20 
Indiana University Studies 
of the characters of individuals depends, of course, on their 
possession of common genes, originating from the accessi- 
bility thru interbreeding to common stocks of genes. The uni- 
formity 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 char- 
acters in the organism, we have an explanation for the occur- 
rence 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 potenti- 
alities, 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 defin- 
ing 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 varia- 
bility 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, hy- 
bridization, or mutation. The definition of species on the 
basis of their fertility or infertility does not delimit phylo- 
genetic units, for while it is true that the individuals within 
a species must be mostly fertile inter se if they are to main- 
tain 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 popula- 
tions 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 popula- 
tions distinct from Mendelian races, local colonies, or the 
