THE FACTOES OF EVOLUTION 203 



animals with new characters. But as animals generally 

 only pair with like and entertain a distinct antipathy to unlike, 

 we Ccannot ascribe to this factor an important role in the origin 

 of species. The race feeling is in most animals so strongly 

 developed that very highly differentiated individuals are fre- 

 quently avoided by their own species and even persecuted. 



It only remains to examine one more point. We heard that 

 variability is one of the fundamental conditions of natural selection, 

 but we have not yet examined the question whether the selection 

 of such variations really can produce new species, and whether 

 such selection is not quickly obstructed by natural impediments. 

 This objection might to many seem irrelevant, because the facts 

 of artificial selection and the history of domesticated plants and 

 animals have already decided this question affirmatively. Never- 

 theless, the doubt is fully justified. Many naturalists even go so 

 far as to regard the results of artificial selection as strong evidence 

 against the possibility of selection in Darwin's sense. In an 

 important (though one-sided) paper 1 G. Wolff sums up his doubts 

 thus : * It is true that the theory of selection proceeded from an 

 empirical fact, the results of an artificial breeding of plants and 

 animals, but from these facts it could never deduce an empirical 

 justification of the evolution theory. The facts of artificial breed- 

 ing are rather to be interpreted as an empirical proof of the 

 constancy of the species, because a change in form can only be 

 produced by it within certain well-defined limits that may not be 

 transgressed. The very fact that in many cases we are able by 

 artificial selection to produce comparatively considerable modifi- 

 cations, often within a few generations, is an empirical argument 

 not for, but against Darwin's doctrine of natural selection, for 

 the manifest limitation of the sphere of effectiveness of artificial 

 selection speaks against one of the first suppositions of the 

 theory of selection, which demands such a plasticity of the 

 living material as to make a constant unlimited alteration of 

 organisms in every possible direction possible by the process of 

 selection. It is precisely because artificial selection performs so 

 much that it proves so little. It would be so much more 

 advantageous for the tactical position of the theory of selection 



1 * Die.Begriindung der Abstamnmngslehre.' 



