VIII] Subtraction-Stages 143 



substances, say the chromogen, is distributed over the whole 

 surface, but in the Dutch-marked, for example, it is reduced 

 in quantity. The reduction however occurs in a fairly 

 definite way, leading to the formation of a type having a 

 recognizably distinct pattern. It does not seem an un- 

 reasonable speculation to suppose that we have here to deal 

 with a condition in which the amount of the substance is 

 insufficient to cover the whole region which it occupies in 

 the self-coloured type, though why it should be restricted 

 to one special region more than another it is impossible 

 to say. 



If the definite pied phases are to be thus regarded as 

 representing quantitative diminution in the development of 

 one of the determining substances, we may make a similar 

 supposition in regard to the diluted colorations already 

 mentioned in the case of mice. In the diluted colours the 

 reduction in quantity, instead of diminishing the coloured 

 area while keeping the intensity of the colour, is effected by 

 diminishing the intensity of the colour while the totality of 

 the distribution is retained. The black Dutch-marked 

 mouse may thus be imagined to be a mouse in which one 

 of the colour-factors exists in its full intensity, though there 

 is not enough of it to cover the skin, while in the blue mouse 

 the factor is generally distributed over the skin but in a 

 dilute condition. In both cases alike the subtraction-stage 

 as we may call it is a fairly definite stage in the reduction of 

 the amount of pigment. 



A physical analogy — doubtless imperfect, but neverthe- 

 less instructive — may be drawn from the way in which 

 various oils distribute themselves over the surface of a 

 liquid with which they do not mix, some forming circum- 

 scribed patches of greater thickness, which may be compared 

 with the patches on the Dutch rabbit, others spreading in 

 a thin layer over the whole surface, like the dilute colours 

 spread over the whole coat. The analogy breaks down 

 at the fact that in the oils the physical distinctions to which 

 the different behaviours are due cannot be transferred 

 from one oil to the other, whereas in the rabbit this is 

 accomplished — a fact which entitles us to represent the 

 several properties as distinct and transferable factors. Thus 

 the results of the cross between a black-and-white Dutch- 



