One point, however: while the Nazis, and the eugenics crowd were indeed inspired by Darwin and met with no check under their understanding of "survival of the fittest", there is, in fact, a biological basis for preserving genetic diversity. Any study of "survival of the fittest" assumes something: it assumes the current climate and resources available to the population. Should those change, it is important to be able to adapt to the new conditions: and the more your genotype ties you to the prior conditions, the more difficult that adaptation will be.
Earth is changeable. We shouldn't assume otherwise. Therefore, Eugenics has a check coming from a little common sense and a little biology.