Among a glut of new works on the great naturalist, Philip Ball finds the claim that a hatred of slavery motivated his studies

It sounds glib to say that every age moulds Charles Darwin to its own preoccupations, but the temptation is hard to resist. To the Victorians, he was an atheistic agitator undermining humankind's privileged moral status. In the early 20th century, he became a prophet of social engineering and the free market. With sociobiology in the 1970s, Darwinism became a behavioural theory, while neo-Darwinist genetics prompted a bleak view of humanity as gene machines driven by the selfish imperatives of our DNA.

Now, 200 years after Darwin's birth and 150 years after the publication of On the Origin of Species, Adrian Desmond and James Moore, whose doorstop 1991 biography seemed to leave nothing more to be said, offer a new vision of the architect of evolution by natural selection. In Darwin's Sacred Cause (Allen Lane £25), they say that Darwin's work on the common ancestry of all living things was motivated not by abstract curiosity, but by a determination to show that African slaves have the same roots as their white masters. They claim that the foundational text of modern biology was spurred by Darwin's repugnance of the slave trade.

In this view, Darwin's championing of the "brotherhood" of all men might even be considered one of the enabling factors in the election of a black man as US president. There will no doubt be sneers at this "politically correct" Darwin, but it is hard to dispute Desmond and Moore's contention that Darwin aimed to overturn the notion, conveniently adopted by slavers, that blacks and Europeans (and other races) were separate species.

Besides, there was little that was PC about Darwin: he concurred with the prevailing belief in the superiority of whites, and of men over women. None the less, the Darwin who emerges from this meticulous analysis is profoundly humanitarian, despising slavery because he abhorred cruelty to any creature.

Others may be dismayed by the new portrait because it threatens to undermine the supposed "purity" of his quest. If Darwin had sociopolitical reasons to insist on the unity of races, was he a reliable judge of the evidence? Certainly, the many-species view of humankind was not then as nonsensical as it now appears. Data on the long-term fertility of progeny from cross-race unions was scant and often distorted. The fossil record was still silent on human origins, and genomics was more than a century away. Many of Darwin's scientific contemporaries held reservations about the one-species picture, including Darwin's friend Charles Lyell, the geologist who helped give geological time the reach that Darwinian evolution required. But if Darwin had an agenda for his theory, this makes it all the more admirable that he didn't leap to conclusions; he agonised over the data when others would have been content to paper over the cracks.

Desmond and Moore's account does, all the same, highlight the extraordinary degree to which Darwin's science has become encumbered by a cult of personality. It's as though evolution by natural selection still seems so challenging that we are forced back on to the man himself. In Darwin's Island (Little, Brown £20), a survey of his lesser-known works on such topics as insect-eating plants and earthworms, Steve Jones explains how mistaken the conventional narrative of Darwin's life is.

His journey on the Beagle, lavishly documented in James Taylor's The Voyage of the Beagle (Conway £20), contributed only modestly to his evolutionary theory - the Galápagos finches created "inexplicable confusion" rather than enlightenment - and his scientific journey was just beginning, not concluding, as the ship returned to Falmouth.

His theory of evolution didn't transform natural history at a stroke. As Bill Price says in his primer, Charles Darwin: Origins and Arguments (Pocket Essentials £9.99), its first brief airing at the Linnaean Society in 1858, alongside the parallel hypothesis of Alfred Russel Wallace, excited barely a ripple. Neither did it immediately mobilise the church in opposition. After all, evolution - the mutability of organisms - was an old idea.

As Jones neatly puts it, Darwinian evolution is a kind of engineering: "Natural selection is a factory that makes almost impossible things." It shows how to design without a designer and requires two elements: random variation and a means of selecting the "best" outcomes. The latter - selective pressure, as biologists call it - comes from the competition for limited resources, an idea inspired by the Malthusian view of population growth. Darwin came to believe that a key aspect of this filtering lay not in who could run fastest, but who got the most sex. He had no qualms about extending this sexual selection hypothesis beyond the peacocks' clumsy yet dazzling tails to the question of adornment and beauty in human populations, giving him a nascent theory of aesthetics.

