No. 575] 



AN APTEROUS DROSOPIJILA 



689 



the flies has not revealed a single copulation or attempt at 

 copulation on the part of an apterous male. 8 Similarly 

 the females have been shown to produce rudimentary eggs 

 in an apparently normal manner, and in two cases females 

 have produced offspring when fertilized by winged males, 

 thus indicating their fertility. But many other cases 

 have been observed in which apterous females were fertil- 

 ized by winged males (or at least in which copulation took 

 place), and yet in these observed cases the females invari- 

 ably died without producing offspring, 9 because, I believe, 

 of their physical weakness. 



From these facts it appears practically certain that the 

 apparent sterility is not due to infertility of either sperm 

 or eggs, but results from a weakness which makes it very 

 difficult for the apterous flies to perform the reproductive 

 processes. 



This explains why no crosses have been secured between 

 apterous and apterous, although each sex has been suc- 

 cessfully crossed to winged. It is also supported by the 

 fact that from the cross between apterous male and 

 winged female a large number of offspring were secured, 

 since the winged female could produce many eggs, — 

 whereas in the two crosses between apterous females and 

 winged males only a very few offspring were secured, 

 because the apterous females could only produce a few 



When the experiments were first begun it was hoped 

 that sooner or later one or more inherently vigorous 

 apterous flies would appear which might give rise to a 

 vigorous race. But nothing of the sort took place, 

 although numbers of the apterous flies were given oppor- 

 tunity to breed all through the course of the experiments. 

 Obviously, then, the physiological characteristics, as 



