Darwin himself was driven by the logic of his evolutionary thesis to foreshadow eugenic science:
With savages, the weak in body or mind are soon eliminated; and those that survive commonly exhibit a vigorous state of health. We civilised men, on the other hand, do our utmost to check the process of elimination; we build asylums for the imbecile, the mamed, and the sick; we initiate poor laws; and our medical men exert their utmost skill to save … life…. There is reason to believe that vaccination has preserved thousands, who from a weak constitution would formerly have succumbed to small-pox. Thus the weak members of civilised societies propagate their kind. No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man…. We must therefore bear the undeniably bad effects of the weak surviving and propagating their kind; but there appears to be at least one check in steady action, namely that the weaker and inferior members of society do not marry so freely as the sound; and this check might be indefinitely increased by the weak in body or mind refraining from marriage, though this is more to be hoped for than expected.
This is not [Herbert] Spencer but Darwin whose suggestion for the application of his evolutionary metaphor to social policy are entirely consistent with later eugenics.
— Philip Sampson, from Six Modern Myths About Christianity & Western Civilization, p. 69