You have to stand on your head, hold your breath, and read through 25,000 letters to accumulate a few scraps of ethnocentric mumblings from Darwin, yet "social darwinism" and eugenics did rule the roost for a few decades.
It's just silly to blame ideas for behavior. No Western religion has ever placed such an explicit prohibition on violence and vengence as Christianity, but it didn't seem to prohibit. Ideas seem to melt away in the face of desire.
A man hears what he wants to hear, and disregards the rest.
(OK, someone put in the Hokie Sixties Song Reference placemarker)