439 



fact that neither the black-and-whites nor the yellow-and-whites 

 produced by tricolors breed true, there are races of black-and- 

 white and of yellow-and-white guinea-pigs which do breed true. 

 It remains to explain why the others do not. A black-and-white 

 animal which breeds true may be considered to possess some 

 chemical substance necessary for the production of color (which 

 we call a color-factor) distributed irregularly throughout its 

 coat. Wherever this substance is wanting no color is formed 

 and a white area results. The specific factor for black (prob- 

 ably ;m enzyme is however everywhere present in the coat so 

 that wherever color forms the color is black. Such races as 

 this breed true. 



The yellow-and-white animal which breeds true may likewise 

 be considered to have an irregularly distributed color-factor, but 

 to lack entirely in its coat the black factor. Hence the color, 

 wherever formed, is yellow. 



Yellow races also exist which do not bear spots of white, hut 

 which have spots of black. In such animals the color-factor is 

 evidently uniform in distribution, whereas the black factor is 

 irregularly distributed. 



Now the tricolor race is a yellow one spotted both with white 

 and with black, i. c, it results from irregularity in distribution 

 through the coat of two different chemical substances, the color 

 factor and the black factor. These two factors are known to be 

 independent of each other in heredity. See Castle (1909). It 

 is therefore not to be supposed that they will commonly coincide 

 in distribution. If the black factor extends over all the colored 

 areas, the animal will be black-and-white. If the black factor 

 falls only on areas which lack the color factor, it will produce no 

 visible effect, and the animal will be yellow-and-white. If, 

 finally, the black factor falls on some of the colored areas but not 

 on all of them, those in which it falls will be black, the others 

 yellow, and the uncolored areas of course white. Hence a tri- 

 color will result. But the gametic composition of these tricolors 

 will not be different from that of the black-and-whites or red- 

 and-whites produced by the same race, since all alike will be 

 characterized by irregularity in distribution of the same two 

 factors. A tricolor race on this hypothesis should be unfixable, 

 as has up to the present time been found to be true. 



The same line of explanation will answer equally well for the 

 ease of the Bassett hounds studied by Galton. These Galton 



