AN ANALYSIS OF THE EFFECT OF SELECTION. 



unfolded enough so that they can be separated on that basis. This 

 is because the anterior pair of dorso-centrals never, so far as I have 

 observed, becomes as large as the corresponding pair in normal flies. 

 The anterior post-alars are also reduced in 8-bristled Dichaets. This 



Table 2. 



separability is a matter of some importance, since, because of the 

 lethal effect of Dichset, any Dichaet culture may produce normal 

 flies. However, the spread wings can be and are used for the separa- 

 tion in all but the rather rare instances of failure to expand properly. 



SEXUAL DIMORPHISM. 



Calculations show that there is a slight but significant sexual di- 

 morphism in bristle number in the Dichset races. Random selection 

 of plus and of minus selected cultures gave the totals shown in table 3. 



Table 3. 



These distributions give the statistical constants shown in table 4. 



The first three columns show that there is a slight difference in the 

 means, the females being higher in both cases. In the case of the plus 

 series the difference is doubtfully significant; in the minus series it is 

 larger and certainly significant. The last column gives the chance 



