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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.
Darwin, "The Descent of Man", Chapter V.

And now, as Mr. Harvey used to say, the rest of the story:

It would be unfair to suggest that Darwin condemned these merciful acts; that was not the intention of the quote presented above. He was simply providing evidence for the theory that humans have evolved a more sophisticated moral sensibility than other animals. In the passage following the quote above he states that eliminating acts of human sympathy would result in the "deterioration in the noblest part of our nature." Even though some intelligence theorists advocating the hereditarian position have used Darwinian ideas to support eugenicist goals like forced sterilization, it is very likely that Darwin himself would have objected to these practices:

…if we were intentionally to neglect the weak and helpless, it could only be for a contingent benefit, with an overwhelming present evil. We must therefore bear the undoubtedly bad effects of the weak surviving and propagating their kind…(Darwin, 1871, 1896, p. 134)