224 THE ORIGIN OF SPECIES 



The difficulty in distinguishing variable species is 

 largely due to the varieties mocking, as it were, other 

 species of the same genus. A considerable catalogue, 

 also, could be given of forms intermediate between two 

 other forms, which themselves can only doubtfully be 

 ranked as species; and this shows, unless all these 

 closely allied forms be considered as independently cre- 

 ated species, that they have in varying assumed some 

 of the characters of the others. But the best evidence 

 of analogous variations is afforded by parts or organs 

 which are generally constant in character, but which oc- 

 casionally vary so as to resemble, in some degree, the 

 same part or organ in an allied species, I have collected 

 a long list of such cases; but here, as before, I lie under 

 the great disadvantage of not being able to give them. 

 I can only repeat that such cases certainly occur, and 

 seem to me very remarkable. 



1 will, however, give one curious and complex case, 

 not indeed as affecting any important character, but from 

 occurring in several species of the same genus, partly 

 under domestication and partly under nature. It is a 

 case almost certainly of reversion. The ass sometimes 

 has very distinct transverse bars on its legs, like those 

 on the legs of the zebra: it has been asserted that these 

 are plainest in the foal, and, from inquiries which I have 

 made, I believe this to be true. The stripe on the 

 shoulder is sometimes double, and is very variable in 

 length and outline. A white ass, but 7iot an albino, has 

 been described without either spinal or shoulder stripe: 

 and these stripes are sometimes very obscure, or actually 

 quite lost, in dark-colored asses. The koulan of Pallas 

 is said to have been seen with a double shoulder-stripe. 



