68 MODES OF RESEARCH IN GENETICS 



gories of variation, and that this fact must never 

 be lost sight of in any discussion of heredity which 

 is to lead to valid conclusions. On the one hand 

 are the variations which are definitely inherited 

 (i.e., reappear in the progeny), presumably be- 

 cause they are in some way represented in the 

 germinal substance ; on the other hand are the 

 purely somatic variations which do not reappear 

 in the offspring and are not inherited, presumably 

 because they are not represented in the germinal 

 substance. Now the "law of ancestral inherit- 

 ance" entirely disregards the existence of these 

 two sorts of variations. In its fundamental 

 thesis that the correlation between parent and 

 offspring in regard to somatic conditions is a valid 

 measure of the intensity of inheritance it definitely 

 and implicitly assumes that all variations are of 

 equal significance in heredity. Upon this funda- 

 mental biological error, which is taken as a basic 

 assumption, the whole superstructure of the bio- 

 metric treatment of inheritance is reared. When 

 the significance and consequences of this initial 

 error are perceived it is seen at once that the whole 

 reasoning, so far as it concerns heredity, falls to 

 the ground. Thus it is assumed that the existence 

 of a definite degree of correlation (say r = 0.40 

 =*= 0.02) between parent and offspring indicates 

 inheritance, providing both generations have 

 been reared under reasonably the same environ- 

 mental conditions, and an absence of correlation 



