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THE AMERICAN NATURALIST [Vol. XLIX 



time, and then removed. During the succeeding ten days 

 the F 2 are counted each day as they hatch. The bottle is 

 then discarded for fear of overlapping of generations; 

 for, the F, might mate and deposit eggs before removal. 



It has, however, been observed that each time a bottle 

 is discarded a considerable number of pupae and even 

 larvae remain behind. This being the case, it is possible 

 that the ratio we get does not always represent what 

 actually happens. In order to count the total output, it 

 was decided to transfer the flies to a second bottle on the 

 fifth day. All eggs deposited, during the five days that 

 the parent remained there, would thus have at least fifteen 

 days to develop. Tt was hoped that, in this way, a more 

 representative ratio would be obtained. 



H. It was shown (Tables III and IV) that large num- 

 bers of larvae fail to develop when a bottle becomes too 

 "dry" or too "wet." Considerable care was taken to 

 avoid either of these conditions. If a bottle showed a 

 tendency to dry up, fresh food was immediately added ; 

 when it was too wet, the moisture was absorbed by filter 

 paper. 



III. The yield of a mass-culture bottle is always rela- 

 tively small as compared with that of the same number of 

 flies mated in pairs. This would indicate probably greater 

 mortality due to overcrowding. For this reason only 

 pairs were used in the later experiments. 



In the first of these experiments pure stocks, both pink 

 and red, were used; for it was believed that if differences 

 existed other than the red-pink distribution, between the 

 two varieties, they would be more emphasized if hybridi- 

 zation had not been effected. The chief purpose, how- 

 ever, was to become familiar with the modes of behavior 

 of the races. The experiment follows : 



A number of flies, both pink and red, were isolated 

 within one to six hours after hatching and the sexes kept 

 apart for 3^ days, after which time they were mated, 

 red to red, pink to pink. Immediately after mating, which 

 took place within five minutes to two hours, the males 



