No. 576] 



VARIATION IN DROSOPHILA 



719 



the counts made in the first few days after the flies of 

 any brood begin to hatch show almost invariably a very 

 much larger percentage of Beaded-winged offspring than 

 do the last counts made. This fact will be considered at 

 some length in the section on environmental effects. 



Enough has been said, at least, to show that, whether 

 the results here described are genetic or environmental 

 effects, the Fj generation is remarkably inconstant with 

 reference to the percentage of Beaded-winged offspring 

 that appear. It is evident that this percentage can be 

 readily altered by (1) changing the length of the period 



daughters, 33% Bd sons, 13% 



Bd & X Wild 9 



during which the brood is allowed to run; (2) by chang- 

 ing the parents from one bottle to another. Extensive 

 studies of environmental effects have shown other ways 

 in which the percentages can be altered, but of this we 

 will treat later. 



2. Behavior in the Second Generation 

 The question at once arises whether the Beaded and 

 normal P, flies are alike genetically. To the solution of 

 this problem two different breeding tests were applied: 

 viz., matings of F, normal by normal, normal by Beaded, 

 and Beaded by Beaded; and back crosses of both normal 

 and Beaded to Wild stock. The results of these tests are 

 given in Tables IV and V. These tables show that when 



