The Hypothesis of Indiscriminate Mutability. 199 



at the moment; in this way alone can their survival be 

 explained. 



According to Wallace's and his followers' modi- 

 fication of the theory of selection that process concerns 

 the individuals of one and the same species only. Accord- 

 ing to the theory of mutation, however, the units with 

 which selection deals are the species themselves. Some 

 survive and extend the limits of their distribution ; others 

 are wiped out ; the former may again be the source of new 

 species, the latter vanish and leave no posterity. The 

 essential idea of this theory may be expressed by saying 

 that by natural selection species are not created but elimi- 

 nated. 



Wallace's theory of selection and the theory of 

 mutation — specializations in two different directions of 

 Darwin's theory — both have to account for evolution 

 without calling in the aid of a theory of variation in a 

 definite direction. Wallace's theory obviously does this 

 inasmuch as according to it the material on which selec- 

 tion operates is individual variability ; moreover the study 

 of this variability rewards the student with a rich harvest 

 of facts which might afford a strong support for this 

 theory were there not other reasons for rejecting it. 



The theory of mutation is in this respect less for- 

 tunate. For mutations themselves can only be directly 

 observed in a very few cases ; and in fewer still have they 

 been properly studied. Mutations are naturally much 

 rarer than individual variations, which every animal and 

 plant exhibits ; they do not lend themselves to investiga- 

 tion in the same way as the latter. Nevertheless they can 

 be made the subject of research, and for many reasons 

 they ought to be investigated just as minutely as varia- 

 tions have been. 



