170 ELMER ROBERTS . 



In none of the series did the individuals breed true for the 

 type of wing possessed. Under high temperature the size, 

 judged by length of wing, was constantly much larger than the 

 normal vestigial wing, but the form was variable, though more 

 wings were produced like the normal long wing than under any 

 other conditions. It is interesting to compare the length of 

 wings in wild Drosophila with the length of vestigial wings 

 found in this experiment. For twenty-four wild males the 

 average length is 57.7 and for twenty-six females 68. Some of the 

 vestigial wings were as long as the long wings of the wild stock. 

 The position of the wing in relation to the body was also vari- 

 able. In the normal vestigial-winged fly, the wing seems to be 

 stationary and projects from the side of the body at an angle of 

 about 60°. In many of the flies with large vestigial wings pro- 

 duced in these series, the wings were in the position of the nor- 

 mal wing and some functioned as normal wings. In fact, a few 

 individuals escaped by flying away. These wings were thinner 

 than those of the wild Drosophila. 



In vestigial-winged Drosophila the balancers are much reduced 

 in size. Normal balancers from long-winged flies are shown in 

 plate 2, figures 21 and 22, while figures 15 and 16 show the bal- 

 ancers from normal vestigial-winged flies. In those flies with 

 large vestigial wings, balancers of the normal type were present 

 (figs. 17 to 20). 



Unfortunately, the experiment was brought to a close by the hot 

 weather and further work on fixing these types was brought to 

 an end. 



The other series, previously noted, obtained by crossing ves- 

 tigial-winged individuals two, four, and eight successive times to 

 long-winged, gave in general the same results as the series reported 

 at length in this paper. 



IV. SUMMARY OF RESULTS 



1. Selection failed to modify the size of vestigial wing in 

 Drosophila (tables 5 and 6, and text figs. 1 and 2). 



2. The size and form of the vestigial wing were affected by 

 crossing to long-winged flies (tables 1 and 3, and text fig. 3). 



