1. 2. Darwin, C. R. The Descent of Man and Selection in Relation to Sex, 2:330. New York: D. Appleton, 1872.
2. 3. Freud, S. Beyond the pleasure principle (1920). Standard Edition, 18:14–11. London: Hogarth Press, 1955.
3. 4. Freud, S. Civilization and Its Discontents (1930). London: Hogarth Press, 1930, pp. 8–22.