VARIATION 15 
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 individuals varia- 
tion 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 expressions which shall always convey 
to one another a concept of the same particular sort of organic difference. 
Classification of Variations.—1. Heritability Character differences 
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 distinction 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 hereafter 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 the next chapter. 
Combinations are heritable differences between the individuals of a 
race or between the offspring of a pair of parents caused by segregation 
and recombination of hereditary units. They also frequently display 
the normal variability curve. 
Mutations are heritable differences between parents and offspring 
which do not depend upon segregation and recombination. 
These three categories, as Baur has shown, are not to be recognized 
and separated merely according to appearances. The cause of any 
individual differences can usually be established only by careful breeding 
experiments; but by this means the separation of the three categories 
is always possible as the boundaries between them are quitesharp. Modi- 
fications are somatic effects of environmental differences and should not 
be confused with germinal changes which are sometimes induced by 
natural or artificial means and which result in the production of muta- 
tions. Within this first category must be included all place-effects in 
plants and somatic environmental effects in animals. Modifications 
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