Darwin's Sacred Cause, By Adrian Desmond and James Moore

Did Darwin develop his theory of evolution for use as a weapon in the fight against slavery?

This is a bold book that attempts to turn our view of Darwin upside down. In the conventional picture, Darwin gradually develops the theory of evolution through the intensely detailed study of plants and animals, before eventually applying it to human beings. In Desmond and Moore's account, "human evolution wasn't the last piece in the evolution jigsaw; it was the first ... It was there in his first musings on evolution in 1837." What drove Darwin, they argue, was not a detached search for scientific truth but a moral passion: a hatred of slavery and a belief in the unity of the human race as a single species descended from a common ancestor.

To support their argument, Desmond and Moore have explored Darwin's family correspondence, his notebooks and jottings, his reading lists and marginal scribblings. The result is a fascinating picture of a man deeply aware of the racial controversies of his era but making his own contribution with extreme caution, determined to prove his findings beyond all reasonable doubt.

Darwin was born into the fight against slavery. The slave trade was outlawed in the British colonies in 1807, two years before his birth, thanks in part to the efforts of his grandfathers, Erasmus Darwin and Josiah Wedgwood. Wedgwood, the potter, created the logo of the anti-slavery movement: a seal featuring a black slave, kneeling, and the words "Am I not a Man and a Brother?". Slavery itself would not be abolished in the colonies until 1833. Darwin's sisters and cousins were passionate campaigners on the issue, and his own hatred for the system was reinforced by the cruelties he witnessed during his travels on the Beagle.

Before then, however, he had had another formative experience. During his spell as a medical student in Edinburgh he had learned the art of taxidermy from a freed slave, spending some 40 hours in his company, later describing him as "a very pleasant and intelligent man". Ironically, Edinburgh University was in those days becoming the centre of a new racial "science" dedicated to defining the races and labelling some of them as "inferior".

Phrenology began as a way of reading individuals' characters from the shape of their skulls. But soon phrenologists began to make pronouncements on whole races, filling the museums of Britain with skulls from around the world. One Edinburgh pioneer of the technique explained Britain's success in ruling the "Hindoos" of India by identifying their "small organs of Combativeness and Destructiveness". By the middle of the 19th century, the scientific racists had their own organisation, the Anthropological Society of London, where they promulgated the idea that the white man was destined and obliged to enslave "inferior" races. "The mace which brought meetings to order," note Desmond and Moore, "was topped by a negro's head gnawing a human thigh bone." In the run-up to the American Civil War, the society's council included a paid agent of the slave states, and educated opinion was moving in his direction. Sound scientific arguments against the "polygenist" view, that the races were created separately, were lacking.

This is where Darwin found his cause. In order to show that human beings, in all their variety, descended from a common ancestor, he needed to demonstrate similar variation in other creatures. The example he turned to was the pigeon, all of whose many variants descended from the same source, the rock dove. The pigeons would form the starting point of On the Origin of Species, illustrating the revolutionary idea of "natural selection".

What the Origin of Species did not include was an explanation of human racial origins. That was because "Darwin lacked the overwhelming evidence to convince a sceptical world". A tireless and punctilious researcher and experimenter, he would not step beyond the bounds of his own findings to apply them to the human realm. But other people – quietly encouraged by Darwin – would, and did, with the result that controversy was ignited and the great debate about "Darwinism" began. It would be another 12 years before Darwin, in The Descent of Man, would make his own views explicit.

Darwin's Sacred Cause is a scholarly, dense but thoroughly readable book. As well as following every twist and turn in Darwin's researches and opinions, it offers a comprehensive account of 19th-century racial theory and a vivid picture of the era's anthropological controversies. My only reservation is about the authors' method of attributing their vast range of sources in the footnotes, making it a struggle to match quotation to source.