37O DON C. WARREN. 



of his findings. The fact that the difference which Moenkhaus 

 found between his two strains can be readily explained to be due 

 to a cause entirely independent of the cumulative effect of selec- 

 tion, makes his work support, rather than oppose the findings of 

 the writer. It must be admitted that negative results in secletion 

 work can never be conclusive, for the criticism may always be 

 offered that selection over a greater period might have yielded 







different results. But, nevertheless, we feel justified in conclud- 

 ing from the data here presented that the sex-ratio in Drosophila 

 ampelophila is not readily, if at all, modifiable by selection. 



LETHALS. 



Since the offspring of over 700 pairs of flies were examined for 

 sex in these experiments, and since extreme ratios were sought, 

 it would not be surprising to find a sex-linked lethal mutation. 

 In fact, there is fairly good evidence that one such mutation 

 occurred. This was mating number of 6 generation G of the 

 low strain of the Bloomington stock (Table IV.). This gave an 

 approximate 2: I ratio (84 males to 180 females). Four matings 

 were made from among the offspring of this mating and three 

 out of the four gave 2 to i ratios while the fourth was doubtful. 

 If a sex-linked lethal mutation had occurred, half of the four 

 should have given 2 to I ratios and the other half normal. Since 

 so few matings were made, it is possible that all the females 

 chosen were lethal bearers. The stock was lost by accident at 

 this time and further tests were not possible. But since the 

 unusual ratio was transmitted it is probable that a mutation 

 occurred here. In the high series of the Saratoga stock, there 

 were several ratios which approximate a 2 to I ratio but none 

 of the flies showing it transmitted this tendency to their off- 

 spring. Also in the high strain of the Warsaw stock, generation 

 E mating 12 gave a 2 to I ratio. A large number of matings 

 were made among the offspring of this pair but this unusual ratio 

 was not transmitted to any of the progeny. 



In all of the matings examined there were comparatively few 

 ratios found which were two to one or more. 1 These extreme 



1 The unusual ratios more frequently occurred where the counts were very small. 

 These cases were considered to have been due to some unfavorable environmental 

 condition. 



