This thread is a bit late, but the two have something in common: they were both abolitionists. In fact, there is a fancy new controversial theory that Darwin was in part motivated by his abolitionist views to consider and discover evolution. He came from a family of hardcore abolitionists, and wrote of his own experiences in witnessing the horrors of slavery while on the journey that took him to the Galapagos. The notion that all humans have the same origins would lend credit to the abolitionist appeal that white men and black men were "brothers", so it seems quite likely that was on his mind when he decided to advance his theory. Thoughts?