Criteria for subspecies, species and genera as determined by researches on fishes. N. Y. Acad. Jordan, D. S. 1905. The origin of species through isolation. Mayr, E. 1940.
Speciation phenomena in birds.
Mayr, E. 1957. Difficulties and importance of the biological species concept. The species problem . Washington, D. C., Amer. Mayr, E. 1959.