CONTROL OF HEREDITY: EUGENICS 389 



pure or mutations may have occurred during 

 the course of his experiments and his selection 

 has served merely to isolate distinct hereditary 

 lines; or, more probably, extent of color in his 

 animals may depend upon multiple factors, 

 which are more numerous in some individuals 

 than in others, as is the case for example in 

 "blending" inheritance (pp. 288-293), and se- 

 lection has merely sorted out some individuals 

 with a larger or smaller number of factors, and 

 consequently with a larger or smaller develop- 

 ment of the character in question, but it has 

 not in the least modified any individual factor. 

 This view finds support in the recent work of 

 McDowell and of Zeleny and Mattoon on the 

 effects of selection on certain characters of 

 Drosophila. 



The crux of this whole controversy lies in the 

 question as to whether inheritance factors fluc- 

 tuate or not; Castle maintains that they do, 

 Johannsen that they do not. If inheritance 

 factors fluctuate they may be changed grad- 

 ually in one direction or another by selection; 

 if they do not fluctuate, but mutate only, 

 changing only rarely and then in one direction. 



