1. Robson, G. C. & Richards, O. W. The Variation of Animals in Nature. (Longmans, Green, and Co.; London, 1936).
2. Darwin, C. Sir Wyville Thomson and natural selection. Nature 23, 32. (1880).
3. Romanes, G. J. Darwin and after Darwin, vol. 2. Post-Darwinian questions. Heredity and Utility, 344 pp. (Longmans, Green, and Co.; London, 1900).
4. Mayr, E. Animal Species and Evolution. 797 pp. (Belknap Press of Harvard Univ. Press; Cambridge, Mass., 1963).
5. Dobzhansky, Th. Genetics and the Origin of Species. 364 pp. (Columbia Univ. Press: New York, 1937).