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- 



