328 THE GERM-PLASM 



in one part or another. The differences between the young 

 support the theoretical assumption that the number of ancestral 

 determinants, which determined the type of the five young ones, 

 was by no means the same in the different germ-cells of the 

 two parents. Their dissimilarity, moreover, admits of a further 

 conclusion, which could likewise be foreseen theoretically, viz., 

 tJiat the deterDiinaiits for the coloration of the different regions of 

 the body are represented in the different germ-cells in fluctuating 

 numbers. Not only should we naturally suppose this to be the 

 case from the structure of the germ-plasm as here assumed, but 

 we can hardly even imagine it to be otherwise. If the various 

 idants in the germ-plasm of a hybrid together contain twenty 

 ids with ancestral determinants, it does not follow that the latter 

 only are present in each id. There is no reason why in any one 

 id the determinants for the colours of the wings, for instance, 

 should not be ancestral, while those for the colours of the tail 

 are racial, or vice versa. The bird would display reversion in 

 the tail or wings, according to whether the former or the latter 

 kind of determinants is in the majority in the germ-cell : for 

 our theory shows that the struggle of the determinants takes 

 place independently in every cell, and consequently also in 

 every part of the organism ; and the result will therefore be 

 independent in each part, and will depend solely on the com- 

 bination of homologous determinants which contend with each 

 other in the cell or group of cells under consideration. It can 

 thus easily be explained how it comes about that though all the 

 offspring of a certain cross show reversions, the latter are never- 

 theless combined in very different ways with the racial characters 

 of the parents. 



We must now consider the third type of reversion mentioned 

 at the beginning of this section, viz., that which concerns in- 

 dividual characters of very remote ancestors., and is seen, for 

 instance, in the stripes on the legs of horses and asses, and 

 more especially of mules. 



It will not be necessary to repeat the arguments which Darwin 

 used to show that the horse was primitively dun-coloured and 

 striped. Indications of such a coloration are seen at the present 

 day in horses of different colours in all parts of the world, in the 

 form of a dark stripe down the back, and of transverse markings 

 on the legs and shoulders ; such markings, however, are not 

 common, and most frequently occur in dun-coloured horses. 



