Janklow locked the thread before I could respond but you misunderstood this Darwin quote:
"The American aborigines, Negroes and Europeans differ as much from each other in mind as any three races that can be named...."
I guess it would help you to better understand what is being said there by observing the context around it. Just before that sentence, Darwin is quoted as saying thus:
"The same remark holds good with equal or greater force with respect to the numerous points of mental similarity between the most distinct races of man."
Darwin is saying that all races of man are similar in intelligence and mental capacity. The quote you pulled says that those 3 races differ in intelligence as much as any 3 races of men that you could name. That means that there is not any difference in intelligence between any race of man on the planet. You could switch out either of those three races in the example Darwin has given for another race and the difference would be the same: 0 difference.
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