170 THE SECOND-CHROMOSOME GROUP 



assumes a creamy tone which in turn becomes pinkish, passing pro- 

 gressively through a j^ellowish pink to pink and to ruby. When the 

 flies hatch, the color is a transparent rather deep ruby. This color 

 rapidly deepens to garnet and then passes on to a purplish tone. The 

 typical purple color at its maximum development — in flies about a 

 day old — while retaining much of its transparency, appears darker in 

 tone than the red of the wild-type, purple being the first of such 

 "dark" eye-colors. As the fly becomes older this "ripe- cherry" color 

 is progressively obscured, apparently by an increase in a flocculent 

 red pigment, hke that of the wild fly. The eye-color thus becomes 

 somewhat lighter than red again, though always distinguishable by a 

 lesser opacity and by a light "fleck" in place of the hard dark fleck 

 seen in the wild eye. With extreme old age the color approaches still 

 closer to red, but does not become strikingly darker, as do pink and 

 sepia, for example. In purples of the same age fluctuations in color 

 are not great. Separations are easy if done, as usual, while the flies 

 are mostly under 2 days old, though the climax in the development of 

 the purplish tone offers the most favorable stage. 



THE DIFFERENTIATION OF PURPLE BY VERMILION— DISPROPOR- 

 TIONATE MODIFICATION. 



While the difference between the color produced by the purple gene 

 and the color produced by its wild-type allelomorph (red) is distinct, 

 it is neither great nor striking, since in tone purple is first slightly 

 darker and later somewhat lighter than red. However, in classifying 

 the eye-colors in F2 from the cross of vermihon by wild, it was observed 

 that the difference between vermihon purple and vermihon not- 

 purple was not only constant in direction, but also conspicuous in 

 extent. The separability of purple versus not-purple is favored by 

 the presence of vermilion, which may therefore be called a "differentia- 

 tor" of purple. Regarded in the converse relation, namely, the effect 

 of purple on vermilion rather than the effect of vermilion on purple, 

 purple is a much stronger modifier of vermihon than of not-vermilion. 

 Purple may be described as a "disproportionate modifier" of vermil- 

 ion, since from the small amount of its effect on eye-color when acting 

 alone one would not have expected the great effect it produces when 

 acting in the presence of vermilion. 



This type of intensification — disproportionate modifier and, con- 

 versely, differentiator — stands midway between the normal relation 

 where combination effects are roughly proportional to the separate 

 effects, so that both genes may be called "general modifiers," and the 

 special relation where a given gene, "specific modifier," produces by 

 itself no visible effect whatever, but which gives a more or less marked 

 effect when acting in conjunction with some other gene, its specific 

 base, sensitizer, or differentiator. 



