40 THE INHERITANCE OF COLOR IN MICE. 



for the production of spotting (panachure). These he designates pi, Pi, pa, 

 Pi, ... . etc. Cu^not is by no means alone in favoring multiple factors as 

 an explanation for spotting. It is easy and attractive to imagine a factor for 

 every observed degree of spotting. The various types of spotted animals seem 

 clear-cut, the colored and white areas seem to form a marked contrast, and the 

 presence of many factors to produce these many forms gives an air of finality 

 to an hypothesis which is alluring. 



The advantages of a multiple-factor hypothesis, however, are rendered 

 dubious by the observed occurrence of strains which show that a certain amount 

 of fluctuation exists in the manifestation of even these assumed multiple units. 

 It is questionable whether it would be possible to distinguish between a result 

 produced by multiple factors of the number necessary to explain the occiu-rence 

 of the various spotted forms and a continuous series formed by fluctuation of 

 of the original modification which produces spotting. 



Observed experimental facts, however, do not favor a multiple-factor 

 hypothesis. Thus, if multiple factors for spotting existed, the black-eyed 

 white variety would possess the greatest number of these spotting factors. 

 Cu^not in mice and Castle in guinea-pigs found that spotting was recessive to 

 self in crosses. Therefore the black-eyed white forms would possess a great 

 number of recessive spotting factors, pi, pi, pz, Pi, Ps, Pe, etc. How, then, 

 could two such animals when crossed inter se give in their young practically the 

 whole gamut of spotted forms, as Castle (1905, p. 45), found was the case. 



In a somewhat similar way the presence-and-absence hypothesis meets 

 with trouble in the same cross, i. e., black-eyed white X black-eyed white. The 

 spotting from which the black-eyed whites are derived is recessive. It is, 

 therefore, according to the presence-and-absence hypothesis, as advanced by 

 Bateson and Puimett, due to the loss or absence of a factor or factors for uni- 

 formity of pigmentation. Yet two animals having, according to this theory, 

 lost the factors for pigment production in the coat, give young which often 

 have a large amount of pigment on the coat. The presence-and-absence 

 hypothesis, in order to explain this case, must imagine a degree of fluctuation 

 in the manifestation of the spotting character which makes proof of the 

 multiple-factor hypothesis difficult if not impossible. 



In the writer's opinion, present knowledge of spotted forms is too scanty 

 to decide whether spotting is due to a process of loss of the factor for uniformity 

 of pigmentation, or to a hypostatic restrictive factor. It seems, however, that 

 such spotting as one encounters in guinea-pigs, hooded rats, and in many mice 

 is a unit character subject to enormous quantitative fluctuation, and that it 

 may be considered due to a modification of the factor Y ov U which is hypo- 

 static to unmodified or "self" forms. 



In view of the fact that the same substance Y is distributed in two different 

 ways to produce spotting and dilution, we may consider it as consisting of an 

 amount of pigment (D) distributed over the entire animal (S) ; therefore we 

 may consider the pigment of the wild mouse as being designated by the letters 

 DS. An animal may then be of one of the following formulae: 

 DS intense self. Ds intense spotted. dS dilute self, ds dilute spotted. 



