It has taken me more than a month, but I have finally made my way through Desmond and Moore's dense, excruciatingly detailed Darwin's Sacred Cause. The subtitle of the book gives a good synopsis of what it's all about: "How a hatred of slavery shaped Darwin's views on human evolution."
The evidence Desmond and Moore marshal is substantial, filling 376 pages of smallish text; not only do they describe how deeply involved the Darwins and Wedgwoods were in the anti-slavery movement, they reveal how so much of Darwin's work was informed and motivated by the desire to refute a then-prevalent justification for slavery—the opinion that Africans actually constituted a separate species, and did not participate in the brotherhood of humanity with the white people who longed to be their masters.
Darwin, of course, did the equivalent of driving this nail with a hydrogen bomb, destroying not only the myth of separate ancestry between blacks and whites, but that of separate ancestry between humans and the rest of the living world. It is unfortunate that he still accepted some form of a hierarchy among races, but, situated in his era—for which one's attitude toward slavery must be the strongest litmus test of humanism1—he was very much a progressive.
That proponents of slavery adapted to Darwin, and sought new justifications within the context of the very science that denied them their old ones, is testament to the limitless corruption of humanity, forever ensnared in tribalism.
1 And yes, friends, Wilberforce aces that litmus test, too—let's give credit where credit is due: on the wrong side of history where science is concerned, but a humanist nevertheless.