340 



MUTATIONS IN EYE-COLOR OF DROSOPHILA 



were 



occurring before the new flies were isolated, existed, since the flies were isolated 

 only each twenty-four hours. If so, even although white alone was present the 

 eggs might have been fertilized by a red fly which died before the flies 

 examined. Although the stock remained pure I found nevertheless an individual 

 whose eyes were different in color from any color with which I was then familiar. 

 The fly was isolated and bred to white-eyed flies of the same stock. In the next 

 generation some flies with the new color appeared, and from these two stocks 

 were subsequently produced; a stock pure for the color, and the other having 

 some flies with the eyes of this color and others with white eyes. For some time 

 I spoke of the new eye color as a mutation upwards ; from white to color, something 

 that I have never seen before. That such a change really occurred may seem 

 improbable, when the relation of the white to the colored eye is considered, as 

 will be shown below. It may seem more likely that at the time the colored flies 

 had not been entirely removed from the culture and that the mutation had 

 occurred before or at the time of separation. Therefore, until mutation upwards 

 appears again such an origin will seem to the advocates of the presence and 

 absence theory well nigh inconceivable. 



Since the preceding account was written the eosin eye has appeared twice as 

 a mutant in red stock. In both cases the single fly that appeared has been tested 

 with Orange and shown to give the same results as does the eosin stock. In 

 one of these cases the fly arose in a mass culture ; in the other case an individual 

 (a male) appeared amongst the offspring of an isolated pair of wild red flies from 

 stock that had been inbred for two years. There can remain no doubt that the 

 mutant arose through the loss of the orange factor in the red-eyed fly as its 

 formulae VPoC would most naturally lead one to infer. 



During the course of the experiments with eye colors we had often commented 

 on the fact that the orange factor had never been lost. Its location in the com- 

 plex could not therefore be determined. When the new eye appeared the 

 suspicion that it might be due to the loss of the orange factor at once arose, and 

 was put to the test. If orange alone was lost the new mutant when crossed to 

 orange should give one of the higher colors in the series. 



One peculiarity of this eye, first noted by Mr. C. B. Bridges, my assistant, to 

 whom the new eye was entrusted for experimentation, is that two distinct colors 

 may be easily distinguished. 



In pure cultures the females, Plate XXVIII, fig. 7, have darker eyes than the 

 males, fig. 9. The "dark" eosin eye of the dark female is more nearly like the 

 pink eye than like any other ; while the light eosin eye of the male is more like 

 the orange eye, but is yellowish, and more translucent. 



If, however, the dark eosin females be crossed with white males, fig. 10, of their 

 own stock the daughters will have light eyes, fig. 8, like those of the eosin males, 

 fig. 9. This must mean that the dark eosin eye is due to some factor or factors in 



duplex, and the fight eosin eye to the same factor in simplex. 



The tests made by 



Mr. Bridges showed, in fact, that this is the explanation, and that the 



factor 



