THE AMERICAN NATURALIST [Vol. LYI 



There is clearly a significant increase, amounting 

 roughly to 10 per cent., in the mean duration of life, or 

 expectation of life at emergence, in the flies in the venti- 

 lated as compared with the unventilated bottles, all 

 other conditions both genetic and environmental having 

 been the same in the two series. 



That the increased amount of fresh air is the cause of 

 the difference is evidenced by the behavior of the flies. 

 In the ventilated bottles the flies tended at all times to 

 congregate on the under side of the bolting cloth going 

 down to the bottom for food occasionally, but otherwise 

 exhibiting a strong preference for the region about the 



