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THE AMERICAN NATURALIST [Vol. XLVIl 



According to the scheme that Mendel followed, red. R, 

 and vermilion, V, are symbolized as complete and con- 

 trasting characters carried by the germ-plasm of the 

 hybrid. They are assumed to separate in the germ-cells, 

 and as a consequence two kinds of these cells are pro- 

 duced. 



According to a more modern interpretation, known as 

 the presence and absence theory, vermilion is supposed 

 to arise through the loss of something from the germ- 

 plasm of the wild fly. This something is not supposi'd to 

 be the factor for vermilion, but another factor. On this 

 scheme the red eye would be represented by the letters 

 RV, and the vermilion eye by rV ; as though the vermilion 

 color arose through the loss of a red factor. 



The relative advantages of these two modes of repre- 

 sentation become apparent when two pairs of factors are 

 involved. For instance, a new eye color — pink — ap- 

 peared as a mutant. It, also, was recessive to red. 

 Mendel's scheme would make the pink character the mate 

 of the red character, just as vermilion had been before. 

 But if pink and vermilion were mated to each other, it is 

 not clear whether vermilion and pink should be treated 

 as contrasted characters, or whether each should still be 

 treated as allelomorphic to red. If either of these alter- 

 natives is adopted, the scheme fails to account for what 

 actually happens. Mendel did not meet with such a situa- 

 tion, for none of his paired characters involved two 

 changes in kind in the same organ, and consequently the 

 problem did not exist for him. 



Bateson did meet with just this situation in the case of 

 the comb of fowls and the coat color of mice. His scheme, 

 if applied to the present case of the eve colors in Droso- 

 phila, would be to represent red by RV, vermilion by rV, 

 and pink by Rv. This scheme illustrates first why when 

 vermilion is bred to pink a red-eyed fly, rVRv, should 

 result ; second, why in the second generation the propor- 

 tion 9:3:3:1 should appear; 1 and third why in the eye 



1 Except in so far as modified by sex-linkage. 



