The modern theory of evolutionary dynamics is founded upon the remarkable insights of R. A. Fisher and Sewall Wright and set forth in the loci classici The Genetical Theory of Natural Selection (1930) and ‘Evolution in Mendelian Populations’ (1931). By the time of the publication of Wright’s paper in 1931 all of the theory of population genetics, as it is presently understood, was established. It is a sign of the extraordinary power of these early formulations, that nothing of equal significance has been added to the theory of population genetics in the thirty years that have passed since that time. Yet we cannot take this period to mean that we now have an adequate theory of evolutionary dynamics. On the contrary, the theory of population genetics, as complete as it may be in itself, fails to deal with many problems of primary importance for an understanding of evolution.