No. 576] VARIATION IN DROSOPHILA 743 



the shorter. (3) The determination of whether or not a 

 fly of a given germinal constitution shall have Beaded 

 wings. The first of these effects is probably not the 

 significant one, in view of the following facts. 



Although as a rule F 2 normal flies give few Beaded off- 

 spring, and Fj Beaded-winged flies relatively many, 

 nevertheless, as has been said, at times normal flies give 

 a high percentage of Beaded offspring and, occasionally, 

 Beaded flies give a low percentage. This can only mean 

 that the dominance of the factor B' is variable, and con- 

 sidering the large number of times that it shows itself as 

 a recessive, it must be that this varying dominance has a 

 marked effect on the percentage of Beaded-winged off- 

 spring that appear. 



The possible amount of variation in the environment 

 surrounding a brood of Drosophila developing under 

 laboratory conditions is enormous, even when the attempt 

 is made to keep conditions constant. These variations 

 depend upon the exact ripeness of the bananas used as 

 food, the length of time the food has been fermenting, the 

 amount of food and filter paper used, the size of the bottle 

 in which the larvae are developing, the tightness of the 

 cotton plug, the temperature of the laboratory, etc. Due 

 to these causes there arise very great differences in the 

 relative moisture content and carbon dioxide content. If 

 the food is not properly prepared it may rot instead of 

 fermenting, or it may mould, or the reaction may be in 

 one bottle quite alkaline and in another very acid. A 

 perfect control thus becomes an impossibility, and there- 

 fore the experiments to be described must be considered 

 as trials only, and not as decisive tests. 



In all the experiments on this subject, Beaded flies of 

 pure stock were mated to normal flies of Wild stock in 

 order to learn the effect of particular environments on 

 the percentage of Beaded offspring in the Fj generation. 

 On our hypothesis, the pure Beaded flies from stock should 

 be of two kinds, viz., those with the lethal gene 1 (i. e., 

 B'IB'L), and those without 1 (i. e., B'LB'L). Correspond- 

 ingly there should be two types of offspring in the F, 



