THEORY OF EVOLUTION 165 



highly specialized domesticated animals have 

 been produced. Selection has accomplished 

 this result not by changing factors, but by 

 picking up modifying factors. The demonstra- 

 tion of the presence of these factors has already 

 been made in some cases. Their study prom- 

 ises to be one of the most instructive fields for 

 further work bearing on the selection hypo- 

 thesis. 



In addition to these well recognized methods 

 by which artificial selection has produced new 

 things we come now to a question that is the 

 very crux of the selection theory today. Our 

 whole conception of selection turns on the an- 

 swer that we give to this matter and if I appear 

 insistent and go into some detail it is because I 

 think that the matter is worth very careful 

 consideration. 



ARE FACTORS CHANGED THROUGH 

 SELECTION ? 



As we have seen, the variation that we find 

 from individual to individual is due in part 

 to the environment; this can generally be 

 demonstrated. Other differences in an or- 



