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THE AMERICAN NATURALIST [Vol. LI 



firms the effectiveness of selection and adds greatly to our 

 knowledge of how it is brought ahout. What are tlic facts? 



For these we may turn to the organism of which the genetics 

 are best known ; to the fruit fly Drosophila; and we may accept 

 the accounts presented by those most uncompromising opponents 

 of the effectiveness of selection, the investigators of Drosophila 

 in the Columbia laboratory. Their account we cannot suspect 

 of being colored to favor the selectionist point of view. We find 

 (lata as to certain known modifying factors in the recent im- 

 portant paper of Bridges (1916) on ''Non-Disjunction of the 



no Irss than seven div(M'sc factors that modify the single primary 



lorated 111 parts of the chromosomal apparatus different from 



each is inherited in Mendelian fashion. One of these factors 

 lightens the eosin color in a fly with eosin eyes, nearly or quite 

 turning eosin to white ; this factor Bridges calls "whiting. ' ' An- 

 other has the effect of lightening the eosin color a little less, 

 triviiitr a sort of cream color; this is called "cream h." A third 

 factor dilutes the eosin color not so much: it is called "cream 

 a." \n addition to these. Bridges tells us that he has discovered 

 Tinve othei- dilnters of the eosin color; we will call them the 



of another factor wIims.. onlv effrrt is to make' e(,sin darker; this 

 factor he calls 'Mark ' We L:et tlieiefore the tolloumg list of 



1. Whiting 



2. Cream h 



7. Dark 



We have then in Drosophiln minutely diilVring conditions of 

 a si ugh' shade of color, brought al.ont hy s,'veii modifying factors. 

 Coin'crning these, Bridges makes the t'ollowing remark, which is 

 worthy of particular attention : 



A remarkably close imit:itioii nf sucli a iiiiilti|ilc factor case as that 



