THE INHERITANCE OF SPOTTING. 41 



We have previously discussed the observable fluctuations in amount of 

 pigment in animals possessing D, for example in yellows. The fluctuations 

 observed in the caseof spotted animals are little if any greater in extent than the 

 fluctuations from deep orange to cream already noted in the case of yellow mice. 

 If there are two ways (d and s) in which the factors D and S may respec- 

 tively be modified, we should expect that the same might hold in the case of the 

 factor P. The quantitative modification of this factor (p), acting upon brown 

 and black pigment to the exclusion of yellow, produces the pink-eyed colored 

 series. 



If a regional modification of black and brown forming substances (Br and 

 B) existed, we should expect a spotted condition to be formed. Here, however, 

 the spots would be black or brown on a yellow ground. To such a class as this 

 we may say that the tortoise varieties of guinea-pigs belong. This condition 

 is in a measure hypostatic to self-pigmented forms, as are the (s) spotted forms. 

 We have supposed that the factor P produced a certain amount of black or 

 brown substance typified by the wild Mus musculus. This factor, then, ap- 

 pears to bear the same relation to the factors Br and B that the factor D does to 

 the factor Y. We may, therefore, logically construct a rough diagram to show 

 the relations of the distributive factors to Br and B, as we did in the case of Y: 



Factors for the 

 production of 

 brown and 

 black pigment 



Distributed in a certain ( Modified in amount to form 

 degree of depth = P \ pink-eyed varieties = p 



Distributed to certain / Modified in amount to form 

 extent (total) = T \ tortoise varieties = t 



If we designate the tortoise spotting by t, we may consider its epistatic 

 allelomorph as T, total or self pigmentation. In this case the factor P becomes 

 complicated, as did the factor D, becoming PT. As in the case of the factor 

 DS, animals may be of any one of the following formulae: 



PT dark-eyed self. pT pink-eyed self. 

 Pt dark-eyed tortoise, pt pink-eyed tortoise. 



In mice we find that the p modification has taken place, while in guinea- 

 pigs the t modification is well known, and Castle (19126) has described the origin 

 of a pink-eyed form of guinea-pig which may turn out to be the p modification. 



It may be seen from the foregoing description that the two forms of 

 spotting (piebald and tortoise) are entirely independent, and it is thus entirely 

 conceivable that they should occur in the same individual. This appears to 

 be the case in the well-known tri-colored breeds of domesticated animals. 



One of the best-known cases of inheritance in tri-colored races is found in 

 the case of "basset" hounds, on which Galton based his "law" of heredity. 

 The character considered was the presence or absence of black in addition to 

 yellow and white. When black was present the tri-color condition was pro- 

 duced. Though there is no direct evidence at hand, Bateson (1909), in dis- 

 cussing these experiments, states that yellow is not usually dominant to black. 

 If this is the case the observed facts may be explained as follows : 



In guinea-pigs the presence of black and yellow spots on any individual 

 depends upon the presence of a spotting character which acts upon a totally 



