BRISTLE INHERITANCE IN DROSOPHILA 85 



(after the 6th) as far as the eleventh, no further advance has 

 been detected. This is what the comparisons of different fami- 

 Ues from the same grandparents, or successive generations of 

 inbred hues, had led one to expect. In the second generation 

 there is a tendency for parents of higher grades to produce chil- 

 dren of higher grade, while this is not the case in the later gener- 

 ations where high and low variates seem to have the same 

 genetic potentiality, and in so far as this is true, the variability 

 must be explained by extra germinal causes. 



THE ROLE OF THE ENVIRONMENT 

 1. INFLUENCE OF FOOD 



In seeking causes for variability in the number of extra bristles, 

 studies have been made on certain factors of the environment, 

 namely, food and temperature. It was early observed that if 

 any normal flies came from a bottle of inbred selected flies they 

 were apt not to appear among the first flies to hatch in that 

 bottle. Also it became evident that more of the high grades 

 did appear among the first flies hatched, than among the last. 

 To make this clear the data was arranged to show the distribu- 

 tions of the flies counted on successive days from individual 

 bottles. In many cases it was clearly shown that successive days 

 showed lower and lower distributions of extra bristles. In 

 other cases there was no such decline. However, in all cases 

 the highest flies appeared among the first flies drawn off. That 

 the falling off in bristle numbers is not due to any differences 

 in the fertilized eggs is shown by the fact that whenever there 

 is found such a decline in bristle numbers at the end of a bottle, 

 the first flies hatched from the next bottle into which the same 

 parents had been placed, show bristle numbers as high as those 

 found at first in the preceding bottle. This was found to hold 

 good for all cases, even though the parents were moved into 

 three or more successive bottles. When the changes from 

 bottle to bottle were frequent there was less falling off at the 

 end of a bottle. Since the facts are so clear it seems needless 

 to publish a large number of examples. Table 12 gives 3 ex- 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 19, NO. 1 



