The logic train for Darwinism to translate to eugenics was:

Darwin touted natural selection of the best to the best, illustrating the weakest or slowest would be eaten or die or not be chosen as a mate. Therefore those creatures would not pass on its genetic material to the next generation.

Eugenicists saw that humans allowed the weakest among us to breed where is nature was involved they would have not been able to. So it was actually un-natural for them to procreate. Allowing certain people to procreate un-naturally allowed certain genes to survive where they should have died out.

Yes, religion has done this too. "If you do not believe what I believe, you must die because you could only be evil if you don't believe in my god."

Maybe some day we will be evolved enough to not over think our science and under think our faith.

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I am more concerned with the intellectual environment

'Those who hammer their guns into plows will plow for those who do not.'

~ Thomas Jefferson

64,999,987 firearms owners killed no one yesterday.