No. 588] EXPERIMENTS IN MASS SELECTION 725 



cms variation signifies only the combined result of several 

 independent agencies. In purely phenotypic variation 

 (such as possibly Johannsen has observed) these agencies 

 are obviously environmental and so do not affect the in- 

 heritance. But in a case of multiple genetic agencies (the 

 existence of which everyone recognizes) a continuous 

 series of variations may result which would be amenable 

 to selection. Pearl and all other pure-line advocates ad- 

 mit the existence of such cases. But the same thing would 

 result if, aside from purely phenotypic variations in a 

 character, its single factorial basis should undergo quanti- 

 tative variation. It is precisely this last named category 

 of cases which alone can explain our rat results. And it 

 is precisely this category of cases which the pure-line ad- 

 vocates, unable to disprove, boldly deny. Driven from all 

 other defences they cling to . this as their last line and 

 solemnly repeat challenges issued years before in mo- 

 ments of greater confidence. Thus Pearl closes his paper 

 with a renewal of the opinion expressed by him in 1912. 



It has never yet been demonstrated, so far as I know, that the abso- 

 lute somatic value of a particular hereditary factor or determinant (/. e., 



Our observations on rats are submitted as a sufficient 

 answer to this challenge. 



I do not suppose that Pearl means to be taken seriously 

 when he says (p. 608) : 



The extreme selectionist appears to believe that in some mysterious 

 way the act of continued selection, which means concretely only the 

 transference of each selected individual from one cage or pen to another 

 to breed, will in and of itself change the lii Tin-plasm. 



I have never heard a selectionist, however extreme, ex- 

 press such a view; certainly I, whose views are attacked 

 in the next sentence, have never entertained such an idea. 

 But Dr. Pearl knows, as well as I do, that while the germ- 

 plasm of the individual remains unmodified upon its trans- 



