AN ANALYSIS OF THE EFFECT OF SELECTION. 



35 



There is no evidence for contamination. With the one exception 

 noted above, all the variations are in the direction for which the 

 Dichsets were being selected. On the multiple-factor view one would 

 expect this result, since it would seem likely that any modifier would 

 usually affect Dichsets and not-Dichaets in the same direction. The 

 one exception, a 7 from 1190 of the crossbred plus series, is scarcely 

 surprising on this hypothesis, in view of the facts that unselected not- 

 Dichset races may produce sevens (see table 2), and that 1190 was prob- 

 ably not homozygous for a large number of plus modifiers. Since 

 this individual was not tested, it would perhaps be futile to argue the 

 case further. 



TABLE 24. 



It may be noted here that in the Star Dichset stock referred to above 

 (p. 31) there were found to be numerous not-Dichaets with 9 and 10 

 bristles. Unfortunately, no counts were made on these flies, and the 

 nature of the extra bristles was not determined. The stock has since 

 been " purified," to rid it of certain other mutations, and the extra- 

 bristled flies, formerly plentiful, have now disappeared. This stock, 

 as stated above, was continued by mating together (Star) Dichaet 

 flies, without regard to bristle number. These extra-bristled not- 

 Dichsets therefore furnish evidence of the same type as that just dis- 

 cussed, except that the race was not selected for bristle number. 



