Those like Arthur Caplan who claim that Nazi ideology did not draw on Darwinism should watch this clip from a 1930s Nazi propaganda film justifying forced sterilization. Near the beginning of the clip the narrator warns that modern society is transgressing against a fundamental law in preserving the unfit. Just what law is he talking about? (Hint: You find it mentioned repeatedly in The Origin of Species and The Descent of Man.)
https://youtube.com/watch?v=0TGyZlXvz7I