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THE AMERICAN NATURALIST 



[Vol. L 



(a) First we may assume that A 38m and A 38m , the par- 

 ents of this A 39m+1 , instead of having a 38 gametes had a 39 

 gametes. This would correspond to what is called a mu- 

 tation. The gamete-soma correlation has been broken by 

 the appearance of a new kind of gamete different from 

 the parental gametes. There has been a sudden definite 

 change in the germ plasm, such that an a 38 germ plasm 

 has changed to an a 39 germ plasm. 



(6) Or, we may assume that A 30m+1 was produced by 

 the union of two a 38 gametes, but that these gametes de- 

 develop a 39 soma instead of a 38 soma. This assumption 

 leads logically straight to genetic indeterminism, a con- 

 clusion which, I think, is repugnant to all that is known 

 regarding the physiology of the hereditary process. 



Embracing alternative (a) then, we may next inquire as 

 to the possible cause of this sudden change of the germ 

 plasm, by an amount of which the somatic equivalent is B, 

 from a. iS to a 30 . If we say that this change has been 

 caused by a selection, we can only conclude that the fact 

 that A 38l , A 382 . . . A 38m have been placed in particular 

 cages or apartments to breed, for this is the only physical 

 thing that selection means in this case, has been the cause 

 of the germinal change. For by hypothesis there has 

 been no mixing of germ-plasms. We have been prac- 

 tising straight selection of the most extreme somatic in- 

 dividuals, all by hypothesis A 38 , and each homozygous. 

 It seems to me a misuse of terms to say in such a case as 

 that postulated that selection has caused the appearance 

 of the variation which it selects, unless we are prepared to 

 say that the physical act of the selection of the individuals 

 for mating physiologically effects the germ plasm. Such 

 an assumption we are all agreed would be nonsense. 

 What has happened in the postulated case is precisely 

 this : a new heritable variation in the direction of selection 

 has appeared while selection was in progress. If we say 

 any more than this we are going beyond our facts. If 

 the selectionist would state his results in this form, and 



