No. 578] DROSOPHILA AMPELOPHILA 101 



It is evident that neither dryness nor moisture has any 

 effect in rendering more favorable the conditions needed 

 for the emergence of either the red or the pink variety. 

 The results show that under unfavorable conditions, large 

 numbers of larvae and pupa? fail to develop, since the total 

 yield of each and every bottle is far below the normal 

 output ; and those which do emerge are but chance sur- 

 vivals. 



It was suggested that the reason the pink flies fell be- 

 hind the expected ratio, was the fact that the mutant 

 was weaker than the wild stock and therefore less likely 

 to come through the larval and pupal stages. If this 

 were the case, they should always fall behind. In many 

 cases, however, they actually exceeded the expectation. 

 Furthermore, they always seemed to be just as vigorous, 

 and to live as long as the wild fly. 



The hypothesis which was formed at this stage, and 

 which determined to a large extent the experiments which 

 followed was, that another factor not related to eye-color 

 was at work. Such a factor, if one is assumed to be pres- 

 ent, by its independent action might be responsible for 

 the disturbance in ratio. It might, moreover, be present 

 in the wild stock which originally gave rise to the pink, 

 since the wild fly is similarly, though less frequently, 

 affected. It is with the search for such factors that the 

 subsequent experiments will chiefly deal. 



Before presenting the data, it will be well to point out 

 some of the possible sources of error which were to a 

 great extent eliminated. 



I. The method usually employed in these experiments 

 is as follows: The flies, which are to be cross-bred, are 

 taken out of the culture bottles as soon as they hatch and 

 before they have time to mate. They are then put into 

 a clean, sterilized bottle in mass cultures of about five or 

 more pairs. There they remain till their offspring (F x ) 

 are ready to emerge : 9-10 days in summer and 11-12 or 

 13 days in winter when the temperature is low. The F t 

 flies are placed in fresh bottles for a similar length of 



