72 INHERITANCE IN GUINEA-PIGS. 



produced at a lower rate of production of I than is the case of I 

 alone. Above the level at which I alone produces yellow, the two 

 kinds of enzymes, yellow- and black-producing, compete with each 

 other for chromogen, producing a mixture of black and yellow, the 

 relative importance depending on the rate at which II is produced. 

 Because of the competition the intensity of black shows two maxima 

 as production increases one just below the yellow threshold and the 

 other at maximal production of I. Intensity of production or inhibi- 

 tion of II in patterns hi the fur are determined by various factors 

 (group 2) which produce self yellow, yellow spotting, agouti, etc. 



(3) There is a third group of substances which, added to the dark- 

 pigment-producing enzyme (II), affect the intensity of dark color pro- 

 duced but not the power of fixing chromogen in competition with the 

 yellow-producing enzyme. They have no effect on the intensity of 

 yellow. In this group are the brown factors of mice and guinea-pigs, 

 and perhaps rabbits and dogs, the pink-eye factor of rats, mice, and 

 guinea-pigs and the new red-eye factor of rats,^.e., the factors of group 3. 



While based to a larger extent on the genetic facts in the albino 

 series in guinea-pigs, the hypothesis explains many cases in other 

 mammals in the sense that apparently complex variations are reduced 

 to a single physiological cause. 



In rabbits, single Mendelian factors produce some rather complex 

 variations. A single factor changes a self black to the gray color with 

 a yellow-ticked back but a pure white belly. Another variation 

 changes a self black to a sooty yellow with a black belly. These varia- 

 tions combined hi one animal give a white-bellied clear yellow. How 

 can each of these apparently complex color changes be determined by 

 a simple physiological change? Let us suppose that in all rabbits I 

 is produced strongly on the back, but so feebly on the belly that it is 

 below the yellow threshold, but not so feebly that black is greatly 

 affected. Let us suppose that II is likewise more strongly produced 

 on back than on belly. A factor which tends to produce an inhibitor 

 of II is added. On the back II (the black-producing enzyme) is 

 inhibited in only a portion of the development of the hair, leaving 

 yellow ticking. On the belly all II is inhibited, leaving white. The 

 result is a gray rabbit. The other factor causes a general slowing up 

 in the production of II. On the back this enables the yellow-pro- 

 ducing enzyme to predominate in competition and sooty yellow results. 

 On the belly below the yellow threshold what little black-producing 

 enzyme does develop has no competition and only black can result. 

 We get a black-bellied sooty yellow. The combination pattern can 

 only be a white-bellied yellow. In many other mammals color phases 

 are found which can be explained as due either to variations in 

 production of II or I. The red phase of the red fox, has a white 

 chest. The level of production of I is below the yellow threshold 



