WRIGHT 



but feeble on the belly in all rabbits 

 due to a regional differentiation. We 

 will suppose that enzyme I is below 

 the yellow threshold on the belly. Sec- 

 ond, we will suppose that factor A 

 determines the production of an in- 

 hibitor with the same subtraction 

 effect on enzyme II everywhere, while 

 factor e determines a general propor- 

 tional reduction in rate of production 

 of enzyme II. On this basis it follows 

 that factor A produces a partial inhibi- 

 tion of black on the back, revealing 

 yellow but a complete inhibition of 

 black on the belly where, however, 

 only white can be revealed. Factor e 

 reduces black on the back sufficiently 

 to permit yellow to predominate in 

 competition while on the belly, where 

 there is no competition from yellow, 

 what little of the black producing en- 

 zyme I-II is produced is fully effective 

 and black, or at least blue, results. 



Class 2b.— The factors which reduce 

 the intensity of black areas in skin, fur 

 and eyes without affecting red areas 

 form a clearly defined class. The 

 brown-eyed chocolate mice, guinea- 

 pigs and rabbits; the pink-eyed pale 

 sepia or "Hlac" mice, guinea-pigs and 

 rats, and the red-eyed rats of similar 

 coat color ^^ differ from blacks by fac- 

 tors of this class. Among the larger 

 animals the difference between liver- 

 colored and black dogs seems to be of 

 this kind. Probably chestnut and liver- 

 colored horses differ from bays and 

 blacks by such a factor. All of these 

 factors are recessive. In the pink-eyed 

 mice, guinea-pigs and rats it is remark- 



23 These red-eyed rats first described by 

 Castle (1914, A?ner. Nat.) under the name 

 of yellows must not be confused with the 

 red-eyed rats described by Whiting (loc. 

 cit.). In the former yellow pigment is un- 

 affected and to a large extent gives the color 

 to the fur in agoutis, owing to the great 

 reduction of black. In the latter, yellow is 

 reduced to white, while black is only slightly 

 affected. 



91 



able to what an extent black is diluted 

 without bringing out any distinctly 

 reddish tinge although in red regions 

 of the fur as in the agouti band of gray 

 varieties the red appears in full inten- 

 sity. There is evidently a different sort 

 of reduction of enzyme II from that 

 in the sooty yellow rabbit. A normal 

 quantity of enzyme II but reduced 

 potency in some other way would 

 seem to be required in class 2b. 



SIMULTANEOUS EFFECTS 



It has been assumed so far that fac- 

 tors act only on one or the other of 

 the hypothetical enzymes I and II. In 

 the great majority of cases this is satis- 

 factory but it is not impossible that a 

 factor may influence both enzymes in 

 the course of development. In fact the 

 writer will soon publish evidence on 

 one such case in guinea-pigs. Tri-color 

 male guinea-pigs of many different 

 stocks agree in showing a slightly 

 greater average area of color as op- 

 posed to white than their sisters, and 

 they also show relatively more black 

 as opposed to red. Maleness seems to 

 determine a higher level of both en- 

 zymes I and II as regards pattern. The 

 effect on color is perhaps due rather 

 to a general metabolic difference in 

 the cells of males and females early 

 in ontogeny than to any specific modes 

 of action on the two enzymes. The 

 same may be true of certain coat pat- 

 terns in which it seems necessary to 

 suppose that the level of enzymes I 

 and II is raised or lowered simultane- 

 ously in some respect by regional dif- 

 ferentiation. We have cited the cases 

 of the rabbit, mouse, and guinea-pig 

 in which it was found convenient to 

 suppose that both enzymes I and II are 

 strong on the back and weak on the 

 belly. Similar cases are common. An- 

 other sort of example seems to be 

 present in the tiger pattern of cats. In 

 yellow cats the pattern is shown by 



