vm] Discussion of Colour-Evidence 141 



Animals and plants are alike in the fact that their colours, 

 however produced, may be modified by the presence of 

 additional factors. In each case therefore we must conceive 

 of one lowest or hypostatic colour, and of epistatic factors 

 superimposed on this which produce their several effects. 

 In the Sweet Pea the lowest colour is red, which is turned 

 to purple or blue if the factor having this power is present. 

 Similarly in the mouse the lowest colour is chocolate, which 

 becomes black if the black factor is added, and so on. The 

 intensity and also the distribution or pattern of colours 

 behave in descent as if they also were governed by such 

 superimposed factors, though as will shortly appear, it is 

 not certain that this mode of representation is strictly 

 correct. However this may be, we are safe in regarding 

 the pigmentation of animals and plants as a character usually 

 resulting from the combined operations of several distinct 

 factors, transmitted separately in heredity. 



Applying conceptions which have lately become current 

 in physiology Cuenot suggested that the determiners which 

 modify colour in the mouse, for instance, may be distinct 

 diastases acting on a single chromogen substance. In the 

 present state of physiological chemistry it is, I suppose, 

 impossible to speak with confidence as to the nature of the 

 bodies concerned and we must keep an open mind. Nothing 

 yet precludes the possibility that there may be one diastase 

 responsible for the production of colour, and another set of 

 bodies which, acting in the presence of the diastase and of 

 the chromogen, determine the quality or shade of the 

 colour^. 



So in the mouse, the wild grey colour results from the 

 joint action of at least three factors: (i) the colour, which, if 

 no epistatic factor is present, would be chocolate; (2) a black 

 determiner, which causes black pigment to appear ; (3) the 

 agouti-factor, G, which gives the hairs their banded appear- 

 ance and also causes some yellow pigment to be formed in 

 them. In rats a black variety exists because the factor G 

 may be absent, but no chocolate variety has been recorded 

 because the factor for blackness has not yet fallen out. If 



* As pointed out above, Cuenot's suggestion that in the case of mice 

 the agouti-factor, G, is allelomorphic to the factor for blackness, B> is not 

 an adequate representation of the phenomena. (See p. 76.) 



