12 Selection and Mutation. 



they correspond precisely to the anticipations of the 

 Transmutationists of that time. 



The weak point of the whole position before Darwin's 

 time lay in the application of the conception of mutation 

 to the Linnean species, for these are not really elementary 

 species but aggregate ones, and the question of their 

 origin is obviously different from that of their con- 

 stituent units. 



The second principle in Darwin's theory was the 

 idea that individual variation could lead to the origin 

 of new species by continued selection. This idea was 

 at that time absolutely new, and found many adherents 

 amongst whom Wallace, whose views are set forth in 

 his book ''Darwinism," must be considered the chief. 

 Moreover it is Wallace who has insisted that this form 

 of the theory affords the only possible explanation of 

 evolution. He absolutely rejects the theory of the origin 

 of species by mutation. ''Single variations" according to 

 him have no significance for the theory of descent. 



Experimental researches on individual variability and 

 mutability hardly existed at all at that time. Investi- 

 gfators had to be content with the information of breeders 

 and general biological considerations. But the latter, 

 although they often afford the .strongest argument for 

 the theory of descent, seldom distinguish between the 

 two processes in question. 



The experience of breeders demands in my opinion 

 the most careful examination before it can be accepted 

 as evidence in a scientific inquiry. Their experiments 

 are neither designed nor carried out with this object in 

 view. A critical revision of the whole range of facts 

 on which the doctrine of selection rests is not only ad- 

 missible, but is urgently called for. Darwin accumu- 



