308 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
selection does not exist in respect to all important field crops although 
they are subject to the general law of variation. That this must be 
so is clear when we realize that many natural species as well as culti- 
vated varieties of plants are really mixtures of sub-species, varieties, or 
races and that upon being isolated these distinct forms reproduce 
their own particular type. This is most easily demonstrated in plants 
normally self-fertilized, yet in all naturally cross-fertilized plants and 
in higher animals this same endless diversity among individuals is 
even more marked. 
The variation concept.—As we have implied in the above remarks 
the term variation may be used in very different senses in referring 
to different phenomena. Thus variation within a species or variety 
means that the group in question is heterogeneous. Among indi- 
viduals variation may consist of differences between members of the 
same generation or between parents and offspring. Even when thus 
restricted, however, the term is apt to prove ambiguous. Hence it is 
necessary to give some thought to the sources, nature and causes of 
these individual differences in order that we may use clear-cut expres- 
sions which shall always convey to one another a concept of the same 
particular sort of organic difference. 
Classification of variations.—1. Heritability. Character differ- 
ences either represent something specific in the germ or they are 
merely the effect of external stimuli upon the individual soma. In 
the first,case they are inherited, although they will not reappear 
necessarily in all later generations or in all the progeny. In the 
second case they will not be inherited. This is a fundamental dis- 
tinction and may well serve as our primary basis of classification. 
According to heritability variations are either germinal or somatic. 
Under germinal variations we recognize two sub-classes, combinations 
and mutations. Purely somatic variations will be referred to here- 
after as modifications. 
Modifications are non-heritable differences between the individuals 
of a race caused by the unequal influence of different environmental 
factors. Such variations frequently approximate continuity and, 
when studied statistically, display the normal variability curve, 
which will be explained in a subsequent chapter. [See chap. xxv.] 
Combinations are heritable differences between the individuals of 
a race or between the offspring of a pair of parents caused by segrega- 
tion and recombination of hereditary units. They also frequently 
display the normal variability curve. 