Wallace, who Darwin came to see more as hindrance than ally, never accepted this idea; he considered animal displays to be primarily forms of camouflage or warning. Indeed, while a new collection of Wallace's works, Natural Selection and Beyond (OUP £30), makes a brave attempt to redeem his also-ran reputation, he emerges as something of a well-meaning gadfly, prone to the mysticism and spiritualism of his age.

Darwin himself could not explain the other ingredient of his theory: how attributes vary in a population. But genomics now does so in terms of the random mutations that occur in copying and combining DNA between generations. The resulting blend of Darwinism and genetics, called the modern synthesis, offers so apparently beautiful a means of generating biological complexity from simple origins that until very recently it blinded biologists to just how complicated, subtle and messy the transmission of our genes is.

Where much of the problems lay, and still lie, however, was in the concept of the "best" outcome. Even Darwin struggled with that. To biologist Ernst Haeckel, his champion in Germany, it implied an almost mystical path towards some pre-existing ideal form, invoking notions of racial perfection that were later blended into Nazism. Herbert Spencer, who coined the misleading term "survival of the fittest", saw implications for society, and soon enough class-based hierarchies and oligarchies were considered the "natural" state of affairs.

Darwin's cousin Francis Galton took that further, arguing that efforts to care for "the imbecile, the maimed and the sick" hindered the weeding process that maintains society's vitality. Galton's eugenics were promoted by Darwinists such as Julian Huxley, grandson of Darwin's bullish supporter Thomas Henry Huxley. Churchill was another enthusiast, drafting what became the 1913 Mental Deficiency Act which aimed to prevent "mental defectives" from having children.

It is often said that Darwin cannot be held accountable for these excesses, but their seeds are obvious in his works, most notably The Descent of Man (1871), in which he finally explained what his evolutionary theory meant for humankind. The book echoes the concerns of Galton and others about overbreeding in "the reckless, degraded and often vicious members of society", such as the "squalid, unaspiring Irishman" who "multiplies like rabbits". There is a clear natural order of class, rank and race and only Darwin's insistence on a moral duty to help the weak partly redeems him.

All this, and not least Darwin's provocative talk of "favoured races in the struggle for life", seems now to be a residue not only of the chauvinism of the times but of a reluctance to abandon belief in abstract "fitness peaks" that natural selection seeks to scale. In fact, evolution can have no targets; races and species cannot be "perfected". That was one of the main objections to Darwinism, for it seemed to knock Homo sapiens off our pinnacle. We have yet to come to terms with our (highly successful) occupation of a evolutionary niche, rather than embodying a supreme destination. Darwin was equally troubled at how descent from non-humans left God no opportunity to invest us with morality. Evolutionary psychology and game theory now offer accounts, persuasive to varying degrees, of how morality itself is a product of natural selection.

If a principled stance on slavery did motivate Darwin's theory, it would be a curiously inapt stimulus, however noble. The separate-species line peddled by slave-trade apologists needed debunking, but it seems highly unlikely that a failure to do so would have altered Darwin's humane convictions on the matter. Likewise, no one would relinquish slavery for that reason - it took a civil war, not a book. That the pro- and anti- camps divided very much along traditional lines - Whig versus Tory, progressive versus reactionary - indicates that science was never really the issue.

All of which is a timely reminder, in these times of creationism, fundamentalism and climate-change denial, of a truth that scientists continually have to struggle with: in human affairs, battles are rarely won by evidence and logic.

Charles Darwin

A life

Born

12 February 1809 in Shrewsbury. Died 19 April 1882 in Downe, Kent.

Personal life

Married his cousin Emma Wedgewood. They had 10 children.

Education

Studied medicine at Edinburgh before dropping out and transferring to Christ's College, Cambridge with the intention of becoming a priest.

Career

1831 Invited by John Stevens Henslow to join HMS Beagle on a survey of South America commissioned by the Crown.

1838-1843 Edited and contributed to The Zoology of the Voyage of HMS Beagle. Consolidated his theory of natural selection, helped by reading TR Malthus's An Essay on the Principle of Population (1798).

1859 On the Origin of Species

1871 The Descent of Man, and Selection in Relation to Sex

1872 The Expression of the Emotions in Man and Animals.

He said

"We must acknowledge, as it seems to me, that a man with all his noble qualities ... still bears in his bodily frame the indelible stamp of his lowly origins."

They said

"Evolution offers a far richer and more spectacular vision than any religion." - Richard Dawkins