The central thesis of their new book Darwin's Sacred Cause (to be published later this month) is that Darwin's abhorrence of slavery, and his support for abolition—the sacred cause of the book's title—heavily influenced his scientific thinking. The book claims that Darwin's belief, held by many abolitionists, that the various races of mankind share common descent, rather than always having been entirely separate species (a position held by many in the pro-slavery lobby), was a key driver in his later work to establish that all species share common descent. Ironically, most of Darwin's fellow abolitionists believed in common descent from Adam and Eve—a view which would be utterly discredited by Darwin.
I must admit that, when I first read of the book's central thesis, I was more than a little sceptical. While it seemed obvious that believing in the unity of mankind might make it easier for someone like Darwin to accept that all species are ultimately related, claiming, as Desmond and Moore do, that Darwin's evolutionary theorising came as a direct result of his stance on slavery seemed to be over-egging the pudding somewhat. But they make a compelling case in arguing that "[t]he social context of Darwin's offensive [against separate creations for separate species] has slipped away and been subsequently lost, but recovering it makes sense of the project in Darwin's moral world".
Darwin's Sacred Cause is a detailed and fascinating study of the Nineteenth Century abolitionist and pro-slavery movements on either side of the Atlantic. Although a fervent abolitionist, Darwin could hardly be described as a particularly active member of the movement. But, as in their earlier biography, the authors place Darwin within context, showing how much of his upbringing, and his family and social connections revolved around the abolitionist cause; how his experiences aboard HMS Beagle strengthened his views against slavery; and how his abolitionist belief in human common descent steered much of his scientific work. They do, however, issue the important disclaimer that "[h]umans were not the sole source of insights into transmutation, but part and parcel of Darwin's project".
Although it is only hinted at in a single sentence in On the Origin of Species, human evolution seems to have been very much in Darwin's mind when he asked many of his apparently innocuous questions about domesticated and wild animal species—and even about seed dispersal. When Darwin's masterpiece was eventually published, both the scientific world and the general public immediately saw its implications for the history of human descent. Desmond and Moore infer that this might have been Darwin's plan all along: make a brilliantly argued case for the evolution of animal and plant species, then let the reader come to their own logical, far more controversial conclusions about human evolution. But they also believe that Darwin was not yet able to make a strong case for human evolution, as many human racial differences did not appear to be adaptive. Darwin believed that such differences could be explained not by Natural Selection, but by his other great idea, Sexual Selection. The case for that, however, would have to wait until his later book, The Descent of Man and Selection in Relation to Sex.
Darwin's Sacred Cause is clearly an important book which will be much discussed, debated and argued about by Darwin scholars over the coming years. It will be interesting to follow the debate. Especially for someone like me, who finds himself considerably more open-minded than previously regarding the influence of Darwin's anti-slavery views on his science.
Definitely one for your Darwin library.
Disclosure: Peter McGrath from the Beagle Project was sent a pre-publication copy of this book, which he forwarded to me for review. It was nice to see Peter's name mentioned in the acknowledgements. He's too polite to say so himself, so I'll say it for him: W00t!