More evidence of Darwin's irrational rationalism doesn't help matters. His naturalistic creation myth cannot explain or account for the emergence of morality itself. If his naturalistic suppositions and speculations actually represented the history of the world there would be no basis for condemnation of racism or eugenics in the first place.
Man thinks, man groups in societies at a much higher level than any other species on Earth. Because of this other things became even more important than pure physical traits as man found strength in cohesiveness is much more powerful than strength in physical form. For example, in the animal world the old and infirm are killed. Human societies can continue to receive value from the old and infirm because they can contribute to society in non-physical ways. Stephen Hawking would have been offed long ago in a eugenics-driven society.
I can try to find another quote for you that explains it more concisely. Basically he said that a sympathetic society will be more cohesive, will look out for all in it. Because of this every person in the society has allegiance to whole and will fight as one to protect it. This society will be selected over another society that may be physically superior individually but has no sympathy, one that kills imperfect children, that sterilizes its unwanted. They will die out in the long run.
With a sympathetic society the whole is truly greater than the sum of its parts, not so with an unsympathetic one. Darwin makes the perfect argument against eugenics.