14 Heredity and Eugenics 



Perhaps the most influential work to enforce the experi- 

 mental method was that of DeVries, in developing his 

 theory of mutation. His great contribution, therefore, 

 must not be regarded as offering mutation as an explana- 

 tion of the origin of species, for that explanation may not 

 stand, but as establishing, or at least powerfully helping to 

 establish, the study of evolution upon an experimental basis. 



The mutation theory needs no extended explanation, 

 for the current literature of organic evolution is full of it. 

 The long series of cultures of Oenothera, under rigid control 

 and in large numbers, are familiar. The appearance, in 

 relatively very small numbers, of widely different indi- 

 viduals, which "came true" in subsequent generations, led 

 to the inference that new species were appearing under 

 observation, suddenly produced by the parent form, fully 

 equipped as species, without any intermediate stages or 

 any building up by selection. It should be noted that this 

 does not banish natural selection as a factor in evolution, 

 but assigns to it a new role, which is not to produce species, 

 but to select among those already produced. 



The study of mutations is one of the vigorous phases of 

 experimental work today, and some of the results will be 

 presented in the subsequent chapters. Objections to the 

 theory have developed, as must be the case in all theories. 

 There are questions as to the extent of mutation as a process 

 going on among plants and animals ; as to its reliability in 

 producing species; as to whether mutants are really new 

 forms, or only old ones derived from a splitting hybrid 

 parent. It is such questions, and others like them, that 

 experimental work today is trying to answer. 



5. Orthogenesis. — The barest kind of evolutionary back- 

 ground would be inadequate without a mention of ortho- 



