160 ELMEK ROBERTS 



duced. The most convenient method of placing the flies under 

 the microscope was by means of a grooved microscopic sHde, 

 using glycerin to support them in a horizontal position. The 

 magnification is 29 expressed in millimeters. This magnification 

 is used in the text and tables without reduction to actual sizes. 

 In generations containing several hundred individuals, one hun- 

 dred of each sex, taken at random, were measured. 



In selecting the parents, the length of the vestigial wing was 

 not the only factor taken into consideration. The goal in view 

 was a perfect wing, hence the breadth and the position of the 

 wing relative to the body of the fly were also considered. The 

 usual method of feeding fermented banana, as described by 

 Morgan, Sturtevant, Muller and Bridges ('15), was used. 



1 . Sources of error 



As a possible source of error, it may be mentioned that the 

 wings were broken off in some cases, rendering the inclusion of 

 such individuals in the tabulations impossible. This was more 

 common among the parents than among the others of the genera- 

 tion, since the parents were kept much longer before preservation 

 than were the others. Some of the parents died in the breeding 

 bottles and occasionally the wings became mutilated to such an 

 extent that measurement was impossible. Also there was a slight 

 tendency to curhng and shrinking among the larger wings, this 

 type being more common among parents selected for wing length. 



For the most part, mass selection was practiced because of the 

 danger of losing the series if only two individuals were used. 

 Consequently, those selected for parents differed in size of ves- 

 tigial w4ng. Therefore, one cannot be sure which of those se- 

 lected were the actual parents of the next generation in case all 

 did not mate or if mating was indiscriminate. Since the average 

 of the parents was always larger than the average of the genera- 

 tion from which they were selected, except in cases where all 

 the offspring were used as parents, this is not a very serious 

 objection. 



