vi] HEREDITY 75 



this is inconsistent with the hypothesis that allelo- 

 morphs exist always in pairs, one possessing a factor 

 lacking in the other. More correctly, then, each 

 colour is allelomorphic with its absence, but the 

 presence of a higher member of the series obscures 

 or prevents the development of the lower. This 

 is expressed by saying that grey is 'epistatic' 

 over black and chocolate, and black over choco- 

 late. Since chocolate is the lowest member of the 

 series, it has been suggested that its factor is indeed 

 the pigment factor represented in the case of the rats 

 described above by the symbol P, and that in other 

 colours the special factors are present in addition. 

 In grey mice yellow, black and chocolate pigments 

 are all present in the hairs, but the factor for ' grey- 

 ness' causes the yellow to be restricted to certain 

 parts of the hair. In black mice both black and 

 chocolate pigments are present, but the black obscures 

 the chocolate, and in chocolate mice this pigment 

 alone is present. 



The object of this rather special digression is to 

 show how the hypothesis of a series of colour-factors 

 acting together can completely coordinate the pheno- 

 mena of colour-inheritance, which very few years ago 

 seemed hopelessly confused and subject to no definite 

 rules. It is now possible to forecast with accuracy 

 the results of a pairing between individuals of different 

 colours, if the constitution of the parents with respect 



