1. Darwin, C. The Expression of the Emotions in Man and Animals (Oxford University Press, 1998).
2. Lorenz, K. On the formation of the concept of instinct. Nat. Sci. 25, 289–300 (1937).
3. Tinbergen, N. The Study of Instinct (Clarendon Press, Oxford, 1951).
4. Von Frisch, K. The Dancing Bees: An Account of the Life and Senses of the Honey Bee (Harcourt, Brace, 1955).
5. Pavlov, I.P. Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex (Oxford University Press, 1927).