166 THEORY OF EVOLUTION 



dinary population are recognized as due to dif- 

 ferent genetic (hereditary) combinations. 

 No one will dispute this statement. But is all 

 the variability accounted for in these two ways? 

 May not a factor itself fluctuate? Is it not 

 a priori probable that factors do fluctuate? 

 Why, in a word, should we regard factors as in- 

 violate when we see that everything else in 

 organisms is more or less in amount? I do not 

 know of any a priori reason why a factor may 

 not fluctuate, unless it is, as I like to think, a 

 chemical molecule. We are, however, dealing 

 here not with generalities but with evidence, 

 and there are three known methods by means 

 of which it has been shown that variability, 

 other than environmental or recombinational, 

 is not due to variability in a factor, nor to vari- 

 ous "potencies" possessed by the same factors. 



( 1 ) By making the stock uniform for all of 

 its factors chief factors and modifiers alike. 

 Any change in such a stock produced by selec- 

 tion would then be due to a change in one or 

 more of the factors themselves. Johannsen's 

 experiment is an example of this sort. 



(2) The second method is one that is capa- 



