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THE AMEBIC AX NATURALIST [Vol. XLVIII 



Seven females of the constitution ir ' DucM gave gametic 

 ratios ranging from 1.5 : 1 to 2.7 : 1, with the modal class 

 at about 2.0 : 1 . 



Seventeen females ir'Mivm gave ratios ranging from 

 1.5:1 to 3.4:1, with a single individual at 4.2:1. The 

 modal class was at about 2.2 : 1. 



It seems highly probable that all these deviations from 

 a 2 : 1 ratio, not due to insufficient numbers, may be satis- 

 factorily explained on the basis of differential viability, 

 which is known to occur here (for a discussion of the 

 vagaries of differential viability see Bridges and Sturte- 

 vant, '14). I do not wish to be understood as arguing 

 that the gametic ratio for any two pairs of genes is abso- 

 lutely constant, but only that it is in most cases uninflu- 

 enced by the way in which the genes arc combined and by 

 heterozygosis for other genes. That it may sometimes 

 show marked differences is now well established. I have 

 myself studied two cases of this sort, and I have good 

 evidence (not yet published in detail) that there are defi- 

 nite genes which cause great differences in the gametic 

 ratios for whole linkage groups. In one case this gene 

 itself shows linkage to those in the group it affects. But 

 even here the intensity of coupling and of repulsion is 

 affected alike, and it makes no difference how few or how 

 many genes a fly is heterozygous for; the linkage is strong 

 or weak according to the form of the linkage-affecting 

 gene which the fly happens to carry. In each of these 

 cases I have been able to obtain about the same extreme 

 values both for coupling and for repulsion. 



In what follows I shall assume that the intensity of the 

 reduplication series is not affected by the way in which 

 the genes are introduced, nor by the number of linked 

 genes involved in the cross. The obvious corollary of 

 this is that reduplication occurs even in homozygous indi- 

 viduals, and that the nature of the series of divisions is 

 in general independent of the constitution of the indi- 

 vidual. This conclusion is directly opposed to the point 

 of view expressed more especially by Punnett, in the 



