of cognitive studies at Tufts University. He takes a seat among the ultras in his tub-thumping tome, Darwin's Dangerous Idea, and generously offers the naturalists drink. "Universal acid", the label reads, vintage 1859: evolution by natural selection. "It eats through just about every traditional concept, and leaves in its wake a revolutionised world-view." It applies to "just about anything I even to itself". As "reductionism incarnate", it promises to "unite and explain just about everything in one magnificent vision".

Just about? Forget the modesty; a fundamentalist would as soon recite: "In the beginning God created just about the heavens and the earth." Dennett's natural selection has all the power of a traditional deity with none of the personal drawbacks. It built the Tree of Life mindlessly, mechanically and singlehandedly, filling "design space" with all the wonderful adaptations touted in William Paley's old Natural Theology (1802). And more: it got life started in primeval times and lately gave rise to minds that understand selection and use it to make other minds - artificial intelligence - as Darwin himself "all but prophesied". "Thus out of next to nothing, the world we know and love created itself." This world, nothing else, is "sacred".

Here, then, we are given to see Reality - the plain unvarnished Truth, uncontaminated by the least perceptible filtrate of wishful thinking. Like rational theology of all ages, Dennett's book offers a profoundly religious vision. It is sincere and self-assured, but wholly secular: Paley manque for the fin de siecle Darwin faithful.

Still, the unholy ghost of creationism hovers about the text and Dennett is determined to exorcise it. He turns the tables deftly. Charles Darwin's dangerous idea is so certain that scientists today must assume objections to it are spurious. The burden of proof now lies with the new unbelievers: "sceptics" who trust in "skyhooks" - miracles, mysteries, heavenly uplifters - or "heretics" who would hamper or tamper with natural selection - people like Eldredge and Gould. Not that any of their odd ideas has a hope in hell. "In due course, the Darwinian Revolution will come to occupy a I secure and untroubled place in the minds - and hearts - of every educated person on the globe." By the good grace of St Charles, we shall all see the light.

With such confidence, Dennett coolly predicts the future of those who religiously doubt his "renewed creed". In the chilling final chapter he warns that "a faith, like a species, must evolve or go extinct when the environment changes. It is not a gentle process in either case". Just as endangered species are found in zoos, so religion is now kept alive in churches and cathedrals, synagogues and mosques, all destined to become "cultural museums". And rightly so, for ancient faiths are among the "superannuated artifacts" that should be preserved for posterity.

But what of resurgent faiths, fundamentalisms Protestant, Catholic, Jewish, and Muslim, as well as "countless minor infections"? Darwin's dangerous idea "in the long run threatens to be just as toxic" to them "as civilisation in general has been toxic to the large wild mammals". Meanwhile, however, we are all in peril; "there are no forces on this planet more dangerous". Just as large predatory species, threatened or not, may have to be caged, so "safety demands that religions be put in cages, too - when absolutely necessary".

"Save the Baptists! Yes, of course, but not by all means. Not if it means tolerating the deliberate misinforming of children about the natural world I Misinforming a child is a terrible offence I Those who will not accommodate, who will not temper, who insist on keeping only the purest and wildest strain of their heritage alive, we will be obliged, reluctantly, to cage or disarm, and we will do our best to disable the memes (cultural traditions) they fight for I Taking a few tips from Darwinian medicine I we can take steps to conserve what is valuable in every culture without keeping alive (or virulent) all its weaknesses."

Thus the paradox of liberalism, unable to tolerate intolerance. It is what fundamentalism feeds on, growing mightier by the day. Epistemological ethnic cleansing will not halt it, even with "universal acid". Quite the contrary. Scientific believers like Dennett would do well to ponder Darwin's own warning in a similar context: "Pause, pause, pause." Their own creed may be caged next.

Darwin's Dangerous Idea, the upshot of 25 years' research, is a brilliant piece of persuasion, excitingly argued and compulsively readable. Its lucid metaphors and charming analogies are reminiscent of the Origin of Species, and like Darwin's critics, Dennett's have ranged from the adulatory to the apoplectic. I cannot begin to assess their arguments, nor have I the authority to, although I do have a strong sense that, theologically, we have all been here before. As Thomas Huxley put it a century ago: "Not a solitary problem presents itself to the philosophical Theist, at the present day, which has not existed from the time that philosophers began to think out the logical grounds and the logical consequences of Theism." It is rather as a reflective historian that I should like just to add one further caveat.

