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 quite sharp. 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 



