The Background of Genetics 7 



from the parent-form in such ways that thc>' might well 

 be called degenerates; the new characteristics shown 

 serve to adapt the mutant more poorly to the environ- 

 ment than the parent was adapted. Other mutants 

 may show changes only of an unimportant type, so that 

 they are neither better nor more poorly adai)ted than 

 was the parent. A few of the mutants (according to 

 De Vries) may show such characteristics as to be 

 better adapted to the environment than was the parent- 

 form. Upon this miscellaneous mass of mutants natural 

 selection immediately comes to play, quickly eliminating 

 the poorly adapted types and preserving the good. 

 Thus De Vries holds with Darwin in invoking natural 

 selection, but the t^pe of variations involved is dis- 

 tinctly different. According to Darwin, natural selection 

 serves gradually to build up a new species; according 

 to De Vries, numerous new species are born full Hedged, 

 and natural selection merely decides which of them shall 

 survive. Objections to the De Vriesian theory will be 

 mentioned later in this text, in the light of some of the 

 rather recent work in genetics. 



In addition to these three great explanations of evolu- 

 tion, there are a few others that should be considered 

 briefly. A number of authors (notably David Starr 

 Jordan in this country) have attached primary impor- 

 tance to the principle of "'isolation" in evolution. A 

 few individuals of a species may migrate successfully 

 to a new locality which is, or may subsequently become, 

 sufficiently isolated, by geographic barriers of one t}pe 

 or another, from the original locality that no extensive 

 return migration can take place. The result is that a 

 new colony is established which is sufficiently isolated 