An idea is not an essence soaring above (or seeping through) history, acquiring temporal accidents. Or at least not on what I take to be Dennett's philosophical premises. At best an idea can be only a brain state, a shifting collocation of neural connections - barely a "thing" at all. Yet Dennett tells us that "cute ideas about evolution had been floating around for millennia"; that Darwin's was a "mutation", the "offspring of earlier ideas"; that he did not understand it "in its entirety even when he had formulated it", but rather did a "monumental job of clarifying the idea, and tying it down so it would never again float away"; that indeed Darwin "lacked the terminology" to describe it fully, but now, with "valuable hindsight", Dennett can "reformulate his fundamental idea" as an algorithm. "This idea, that all the fruits of evolution can be explained as the product of an algorithmic process, is Darwin's dangerous idea."

No. It may be Dennett's dangerous idea but it is not Darwin's. Anyway, ideas are not dangerous in themselves, though those who canvass them may be. An idea alone cannot serve as "universal acid". Much of the bite of Darwin's Dangerous Idea (but none of the delicious tang) is neutralised by recognising, from the first page to the last, what Alfred North Whitehead called the Fallacy of Misplaced Concreteness.

The authors of the interdisciplinary essays in Charles Darwin's "The Origin of Species" are more circumspect. But then they have to be, seated among hoi polloi, deprived of rich high-table fare and the right to sound off in the name of science. Literature, sociology and history feed on texts: these seven academic reps tackle Darwin's most "dangerous" one and make a sumptuous meal of it. None writes with Dennett's panache, but all resist the temptation to which he subtly yields: to understand the Origin's cultural impact "by reducing the text to the hypostatised expression of Darwin's scientific mind".

We learn, for instance, that the book was not about "genetic" origins or the origin of life. Natural selection was only its main evolutionary mechanism, and successive revisions re-emphasised nonselective causes. The laws under which evolution took place were "impressed upon matter by the Creator", a phrase that enabled, if not invited, readers to understand selection as a Purposive Agent.

Harriet Ritvo sees the Origin's analogies and antiquated language "partly in public relations terms", as Darwin's effort to integrate his argument "into the context of existing understanding". Fiona Erskine ably shows how the book supplied late-Victorian readers with a "mechanism for converting culturally entrenched ideas of female inferiority into permanent, biologically determined, sexual hierarchy". David Amigoni and Kate Flint trace the "proliferation" of the Origin's language in philology, philosophy, and literature, while Dermott Killingley caps the volume with a ground-breaking study of Darwin's legacy in late 19th-century India.

Only Ted Benton's thoughtful essay offers Dennett any comfort, by separating the analysis of "Darwin's language" from the analysis of "his theory", itself a "profoundly innovative scientific advance". To this the editor Jeff Wallace shrewdly replies that "linguistic analysis appears inseparable from questions of scientific epistemology and procedure once we restore a text like the Origin to its earlier contexts of reception and debate."

For there was an evolutionary high table in Darwin's time, and it is to the first occupants that we must look for the historic measure of his achievement. The table had many sides, around which sat the aristocrats of intellect, the philosophers and the divines, the poets and the politicians, as well as a rag-tag group of arriviste Darwinians. The debates were intense. Darwin, the Kentish "farmer," the scientist of sweeping vision, not natural selection, was the hands-down winner.

Today that table is gone, worm-eaten and worn; the new one is straight and narrow, with space for just two opposing parties. Darwin's portrait, once hung, then removed, has been hung again above it, and underneath the ultras now hold forth in the name of competitive individualism and natural selection. The debates are intense. There is yet no clear favourite, no final contemporary measure of Darwin's achievement.

So far as his theories are concerned, I doubt that there ever will be. For not since the days when Huxley battled bishops and prime ministers poked fun at apes has there been an unclad essence of Darwinism, a textless, irresistible abstraction to compel universal assent. Always instead we find controversy - scientific resistance, religious dissent - as finite language is loaded, again and again, with new evolutionary tasks. Our social world is not Darwin's, nor our natural world, so we should not suppose that our theories can be his either. Like Darwin's very life, his vision of Life's Tree must be reinvented in the discourse of our time.

This is a collective, even a cultural task. "Evolution belongs to no one - no single individual, certainly, but also no particular discipline," Eldredge points out in Reinventing Darwin. "The breadth of discourse is sweeping indeed, and there are very many voices contributing to the conversation." So will today's "naturalists" and "ultras" take a tip from Darwin College, Cambridge, and level the refectory? Or must we again instal an evolutionary high table of many sides? It is better, I propose, that dear old Darwin should gaze down equally upon us all.

James Moore is a lecturer in history of science and technology, Open University.