THE PHENOMENA OF REVERSION 329 



The ass has been proved to be descended from a species pos- 

 sessing stripes on the legs, and corresponding to the existing 

 wild species {Ashius tcvniopits^ of Abyssinia. The domesticated 

 ass has, as a mle, only retained the crossed stripe on the back 

 and shoulders, but occasionally transverse markings occur on 

 the legs, and this is most often the case in animals of a pure 

 grey ground colour. The stripes on the legs are very rare both 

 in horses and asses, as I know from personal observation, and 

 even when present they are very faint. 



In mules these stripes occur much oftener and much more 

 distinctly — particularly in light grey animals, and are some- 

 times seen on the hind-legs as well as on the fore-legs and 

 shoulders. Thus reversion occurs to the ancestral form of both 

 sides. 



The theoretical explanation of this fact must be based on 

 the assumption that a certain number of unmodified ancestral 

 determinants for the hairy coat have been retained in the germ- 

 cells of the two species ; and that these, when they are brought 

 together in the germ-cells of the two parents, may predominate 

 over the modified determinants of the parents. The fact that 

 such reversion does not by any means always occur in mules, 

 points to the conclusion that the number of ancestral deter- 

 minants varies very considerably in the germ-cells of individuals, 

 and that these may even be entirely absent, or only sparsely 

 represented, or, on the other hand, may be present in large 

 numbers. In the latter case, when two of these germ-cells come 

 together in fertilisation, partial reversion will occur, and will be 

 more marked and extensive the greater the number both of the 

 ancestral determinants which meet together and the parts of 

 the body to which these belong. The determinants for the 

 stripes on the hind-legs are evidently much less numerous in 

 the combined germ-plasm of the two existing species than are 

 those for the marks on the fore-legs as is shown by the manner 

 in which these stripes have been relatively retained by several 

 existing wild species of Equus. All these determinants must, 

 however, be present in very varying numbers in the germ-plasms 

 of different individual horses and asses, for, as already stated, 

 reversion does not by any means always occur in mules ; in Italy, 

 where these animals are used in large numbers, it is not common, 

 and, as far as my experience extends, is perhaps only exhibited 

 in one or two animals out of a hundred. Gosse, however, states 



