1. Ayllon, T. & Azrin, N. H. (1968). The token economy. New York, NY: Appleton Century-Crofts.
2. Carroll, S. B. (2005). Endless forms most beautiful: The new science of evo devo. New York, NY: Norton.
3. Darwin, C. (1859). On the origin of species by means of natural selection. London, United Kingdom: Murray.
4. Darwin, C. (1871). The descent of man. London, United Kingdom: Murray.
5. Dawkins, R. (1976). The selfish gene. New York, NY: Oxford University Press.