CLASSIFICATION OF VARIATIONS 85 



Setting aside the various structural differences between the two 

 breeds, the Silky is immediately distinguished from the Leghorn 

 by the fact that the skin of the whole body including that of the 

 face and comb appears to be of a deep purplish colour. The 

 face and comb of the Leghorn are red and the skin of the body 

 is whitish yellow. On examination it is found that the purple 

 colour of the Silky is in reality due to the distribution of a deep 

 black pigment in the mesoblastic membranes throughout the 

 body. The somatopleura, the pleura, pia mater, the dermis, and 

 in most organs the connective tissue and the sheaths of the blood- 

 vessels, are thus impregnated with black. No such pigmentation 

 exists in the Leghorn. As the result of an elaborate series of 

 experimental matings we have proved that the distinction be- 

 tween the Leghorn and the Silky consists primarily in the fact 

 that the Silky possesses a pigment-producing factor, P, which is 

 not present in the Leghorn. 



This variation must undoubtedly have been one of addition. 

 But besides this there is another difference of an altogether dis- 

 similar nature; for the Brown Leghorn possesses a factor which 

 has the power of partially or completely restricting the operation 

 of the pigment-producing factor, P. Moreover in respect of 

 this pigment-restricting factor which we may call D, the sexes 

 of the Brown Leghorn differ, for the male is homozygous or DD, 

 but the female is heterozygous, Dd. Thus in order that the 

 black-skinned breed could be evolved from such a type as a 

 Brown Leghorn it must be necessary both that P should be added 

 and that D should drop out. We have not the faintest concep- 

 tion of the process by which either of these events have come to 

 pass, but there is no reasonable doubt that in the evolution of 

 the Silky fowl they did actually happen. 



We may anticipate that numerous interdependences of this 

 kind will be discovered. 



Before any indisputable progress can be made with the prob- 

 lem of evolution it is necessary that we should acquire some real 

 knowledge of the genesis of that class of phenomena which 

 formed the subject of the last chapter. So long as the process 

 of division remains entirely mysterious we can form no conception 



