172 MENDELISM chap. 



The distinction between these two kinds of varia- 

 tion, so entirely different in their causation, renders 

 it possible to obtain a clearer view of the process of 

 evolution than that recently prevalent. As Darwin 

 long ago realised, any theory of evolution must be 

 based upon the facts of heredity and variation. 

 Evolution only comes about through the survival 

 of certain variations and the elimination of others. 

 But to be of any moment in evolutionary change 

 a variation must be inherited. And to be inherited 

 it must be represented in the gametes. This, as we 

 have seen, is the case for those variations which we 

 have termed mutations. For the inheritance of 

 fluctuations, on the other hand, of the variations 

 which result from the direct action of the environ- 

 ment upon the individual, there is no indisputable 

 evidence. Consequently we have no reason for 

 regarding them as playing any part in the pro- 

 duction of that succession of temporarily stable forms 

 which we term evolution. In the light of our present 

 knowledge we must regard the mutation as the basis 

 of evolution — as the material upon which natural 

 selection works. For it is the only form of variation 

 of whose heredity we have any certain knowledge. 



It is evident that this view of the process of 

 evolution is in some respects at variance with that 

 generally held during the past half century. There 

 we were given the conception of an abstract type 

 representing the species, and from it most of the 

 individuals diverged in various directions, though, 

 generally speaking, only to a very small extent. It 

 was assumed that any variation, however small, 

 might have a selection value, that is to say, could be 



