CHANGING ANIMALS 19 



Schmalhausen, I. I. Factors of Evolution. The Theory of Stabilizing Selection, 

 Philadelphia: The Blakiston Company, 1949. 



Simpson, G. G. The Meaning of Evohition. New Haven: Yale University Press, 

 1949. 



Simpson, G. G. The Major Features of Evolution. New York: Columbia Uni- 

 versity Press, 1953. 



