130 THE ORIGIN OF SPECIES 



varying offspring of a species assuming characters which are al- 

 ready present in other members of the same group. And this un- 

 doubtedly is the case. 



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 created 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 occasionally 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 dis- 

 advantage of not being able to give them. I can only repeat that 

 such cases certainly occur, and seem to me very remarkable. 



I will, however, give one curious and complex case, not indeed 

 as affecting any important character, but from occurring in sev- 

 eral 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 be- 

 lieve this to be true. The stripe on the shoulder is sometimes 

 double, and is very variable in length and outline. A white ass, 

 but not 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. Mr. Blyth has 

 seen a specimen of the hemionus with a distinct shoulder-stripe, 

 though it properly has none; and I have been informed by Colonel 

 Poole that the foals of this species are generally striped on the 

 legs and faintly on the shoulder. The quagga, though so plainly 

 barred like a zebra over the body, is without bars on the legs ; but 

 Dr. Gray has figured one specimen with very distinct zebra-like 

 bars on the hocks. 



With respect to the horse, I have collected cases in England of 

 the spinal stripe in horses of the most distinct breeds and of all 

 colors; transverse bars on the legs are not rare in duns, mouse- 

 duns, and in one instance in a chestnut; a faint shoulder-stripe 

 may sometimes be seen in duns, and I have seen a trace in a bay 

 horse. My son made a careful examination and sketch for me of 



