LAWS OF VARIATION 147 



what are new but analogous variations, yet we ought, 

 on my theory, sometimes to find the varying offspring 

 of a species assuming characters (either from reversion 

 or from analogous variation) which already occur in 

 some other members of the same group. And this un- 

 doubtedly is the case in nature. 



A considerable part of the difficulty in recognising a 

 variable species in our systematic works, is due to its 

 varieties mocking, as it were, some of the other species 

 of the same genus. A considerable catalogue, also, 

 could be given of forms intermediate between two other 

 forms, which themselves must be doubtfully ranked as 

 either varieties or species ; and this shows, unless all 

 these forms be considered as independently created 

 species, that the one in varying has assumed some of 

 the characters of the other, so as to produce the inter- 

 mediate form. But the best evidence is afforded by 

 parts or organs of an important and uniform nature 

 occasionally varying so as to acquire, in some degree, 

 the character of 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 a great disadvantage in not 

 being able to give them. I can only repeat that such 

 cases certainly do occur, and seem to me very remark- 

 able. 



I 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 apparently of reversion. The ass not rarely 

 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. It has also 

 been asserted that the stripe on each shoulder is some- 

 times double. The shoulder-stripe is certainly 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-coloured asses. 



