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THE AMERICAN NATURALIST [Vol. XL VII 



Again, when the pink eye mutant appeared, it would 

 have been assumed, on the presence and absence theory, 

 that something was lost, so that the formula is PvO. 

 Here again the pink color is the result of all that is left 

 when something (V) is lost. Pink is not assumed to be 

 produced by a factor P, but by what is left when a factor 

 V is lost. An egg is supposed to have lost something and 

 vermilion developed, another egg is assumed to have lost 

 something else and pink developed. It was the loss of the 

 vermilion factor that allowed pink color to develop, and 

 the loss of the pink factor that allowed vermilion color to 

 develop. When pink and vermilion are mated together, 

 the original color — red— is restored, because on this 

 scheme what each has lost is made good by what is found 

 in the other. 



To my series of eye color factors the letter was 

 added to indicate the nature of the color produced when 

 two factors, P and V, were assumed to be absent. The 

 symbol at that time did not seem to stand in the formu- 

 lae on the same footing as P and V, because it stood for a 

 color, and not for a factor that had been lost from the 

 germ-cells of the wild fly. But since on the presence and 

 absence scheme stood for the residuum after P and V 

 were lost it stood for the same sort of thing as did P and 

 V, for P and V also stood for residua, when they were not 

 used as symbols for factors. This will be made clearer 

 later. 



When the experiments had progressed to this stage, a 

 new eye color appeared that was called eosin. Mated to 

 orange it gave red ; therefore, it seemed that this mutant 

 must have contained P and V, and I inferred that it owed 

 its color to the loss of an imaginary factor. Eosin was 

 represented, therefore, by PVo. But a moment's thought 

 will show that on this scheme, as long as P and V are 

 present, any loss from the germ-plasm (giving a new eye 

 color) added to orange should give red, because orange 

 would contain what the new mutant had lost. 



The history of this case will show how, with the best of 



