Sir B. Brodie, after observing that man is a social animal ('Psychological Enquiries,' 1854, p. 192), asks the pregnant question, "ought not this to settle the disputed question as to the existence of a moral sense?" Similar ideas have probably occurred to many persons, as they did long ago to Marcus Aurelius. Mr. J. S. Mill speaks, in his celebrated work, 'Utilitarianism,' (1864, p. 46), of the social feelings as a "powerful natural sentiment," and as "the natural basis of sentiment for utilitarian morality;" but on the previous page he says, "if, as is my own belief, the moral feelings are not innate, but acquired, they are not for that reason less natural." It is with hesitation that I venture to differ from so profound a thinker, but it can hardly be disputed that the social feelings are instinctive or innate in the lower animals; and why should they not be so in man? Mr. Bain (see, for instance, 'The Emotions and the Will,' 1865, p. 481) and others believe that the moral sense is acquired by each individual during his lifetime. On the general theory of evolution this is at least extremely improbable.
Darwin, Descent of Man (1871), p. 71n5. Darwin on moral sense is actually quite interesting; he is taking a position in a very fierce nineteenth century dispute between the moral intuitionists (who believed in an innate moral sense) and the utilitarians (or inductivists, as Mill sometimes calls them), of which this footnote is only one part. In Descent of Man Darwin discusses at length the possibility of an evolutionary account of morality and, in particular, argues that evolutionary biology strongly suggests that there is an innate moral sense (derived from social sentiments). Like all intuitionists of the time, he seems to hold that utilitarians are not completely wrong: but what utilitarianism gets right is to be explained by moral sense theory (which is in turn explained by an evolutionary account of the development of the underlying emotional basis of social life), not vice versa. Part of the reason Darwin thinks it can't be the reverse is that principles of utility are too intellectual; they occur too far downstream in evolutionary history to be the real foundation of moral life, rather than just a derivative branch of it. (This would separate Darwin from some intuitionists, too; intuitionism was a much less unified philosophical camp than utilitarianism.)