86 THE THEORY OF THE GENE 



tants are present in the vicinity at the time. There are a 

 few recorded cases in our stocks that fulfill these condi- 

 tions, and the evidence, as far as it goes, shows that 

 reversal may take place. There is also another possibility 

 that has to be guarded against. Some of the mutant 

 stocks have, after a time, seemed to lose more or less the 

 characteristics of that stock, yet when outbred it has been 

 found that the mutant character can be recovered in its 

 original strength. The fourth chromosome character, bent 

 wings (Fig. 30), that is itself variable, and subject to 

 environmental influence, has shown, when not selected, a 

 tendency to return to the wild type in appearance. If a 

 fly of this kind is outcrossed to wild stock and the F^s 

 inbred, the bent character reappears in many of the indi- 

 viduals of the expected bent class. A similar result has 

 been found in another mutant stock called scute, charac- 

 terized by the absence of certain bristles on the thorax. 

 Individuals appeared in certain pure cultures in which 

 the " missing' ' bristles were present. Apparently the 

 mutant had reverted to wild type. But that this had not 

 occurred was shown by breeding such flies to wild type 

 stock. In the second generation, scute flies reappeared. 

 A study of this case has shown that the return to normal 

 was due to the appearance of a recessive factor which, 

 when present in homozygous condition in scute stock, 

 brings about the development there of the missing bris- 

 tles. Aside from the bearing of this result on the question 

 under discussion, the fact of a new recessive mutation 

 occurring, that brings back the mutant character to the 

 original type, is in itself an interesting and important 

 fact. 



Finally, there is the peculiar reversion to normal of the 

 dominant or semi-dominant character, bar-eye (Fig. 49, 

 1 and 2). For some years it has been known from the 

 observations of May and Zeleny that bar-eye reverts 



