214 



CAUSES OF VARIATION 



of extreme improbability. 1 On the other hand, if influences are 

 discovered which are really efficient in setting bounds to varia- 

 bility, then they make the transfer of characters from parent to 

 offspring so absolutely certain, regular, and fixed, as to seem to 

 leave little or no possibility of variation. 



This latter is the case with the hypothesis of physiological 

 units. Weismann recognized its limitations and proposed the 

 theory of germinal selection 2 to account for variation as well as 

 inheritance. 



This theory assumes that the " biophors," or the physiological 

 units by whatever name they may be called, are engaged in a 

 kind of struggle among themselves within the germ, much as 

 are plants and animals in the larger world outside. 



Any theory of physiological units must include their absorp- 

 tion of food and their power of self-multiplication. If these 

 activities proceed at a uniform rate for each unit involved, then 

 no variation would result from this multiplication ; but if propor- 

 tions change, or if the vitality varies, then variation would neces- 

 sarily result from these causes alone. Now these activities must 

 be either constant or variable. Weismann assumes that they 

 are variable ; that these units of various relative numbers and 

 strengths are competitors among themselves, one with another, 

 for food ; and that those most energetic in food absorption and 

 capable of the most rapid multiplication will not only be the 

 most vigorous but they will also exist in relatively the largest 

 numbers. They will therefore tend the more to impress their 

 characters upon the later development of the individual. 



Under this view of the case the " balance of power " is con- 

 stantly shifting, always in favor of the most vigorous and 

 rapidly multiplying units. Believers in physiological units must 

 either follow Weismann in this conception or else assume on 

 the part of the units absolutely equal powers of nutrition and 

 multiplication, for multiplication there must be if such units 

 avail anything in the r61e of inheritance. 



1 All things considered, inheritance and not variation is the mystery. The 

 wonder is, not that individuals vary, but that they follow as closely as they do 

 the type of the race to which they belong. 



2 Weismann, On Germinal Selection as a Source of Definite Variation (pam- 

 phlet, second edition) [Open Court Publishing Company]. 



