INTERNAL FACTORS IN EVOLUTION. 251 



arise a practical isolation equal in its efficacy to an 

 actual isolation. 



Internal Factors in Evolution. 



Most of the other theories of evolution which 

 exist at the present day attempt to answer a more 

 fundamental question than that covered by natural 

 selection. Natural selection is impotent unless it 

 has something to select, and all later theories at- 

 tempt to explain the origin of variations. American 

 scientists have, more than any others, emphasized 

 the fact that the survival of variations is a matter 

 secondary to their origin, and it is becoming realized 

 more and more that this is the point to be explained. 

 Darwin hardly attempted to answer the question at 

 all. He recognized the existence of variations, and 

 as an explanation assumed that animals and plants 

 have a tendency to vary. It must not, however, be 

 understood that he was satisfied with this explana- 

 tion. He recognized that each variation had its 

 cause, and thought that these causes existed in the 

 environment. But his inability to discover them 

 taused him to make the above assumption provision- 

 ally, and to call the variations due to chance. We 

 have, however, seen that it is no longer possible to 

 believe in an evolution founded on the selection of 

 chance variation. It becomes necessary, therefore, 

 to reduce this material to law, and the theories which 

 we are now to consider endeavor to do this. 



It is plain that if evidence can be found for be- 

 lieving that variations have been definite in direc- 

 tion, and have effected numerous individuals simul- 



