738 



THE AMERICAN NATURALIST 



[Vol. L 



2. Environmental influences may possibly act in such 

 a way that only occasionally does a selected in- 

 dividual carry germ plasm more homozygous 

 then the average. 

 B. The implied claim that the facts do not support the 

 supposition that selection has decreased the number 

 of modifiers, or has reduced the heterozygosity in 

 the two races of rats, has been answered by the fol- 

 lowing points : 



1. Selection reduces the variability. 



2. The rate of advance declines as selection is con- 



tinued. 



3. Parental regression is lowered by selection. 



4. Return selections argue that heterozygosis Is still 



present; they indicate that there is less hetero- 

 zygosis after longer selection, since selection re- 

 duces the effectiveness of return selections. 



5. Crosses between the plus and minus races strongly 



suggest that heterozygosity is still present by the 

 increase in variability in Fj ; they also appear to 

 show that there is less heterozygosity in a later 

 generation, since the increase in F 1 is less in a 

 cross after longer selection. 



6. Crosses between the selected races and the wild or 



the Irish race show that more modification ap- 

 pears in the F 2 hoodeds when crosses are made 

 after longer selecting. 



The reader is now in a position to judge whether the 

 writer is justified in concluding that there is still a " pos- 

 sibility that other as yet undiscovered factors might be 

 responsible for the apparent changes observed" (Castle, 

 :15, p. 722) and that the claim that "all the evidence we 

 have thus far obtained indicates that outside modifiers 

 will not account for the changes observed" is too sweep- 

 ing. 



