Chap. XXVI. ANALOGOUS VARIATION. '351 



other in the same characters by which the skulls of the various species of 

 the genus Lepus differ. 



I will only allude to one other case, already discussed. Now that we 

 know that the wild parent of the ass has striped legs, we may feel con- 

 fident that the occasional appearance of stripes on the legs of the domestic 

 ass is due to direct reversion; but this will not account for the lower 

 end of the shoulder-stripe being sometimes angularly bent or slightly 

 forked. So, again, when we see dun and. other coloured horses with 

 stripes on the spine, shoulders, and legs, we are led, from reasons for- 

 merly given, to believe that they reappear from direct reversion to the 

 wild parent-horse. But when horses have two or three shoulder- stripes 

 with one of them occasionally forked at the lower end, or when they 

 have stripes on their faces, or as foals are faintly striped over nearly their 

 whole bodies, with the stripes angularly bent one under the other on 

 the forehead, or irregularly branched in other parts, it would be rash to 

 attribute such diversified characters to the reappearance of those proper to 

 the aboriginal wild horse. As three African species of the genus are much 

 striped, and as we have seen that the crossing of the unstriped species 

 often leads to the hybrid offspring being conspicuously striped— bearing 

 also in mind that the act of crossing certainly causes the reappearance of 

 long-lost characters— it is a more probable view that the above-specified 

 stripes are due to reversion, not to the immediate wild parent-horse, but 

 to the striped progenitor of the whole genus. 



I have discussed this subject of analogous variation at consi- 

 derable length, because, in a future work on natural species, 

 it will be shown that the varieties of one species frequently 

 mock distinct species— a fact in perfect harmony with the fore- 

 going cases, and explicable only on the theory of descent. 

 Secondly, because these facts are important from showing, as 

 remarked in a former chapter, that each trifling variation is 

 governed by law, and is determined in a much higher degree 

 by the nature of the organisation, than by the nature of 

 the conditions to which the varying being has been exposed. 

 Thirdly, because these facts are to a certain extent related to a 

 more general law, namely, that which Mr. B. D. Walsh 31 has 

 called the "Law of Equable Variability:' or, as he explains it, 

 "if any given character is very variable in one species of a 

 "group, it will tend to be variable in allied species; and if any 

 "given character is perfectly constant in one species of a group, 

 "it will tend to be constant in allied species." 



This leads me to recall a discussion in the chapter on Selec- 

 tion, in which it was shown that with domestic races, which are 

 3i 'Proc. Entomolog. Soc. of Philadelphia,' Oct. 1863, p. 213. 



