But this can't refer to human races. Darwin doesn't address human evolution at in The Origin. "Races" in the title is a synonym for "varieties," or what we would probably call today "subspecies" or "populations".
Later, when Darwin does directly address human evolution in The Descent of Man, he explicitly rejects the notion that there are well defined human "races". He notes that they have been numbered variously from 5 to (I forget the high figure, 30 or 40 something I think) by various authorities. He points out that the so-called races of humans intergrade with each other in all cases, and that there are no constant and distinct features that define any of them.
In fact Darwin systematically undercut the entire basis for scientific racism in his own day. He's not responsible for the fact that later scientific racists would "adapt" to evolution by completely recasting their justifications.