198 DISTINCT SPECIES PKBSENT [Chap. V. 



variation. More especially we might have inferred this, 

 from the blue colour and the several marks so often 

 appearing when differently coloured breeds are crossed. 

 Hence, although under nature it must generally be left 

 doubtful, what cases are reversions to formerly existing 

 characters, and what are new but analogous variations, 

 yet we ought, on our theory, sometimes to find the 

 varying offspring of a species assuming characters which 

 are already present in other members of the same group. 

 And this undoubtedly 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 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 

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

 same part or organ in an allied species. I have col- 

 lected 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 oc- 

 cur, 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 several species of the same genus, 

 partly under domestication and partly under nature. 

 It is a ease almost certainly of reversion. The ass 

 sometimes has very distinct transverse bars on its legs, 



