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 Dichsets. This 



Table 2. 



Stock. 



Wild: 



Falmouth, Massachusetts . 



Berkeley, California 



Mitchell, South Dakota. . . 



Amity, Oregon 



Sydney, Australia 



Pink band 



Black 



Ebony 



Blistered 



White 



d' 



& 



186 

 95 



226 

 59 

 16 



103 

 26 

 80 



114 

 74 



cf 



118 

 104 

 213 

 51 

 21 

 99 

 38 

 92 

 67 

 77 



6" 



10 



Total. 



318 

 199 

 444 

 112 



38 

 209 



64 

 172 

 181 

 153 



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

 lethal effect of Dichaet, any Dichset 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 



