40 THE INHERITANCE OF COLOR IN MICE. 



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

 Pa, ... . etc. Cuenot 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 occurrence 

 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. 

 Cuenot 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, p^, ps, 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-ej^ed whites are derived is recessive. It is, 

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

 Bateson and Punnett, 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 or 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. 



