SEXUAL DIFFERENCE IN WING LENGTH 273 



The difference between these males and the normal males with 

 respect to the wing-to-femur ratio must be considered as an 

 environmental effect since they are supposed to be germinally 

 the same. It is 1.01 ± 0.15 and is doubtless significant in spite 

 of the precautions taken. However, as was pointed out above, we 

 escape even this difficulty when comparing brothers and sisters 

 since there is no evidence and it is not believable that a given 

 environmental condition will operate to increase the relative size 

 of the brothers' wings and decrease that of the sisters' as is the 

 case in this cross. The sexual difference is only 5.64, that is, 

 1.78 ± 0.25 less than normal. The sexual difference in these 

 flies is still nearly thirty times its probable error but in the normal 

 lot it is about fifty. 



An explanation of this remaining and still considerable sexual 

 difference is not difficult of framing on the hypothesis here fol- 

 lowed. In fact, a wiping out of all sexual difference would have 

 proved too much. It is only recently that any sex-limited char- 

 acters have been known. It is more than likely that there are 

 many factors concerned in wing development. Certainly all 

 the factors have not been isolated since wings have not been en- 

 tirely done away with even in the so-called wingless strain. We 

 are free, then, to postulate that it is these remaining factors which 

 cause the remaining sexual dimorphism of wing length. 



Therefore it seems that, while proof is lacking, indications 

 have been found that the greater wing development in the normal 

 females of this fly than in the males is due to the females getting 

 two sets of those wing factors which are connected with the X- 

 chromosome while the males get but one, the double set causing 

 a greater somatic effect than the single. 



