Darwin's racism is so simple and obvious, it can easily be explained without beating around the bush. Natural selection acts not only on corporeal structures, as Darwin would put it, but on the mental faculties as well. There is a continuum of intelligence from the lowest animals up through man. Darwin was not the first to make this point. Notably, before Darwin, Robert Chambers in 1844 in Vestiges of the Natural History of Creation also saw gradations of intelligence from animals to mankind. We are bound up, said Chambers, "by an identity in the character of our mental organization with the lower animals ..." Man is different in degree, not in kind.
Are all human beings on the same level of intelligence and moral or social values? Darwin didn't think so. He believed the savage races were less intelligent and less moral than Europeans, and, as a direct result of this, they would soon become extinct. That's what happens to inferior groups. He used 'extinct' and 'exterminated' interchangeably.
Darwin was not vicious about it. He did not exult in the white man's superiority (actually, there is one slight exception to that in one of his letters). He did not use nasty epithets to describe savages, though he made his revulsion at their way of life quite plain. He simply used, or misused, his science to claim they were inferior in the struggle for survival. Their extermination was inevitable and he expressed no qualms about it. It cannot be stressed enough that Darwin was convinced this extinction was the result of a biological process, namely, natural selection, and not injustice.
Darwin did believe all human beings had a lot in common (like emotions) and he thought many differences were trivial (like skin color and hair texture). But intelligence was not a trivial difference. It had serious consequences.
It also has to be remembered that even though Darwin believed we were all evolved or descended from a common ancestor, this did not confer any kind of equality in his view. Natural selection may proceed from shared origins, but it goes in the direction of incredible diversity. Apes and human beings were related too, but Darwin in no way believed they had the same degree of intelligence (he thought apes would become extinct too).
It's true that Darwin was opposed to slavery—but not because he thought all men were equal. He opposed it because it was cruel and he hated all forms of cruelty. Wasn't colonialism also cruel? Yes, but here he was prepared to look the other way. He seems to have accepted imperialist cruelties the way he accepted the many harsher aspects of nature (like the wasp paralyzing its prey so that it could be used as live food for the wasp larvae).
There are two extremely puzzling things about Darwin: 1) how he could tolerate certain kinds of human cruelty, but not others, and 2) how he could use science to sanction the extermination of entire peoples, savage or not.
There is a lot more than can be said about all these details, but that's the story in a nutshell.
Copyright 2012 L. Zitzer