||Darwin, Charles. 1871. The Descent of Man. London: John Murray.
Darwin, Charles. 1872/1965. The Expression of the Emotions in Man and Animals. Chicago, IL: University of Chicago Press.
The modern reader will still find much of value in Darwin's original Descent of Man. Darwin's theory of the origin of the moral sense, or the feeling of duty to help one's fellows (Chapter 3), drew on four elements:
Darwin's work has many flaws by modern standards. Yet some comments also seem remarkably prescient — for example, his hesitant speculations about group selection, considerations of punishment, and the role of social selection in biological fitness. It is fascinating to see how Darwin articulated the first informed approach to morality from the unique perspective of, as he put it himself, natural history.
- social instincts, a product of a social species able to provide mutual benefit;
- memory, a foundation for reflection, remorse, and conscience;
- language, allowing each organism to communicate its needs more effectively; and
- habit, the product of learning, which will generate immediate, unreflective responses.
In his "sequel" work, The Expression of Emotions, Darwin delved more fully into the roots of the third element, communication. Namely, how did organisms exhibit externally their internal emotions in such a way that others could respond to them, even without language? Darwin considered body postures and the muscles involved in various facial expressions, noting, for example, how their opposite extremes were associated with contrary emotions. His work drew in part on studies by Ernest Duchenne, who photographed subjects when specific muscles had been activated with electrodes. It is a landmark work in showing how mental states may be investigating physiologically — still accessible and provocative to the general reader.