COLOR. 71 



C r C a — but the sequence can not be made to agree with either the eye- 

 color or yellow series. C d C a is distinctly lighter than C r C r in black 

 fur but distinctly more intense in eye color while, in yellow fur C d C a 

 is above, C r C r below the threshold of any color. The effects could be 

 explained by a complicated linkage hypothesis. We would need to 

 suppose that there are separate series of allelomorphs acting on yellow, 

 black of fur, and black of eye, respectively, and that C r and C a are 

 complexes identical in the yellow-dilution factor, C d and C r identical 

 in the black-fur-dilution factor and perhaps C and C d in the black-eye- 

 dilution factor. But an hypothesis according to which it is a mere 

 accident that the factors which dilute yellow, black of fur, and black 

 of eye are perfectly linked in inheritance can hardly be taken seriously. 

 Another escape would be to suppose that our four factors, C a , C r , C d , 

 and C, are, indeed, variations of the same thing but not linear quantita- 

 tive variations. However, it seems most satisfactory to the writer to 

 attempt to explain the results on the basis of four quantitative gradations 

 of one factor, which determines the amount of the basic color-producing 

 enzyme, if it is in any way possible. Let us see what assumptions 

 must be made to do this. First, it will be convenient to assume with 

 Little (1913) that the basic color-producing enzyme (I) acting by 

 itself on chromogen, produces yellow pigment. The addition of a 

 second substance (II) makes it a black-producing enzyme (I-II). We 

 will further assume that I is relatively unstable and must be produced 

 above a certain rate (that determined by C r C r ) in order to reach and 

 oxidize the chromogen in the cytoplasm. United with II it becomes 

 more stable and even produces some effect at the rate of production 

 determined by C a C a . The next assumption is that above the thresh- 

 old for yellow, I-II and the excess of I compete for the chromogen. 

 As a result of partial displacement by the paler color, the intensity of 

 black decreases just above the yellow threshold. C d C a seems paler 

 (and somewhat browner) than C r C r . In the eye, no factor ever 

 brings out a yellow color. There is perhaps never an excess of I here 

 and the intensitj^ of black follows the normal sequence. 



Summarizing, the hypothesis to which consideration of the physio- 

 logical and genetic seems to lead is as follows : 



(1) There is a basic color-producing enzyme (I) which acting alone 

 on chromogen produces a diffuse or finely granular pigment which 

 appears yellow. It is relatively unstable. Intensity of production 

 and absence or inhibition in parts of fur and eye are determined by the 

 various factors of group 1 — the albino series, " blue "-dilution factors, 

 and recessive and dominant white-pattern factors. 



(2) There is a second substance (II) which may unite with I to 

 produce a more stable enzyme, which reacts with chromogen to produce 

 a coarsely granular pigment which appears sepia, brown, or black. 

 When II is present, I is stabilized to such an extent that pigment is 



