76 



Conclusions. 

 The data obtained are of interest in a number of ways: 



1. Bar-eye may return to the full-eyed somatic condition by two dis- 

 tinct routes, (a) Reverse mutation in the bar gene may bring the in- 

 dividual back to the condition of the full-eyed stock not only in somatic 

 appearance but also in genetic behavior. (b) A similar somatic ap- 

 pearance of full-eye mcjiy be produced by a mutation in one of the auto- 

 somes which leaves the original bar gene unchanged, as proven by the 

 fact that the crosses between new full-eyed females and full-eyed wild 

 males yield low bar individuals. Change in a gene and production of 

 new genes without change in the principal gene may produce the same 

 result somatically. Breeding tests alone can show the difference. The 

 change in the principal gene brings the individual truly to its original 

 condition. 



2. Both of these mutations occurring as they did in the course of 

 upward selection furnished material of immediate value in aiding the 

 progress of upward selection so that it is proper to say that with the 

 aid of mutations occurring during the course of the experiment the bar- 

 eyed mutant was returned to its original full condition. It is not in- 

 tended, however, to emphasize the fact that these mutations have so far 

 appeared only in the high line and not in the low line. Whether this is 

 merely a matter of chance or has a fundamental significance can be de- 

 termined only by further observation. 



3. The genetic behavior of the type of full eye due to the addition 

 of an accessory factor is similar to that of the individuals of the high 

 selected line before the appearance of the mutants of large degree. The 

 difference between high bar and low bar is due to accessory factors in 

 the same way. In other words the accessory factors with pronounced 

 somatic effect are different in no respect but degree from the the acces- 

 sory factors with small effect which form the ordinary materials for the 

 action of selection. 



4. It is evident that with respect to this one character, facet num- 

 ber, three separate conditions contributed to the effectiveness of selec- 

 tion; first, the differences in accessory autosomal genes present at the 

 beginning of selection; second, the new autosomal genes arising during 

 the course of selection, and. third the mutations in the bar gene. The 



