1. Newell, N. D. Evolution 3, 103–124 (1949).
2. Hallam, A. in Causes of Evolution (eds Ross, R. M. & Allmon, W. D.) 249–269 (Univ. Chicago Press, 1990).
3. Stanley, S. M. Evolution 27, 1–26 (1973).
4. Cope, E. D. The Primary Factors of Organic Evolution (Open Court, Chicago, 1896).
5. Gould, S. J. J. Palcontol. 62, 319–329 (1988).