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 to the same human family. Slavery was a “sin,” and abolishing it became his “sacred cause.” By extending the abolitionists’ idea of human brotherhood to all life, Darwin developed our modern view of evolution.

Drawing on a wealth of fresh manuscripts, family letters, diaries, and even ships’ logs, Desmond and Moore argue that only by acknowledging Darwin’s abolitionist heritage can we fully understand the development of his groundbreaking ideas.