Interesting, author William Shirer referred to Mein Kampf as "crude Darwinism" among other things.
Well, he wasn't a 'real author', to quote certain goofballs.
In all seriousness, the credit should be given to Francis Galton, a cousin of Charles, whose theorizing provided the foundation for eugenics. In fact, early in the 20th century that concept was in vogue in the tonier circles--Nazi Germany pointed to the eugenics program in the United States as a justification for its own programs.
As a side note, a good read is the "Nazi Doctors". The Holocaust History project has his work online. This link is interesting:
Eugenics
I post this not as a 'guilt by association', but to point out that the mudslinging about 'who killed how many' is ridiculous.