Charles Darwin's Descent of Man
The most significant book published by Charles Darwin after On the Origin of Species (1859) was The Descent of Man in 1871. Darwin saw the mechanism of natural and sexual selection as applicable to human civilization and societies. The extrapolation from Darwin’s natural selection to Herbert Spencer’s Social Darwinism was much less pronounced than is generally acknowledged.
In The Descent of Man, Darwin writes:
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 maimed, and the sick; we institute poor-laws; and our medical men exert their utmost skill to save the life of every one to the last moment. 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. It is surprising how soon a want of care, or care wrongly directed, leads to the degeneration of a domestic race; but excepting in the case of man himself, hardly any one is so ignorant as to allow his worst animals to breed.
[Volume 1, page 168]