376 CALVIN B. BRIDGES 



but they fail to have a cumulative effect, the double type being 

 practically indistinguishable from either single type instead of 

 being as much lighter as each is lighter than the wild-type (dia- 

 gram 2, e). 



It has been shown for the case of cream II that specificity of 

 a very extreme nature obtains; since cream II is incapable of 

 modifying any of the other mutant eye colors tried in place of 

 eosin. The double recessives (not-eosin) cream II vermilion, 

 and (not-eosin) cream II pink are the same in eye color as simple 

 vermilion and pink, respectively. In the case of whiting the 

 specificity is even more striking; for cherry, an allelomorph of 

 eosin so similar as to be distinguished (in the females) only with 

 great difficulty, is entirely undiluted by the whiting gene. 



Tone. Pinkish is aberrant in still a third respect: the other 

 modifications can all be described as grades of dilution of the 

 yellow-pink color of eosin, the lighter grades being especially 

 dilute in the pink component of the color. Pinkish, on the other 

 hand, gives one the impression that the yellow component has 

 faded more than the pink, so that there remains a slightly greater 

 proportion of pink in pinkish than in eosin or in the other 

 modifications. 



Dominance. The only one of the modifiers which gives an 

 appreciable effect in heterozygous form is cream II. The amount 

 of dilution of eosin due to heterozygous cream II is of about the 

 same grade that the weakest of the other modifiers, namely, 

 pinkish, gives when homozygous. 



Fluctuation. The amount of fluctuation in the eye color of 

 the creams due to differences in age, food, etc., was about the 

 same as that observed in the other eye colors and their combi- 

 nations. This question is of some practical importance in the 

 making of classifications and is of theoretical interest in con- 

 nection with the completeness of the seriation obtainable with 

 a relatively few modifiers. 



Viability. All of these creams are of excellent viability and 

 the observed ratios are very close approximations to expectation. 



Chromosome. Because of the difficulty of putting the modi- 

 fiers to practical use, it was not at first thought worth while to 



