castle: r6le of selection in evolution 375 



On the other hand, the experimental breeder, though he lacks 

 perspective, is dealing with the actual material concerned in 

 organic evolution. He can see and handle it and observe it 

 change under his hands, as no other student of evolution can. 

 But the changes which he observes taking place must be cor- 

 rectly interpreted if valid conclusions are to be reached concern- 

 ing the general process of evolution. At present experimental 

 breeders are divided in their views. The very same facts are 

 interpreted by some as indicating an orderly progress toward 

 definite end results, and by others as nothing but haphazard 

 unrelated chance occurrences. Just now the latter method of 

 interpretation, embodied in the mutation theory, is very 

 popular among experimental breeders, although it has few 

 adherents among students of paleontology, classification, or 

 geographical distribution. 



The principal tools of the experimental breeder are hybridiza- 

 tion and selection. All are agreed that hybridization (using 

 the term in its broadest sense) is, in the hands of the breeder, 

 a very potent agency in producing variability, upon which selec- 

 tion may then be brought to bear for the production of new or 

 modified types. Lotsy even goes so far as to suggest that all 

 genetic variability is the result of hybridization, but this is 

 flatly disproved by observations of Johannsen who reports the 

 occurrence of mutations in genotypically pure lines of beans, 

 as also by the remarkable series of variations observed by Mor- 

 gan in an inbred race of Drosophila. 



As regards the action of selection, the most widely divergent 

 views are held by experimental breeders. The mutationists 

 hold that it can do nothing but isolate variations which may 

 sporadically put in an appearance or which may by hybridiza- 

 tion be brought together into new combinations. Those who 

 differ from them, and whom they call selectionists, maintain 

 that selection can accomplish more than the mere isolation of 

 variations because it can, by a series of selections, influence 

 further variability. I confess myself an adherent of this at 

 present somewhat unpopular view. I hold it, not because 

 Darwin, held it, nor merely because paleontologists, systematists, 



