1. Baerends, G. P. (1958). Comparative methods and the concept of homology in the study of behavior. Archives neerlandische Zoologies, 13, 401–417.
2. Barlow, G. W. (1968). Ethological units of behavior. In D. Ingle (Ed.), The central nervous system and fish behavior (pp. 217–232). Chicago: The University of Chicago Press.
3. Barlow, G. W. (1977). Modal action patterns. In T. A. Sebeok (Ed.), How animals communicate (pp. 98–134). Bloomington: Indiana University Press.
4. Darwin, C. (1859). On the origin of species by means of natural selection. London: John Murray.
5. Darwin, C. (1872). The expressions of the emotions in man and animals. London: Murray.