THE INHERITANCE OF SPOTTING. 39 



If, as Riddle states, it seems absurd to postulate the complete loss of chro- 

 mogens in albinos, we must suppose that an enzyme or enzymes have been lost, in 

 order to prevent the formation of pigment. If, now, the work of Mudge shows 

 that the white of spotting lacks the same class of substance as the white of 

 albinism we must say that here also the enzyme or enzymes are absent from the 

 white areas. But by the work of Darbishire, already alluded to, and that of 

 later investigators as well, it is shown that the processes concerned in the 

 origin of the two types of whites are not identical. We must then, by elimi- 

 nation, come to the conclusion that the nature of the processes which control 

 the distribution of the enzyme or enzymes differs in the two categories. 



We have seen that the amount of pigment in the hair of intense pigmented 

 animals is dependent upon the cooperation of two factors, D and P. In the 

 wild Mus musculus the pigment occurs distributed over the entire coat, and the 

 animal is said to be self or uniformly pigmented. 



We have supposed that the distribution of pigment is controlled by certain 

 factors. Thus we have supposed that density of pigmentation is controlled by 

 a factor D, by the activity of which a certain amount of pigment is formed in 

 eyes and skin and hair. We have recognized a quantitative modification of 

 this factor in the case of dilute varieties, possessing less pigment. Since this 

 modification affects all three pigments (yellow, brown, and black) and since 

 we have supposed brown and black to be higher oxidation stages of yellow, it 

 seems logical to consider D a factor modifying Y, and in this way also modify- 

 ing Br and B, which are, so to speak, converted Y. When D is modified in 

 amount, less Y is converted to the brown or black stage. This is the condition 

 seen in dilute animals. Logically we should expect another type of modifica- 

 tion of Y. In self animals Y is distributed over the whole coat. We should 

 expect some forms in which Y was distributed only to certain portions of the 

 body. This modification is seen in the forms in which white spots occur. Such 

 white areas may be considered as those from which the F-forming substance is 

 absent. Thus the relation of these factors to one another may be roughly 

 shown as follows: 



Color producing 

 substance Y 



Distributed in a certain f Modified in amount to 

 degree of density = D \ form dilute varieties = d 



Distributed to a certain ( Modified in amount to 

 extent (self) =S \ form spotted varieties =s 



If we designate the spotted (piebald) forms by the letter s, we may logi- 

 cally consider the unmodified self forms as being of the constitution S. We 

 would expect, then, that in crosses between a spotted race of known gametic 

 formula (s) and a self race of formula (S) a mendelian monohybrid ratio would 

 be observed. That such is the case has been recorded by Doncaster (1906) and 

 by Castle and MacCurdy (1907) in the case of "hooded" and self rats. Such 

 a type of spotting as that seen in the hooded rats is, then, a unit character so 

 far as its behavior in crosses is concerned. 



Cuenot, working with mice, has found that spotted forms exist, varying 

 greatly in proportion of white to colored areas. This series of spotted forms 

 he accounts for by supposing that there are present a number of small factors 



