INBREEDING AND SELECTION IN DROSOPHILA AMPELOPHILA 127 
occasional pair to produce young had hitherto been attributed to 
accidental conditions of food, etc., but this no longer seemed a satis- 
factory explanation of all the failures to produce young. This con- 
dition, was, therefore looked into more thoroughly. This was done 
by laying out instead of five pairs a much larger number from the 
offspring of a given productive pair. The greatest care was taken 
with the food, temperature etc. and it soon developed that a 
variable per cent of the pairs were sterile. These sterile pairs were 
to all appearances normal. It was clear now that, while inbreed- 
ing had not reduced the general vitality of the strain thus far, 
there had appeared a high degree of sterility. 
2. Sterility 
* a. Character of the sterility. Examination of all the matings 
brought out the fact that in all cases eggs were present in large 
numbers. This seemed to suggest that the difficulty lay in the 
larvae either failing to emerge from the egg envelope or, succeeding 
in this, failing to carry themselves through the feeding stage or the 
transformation. 
By a careful search of the food of the sterile pairs, after suffi- 
cient time for the larvae to mature had been allowed, it became 
evident that the difficulty lay at a time earlier than the pupal 
stage for none of the latter could ever be found. The food sup- 
plied these sterile pairs was the same as that of the fertile ones since 
it could not be foretold which pairs were going to prove infertile. 
Furthermore, the infertile pairs were usually kept for from 
twenty to thirty days, the best of food being supplied them from 
time to time. The same search showed that no larvae were pres- 
ent, at least so far as direct inspection of the food under a dis- 
secting microscope could be depended upon. 
It was always possible, of course, that the larvae failed to carry 
their development very far, and, thus, being small when they first 
emerge from the egg, might have been overlooked. It became 
necessary, consequently, to take the eggs as they were laid from 
time to time and keep them under observation to see whether the 
larvae ever emerged. This was done by placing a piece of banana 
