of the characters of individuals depends, of course, on their possession of common genes, originating from the accessibility thru interbreeding to common stocks of genes. The uniformity of the limits of variation within a species may be the result of similarly mutating genes, or of genes derived from hybridization with some exotic influence. Such hereditary property may become fairly well distributed in the course of time thruout any interbreeding population, and if the genes belong to groups of multiple factors controlling single characters in the organism, we have an explanation for the occurrence of every degree of variation between definite limits within a population. The other possible explanation of these common limits of variability is that similar genes may have not only the same potentialities but the same lack of potentialities, reaching similar limits to their capacities for directing the growth of the individual organism or to withstand the effects of environmental factors. As Bateson put it, the degree of variation of an organism may be inherited as much as its degree of uniformity.
We may, then, allow for all individual variation while defining species as populations with common heredity. The older definition of a species as a group of similar (implying nearly identical) individuals fails because of the amount of variability actually found in nature. Definitions of species as groups of individuals distinguished by a definite number of diagnostic characters, or by certain degrees of difference from other species lead to artificial concepts that take no adequate account of individual variation, Mendelian inheritance, hybridization, or mutation. The definition of species on the basis of their fertility or infertility does not delimit phylogenetic units, for while it is true that the individuals within a species must be mostly fertile inter se if they are to maintain any sort of hereditary relationship, the failure of distinct species to interbreed may be due to geographic or seasonal occurrence or to other factors not directly concerned with reproductive physiology. But if species are defined as populations with common heredity, we obtain a concept which seems genetically sound and which, we shall try to show, is a reality in nature.
An appreciation of the fact that species are great populations distinct from Mendelian races, local colonies, or the
