142 Patterns of Colour [CH. 



a chocolate rat were to be produced, then by crossing it with 

 the wild grey type, blacks must occur in 7%, just as they are 

 known to do in the case of the same mating in mice. 



The facts compel the recognition of such a series of 

 determining elements, and it is perhaps simpler to imagine 

 these elements as distinct from the exciting cause, and 

 additional to it, while remembering the possibility that they 

 may in reality be only modifications of it. 



Similarly in attempting to express the genetic inter- 

 relations of the several patterns of a colour as depending on 

 the existence of definite factors, we have to bear in mind 

 that we are only using a convenient symbolism. It is not 

 incumbent on us to believe that there are any physiological 

 substances which have the power of governing the distribu- 

 tion of the colour. Experiment shows that the power to 

 cause the colour to be uniformly distributed as in the 

 "self" type, or to be restricted to special regions of the 

 body as in the Dutch rabbit, for instance, can be carried by 

 the gametes, and that when these two possibilities are com- 

 bined in heterozygosis, they segregate in gametogenesis. 



This being so, the two possibilities may thus be repre- 

 sented symbolically as two factors, having regard to their 

 effects on the configuration of the resulting zygote ; but if 

 we must attempt to imagine an answer to the question, 

 wherein does the distinction between self pattern and Dutch 

 pattern physiologically consist, we should, I suppose, refer 

 it rather to differences in the distribution of one of the 

 chromogenic factors than to the presence or absence of an 

 additional element. In the self-coloured rabbit the two 

 colour-producing elements are generally distributed over the 

 skin, while in the Dutch rabbit either the chromogen or the 

 diastase if these be the critical substances is restricted to 

 certain areas. The colours in the pied animal thus come 

 out in certain patches just as do lithographic colours upon 

 the prepared parts of the stone when the ink is applied to 

 the whole surface. 



As the black rabbit or mouse is an animal from which 

 the grey determiner, G, is absent, so the pied animal is one 

 from which the self-coloured distribution is absent. Never- 

 theless the essential distinction between the two forms must 

 surely be quantitative. In the self-coloured type one of the 



