There has always been a mystery surrounding Darwin: How did this quiet, respectable gentleman come to beget one of the most radical ideas in the history of human thought? It is difficult to overstate what Darwin was risking in publishing his theory of evolution. So it must have been something very powerful--a moral fire, as Desmond and Moore put it--that helped propel him. That moral fire, they argue, was a passionate hatred of slavery.

In opposition to the apologists for slavery who argued that blacks and whites had originated as separate species, Darwin believed the races belonged...