Book of the week: Darwin's Sacred Cause

Early in 1832, Charles Darwin nearly got himself thrown off HMS Beagle. He was in Bahia in Brazil, where black slaves worked the sugar plantations. The ship's captain, Robert FitzRoy, reported visiting a slave owner who asked some of his slaves whether they wanted to be free. All said no - to which Darwin replied that maybe, in the master's presence, the answer was worthless. He survived the furious reaction and later wrote home that his "Whig principles" survived too. He would not be tempted away by Tories such as FitzRoy, with "their cold hearts about that scandal to Christian Nations, Slavery".

In Darwin's Sacred Cause, Adrian Desmond and James Moore present Darwin's evolutionary theorising - the most famous fruit of his Beagle experiences - as his anti-slavery politics continued by scientific means. Readers of the pair's great 1991 biography of Darwin will recall that it too sought to connect Darwin's science to the social debates of the day, and Whig reform politics in particular. Once again, extensive and often painstaking research has been turned into a first-rate read that transforms and refreshes even the most familiar parts of Darwin's story. Surprises abound as slavery slips from the background to assume a leading role.

Take his time at the University of Edinburgh, where he studied medicine from 1825 to 18. In standard versions, this was where Darwin decided against medicine and for a new seriousness about natural history, but here it is a place less of intellectual than moral education. Among his teachers and fellow students, the fashion was for the skull-based ranking of the races. Yet Darwin resisted, loyal to his family's anti-slavery, brotherhood-of-man Whig tradition. How strong it was has never been rendered more vividly. For generations, Darwin's people publicised slavery's evils, bankrolled its opponents, petitioned and campaigned. Anti-slavery was "the air Darwin breathed".

The Beagle voyage, too, comes in for startling treatment. The visit to the Galapagos Islands takes up a short paragraph. It is Darwin's encounters with human diversity, not finch or tortoise diversity, that matter here. When he wondered about life on the South American mainland and how it related to that on nearby islands, it was Patagonians and Fuegians who occupied him - and a French author who claimed that they were different species. Darwin didn't believe it. As much as the blacks, free and slave, who he saw all over the world, the savages belonged, he reckoned, to the same species as himself.

Back in England, Darwin soon came to accept evolution on the strength of reflections that, even on Desmond and Moore's showing, hardly featured slavery. But Darwin's image of evolution's pattern is something else. He likened it to a gradually branching tree. This family tree of life, they suggest, was in effect an extension to all animals and plants of the tree that anti-slavery ethnologists had reconstructed for human races. All could be understood as having gradually diverged from a common ancestral stock, with the different human varieties becoming adapted to different environments - much like Darwinian species.

That was in 1837. Nearly 20 years passed before Darwin started to write up his evolutionary theory. "How like my Book all this will be," he scribbled on the latest edition of a classic study of human unity. But the race discussion had moved on. The US was now home to a flourishing, planter-friendly, "pluralist" anthropology. Against the unifiers, pluralists marshalled not just skulls but statistics, and, as Desmond and Moore innovatively stress, work on domesticated animal breeds. If each kind of pigeon could be shown to be a distinct species, with a wild ancestor all its own, why should humans be different?

Worse, the pluralists' efforts came with the sanction of the highest biological authority in the land, the Swiss-born Harvard professor Louis Agassiz. Whereas Darwin in Edinburgh had signed up for taxidermy lessons with an ex-slave, Agassiz found blacks disgusting. In 1850, in the slave city of Charleston, he affirmed that the human races were different species - which, like all other species, did not adapt or evolve. "Agassiz's Lectures in the US", Darwin remarked in a letter, upheld "the doctrine of several species - much, I daresay, to the comfort of the slave-holding Southerns". On Desmond and Moore's reading, Darwin loathed Agassiz, and wrote On the Origin of Species largely against him.

Nothing is more thrilling in this book than Darwin turning his home in the Kent countryside into an anti-Agassiz laboratory. To undermine the notion that similar but distantly located species had to have been separately created, Darwin experimented with seeds. Even after periods in salt water or the guts of birds, they often germinated, suggesting that sea and seagulls alike could spread plants much farther afield than was widely credited. And then there were his pigeon lofts, where Darwin bred a number of varieties. His bravura argument for the common wild origin of domesticated pigeon breeds went into the first chapter of Origin.

When the book was published in November 1859, all eyes, especially in Darwin's set, were on events across the Atlantic. After civil war finally broke out over Southern slavery, the Confederacy sent agents to Britain to ensure that racist, pluralist anthropology remained influential. Darwin didn't attack it directly, although his ally Thomas Huxley did. Not until 1871 did Darwin himself publish on the human races, in The Descent of Man, and Selection in Relation to Sex.

Like Origin, Descent was a showcase for natural selection, which explained adaptive, branching evolution as the result of competition induced by population pressure. In their biography, Desmond and Moore argued that natural selection was a Whig product in much the same style that they now argue the tree of life was. Darwin family friend and Whig stalwart Harriet Martineau, for example, figures in the genesis of both, as champion of the workhouse (natural selection) and as anti-slavery advocate (tree of life). Yet the upshot for natural selection is noticeably different. Much of the time one feels like booing when it shows up here, as it leads Darwin to speculate about stronger, higher races exterminating weaker, lower ones.

"Emotionally confused and ideologically messy" is how the authors describe Darwin's science. But the workhouses expressed the same Whiggish faith in human progress found in the missions to savages Darwin supported. Be that as it may, it is sexual selection, not natural selection, that here binds Descent to Darwin's humanitarian project, as explaining how the races evolved without ceasing to belong to one species. In Darwin's view, as humans had spread, different ideals of beauty had taken hold among different groups. People picked what they fancied, and their efforts added up to new varieties of people. Men and women thus stood to humankind as pigeon breeders stood to pigeonkind, bringing forth many within one.

The tributes to Darwin are in full flow this year, marking 200 years since his birth and 150 since Origin. None will be more generous, to him or us, than Darwin's Sacred Cause. Here is Darwin's achievement recast magnificently for a world aware as never before of the enormity of our slaving past, but also the distance travelled since. Masterpieces come no more timely.

THE AUTHORS

Adrian Desmond, honorary research fellow at University College London, is no stranger to Darwin. His book Darwin, also written with James Moore, won prizes across the world, including the James Tait Black Memorial Prize. He has published five other books on 19th-century science and culture and has four books on the go. In his spare time, he stays in Scotland studying armour-plated fossil fish, "worrying that I'm not writing those books!"

James Moore, originally from Chicago, now sees Britain as his home, saying his move was prompted by Richard Nixon's "new American majority" and that he considers himself "an intellectual refugee". He gained his PhD from the University of Manchester, and has taught for nearly 35 years at The Open University, where he is professor of the history of science. Despite camping in the Amazon and enduring an attempted plane hijack, he says it was his move to the UK that was life-changing, calling it "my own Beagle voyage".