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THE AMERICAN NATURALIST [Vol. XLVIII 



shown by vigor and viability, are directly associated with 

 morphological characters and are not to be separated from 

 them by selection. In other words, the " factor" respon- 

 sible for lack of wings is also responsible for physiological 

 disturbances. 



The only suggestion of an inherent difference between 

 different races, or strains of apterous, is the slight differ- 

 ence in the percentage of apterous offspring in Experi- 

 ments I and III as compared with II and IV. In I and III 

 the ratios of winged to apterous are 3.12 : 1 and 3.24 : 1, 

 respectively, while in Experiments II and IV they are 

 4.06 : 1 and 4.02 : 1. This deviation is not great, but it is 

 fairly constant, and is sufficient, I believe, to indicate a 

 real difference. But whether it is to be explained upon 

 the assumption that in I and III the apterous parents were 

 inherently stronger than in II and IV is not so clear. It 

 might equally well be explained upon the basis of differ- 

 ences in the winged races to which apterous was crossed. 

 Unfortunately, an experimental analysis of the question is 

 prohibited by the difficulty of breeding the apterous flies, 

 and it must, therefore, be left open. One fact, however, 

 is clear, namely that there is no progressive increase in 

 viability of the apterous flies, for the apterous parent in 

 Experiment II, where the viability appears to be low, was 

 descended directly from that in I where it appears to be 

 high, and likewise the parent from the apterous side in 

 IV was obtained directly from III. 



In conclusion it may be profitable to call to mind 

 briefly the bearing of certain of the above data on the 

 question of the nature and behavior of Mendelian 

 "factors." 



The present case of a definite correlation between lack 

 of wings, reduction in size of balancers, and weak physical 

 constitution in the apterous race of Drosophila, shows 

 clearly that one factor may have far reaching effects, and 

 not be limited to any particular part or organ,— a fact 



