RELATIONSHIP OF SPECIES. 251 



E. zebra and E. asinus suggests that the former is more nearly 

 related to the latter than to the other Zebras. 



Some years ago it was pointed out by Mr. E. I. Pocock* that 

 there were certain markings which are possessed by all the 

 striped forms, generally referred to as Zebras, which are not 

 found in any other of the Equidce. 



It has struck me that some such explanation as the following 

 might be as permissible as such alternatives as that of these 

 markings being caused by some peculiarities of the environment 

 (such as are considered by some to induce these parallel cases), 

 or that of being caused by mimicry. 



Let us assume instead an evolution of the more specialized (?) 

 rock-haunting forms, E. zebra and E. asinus, from intermediate 

 types of Equines. Let us admit it as possible, if not probable, 

 that E. zebra is a specialized form descended from some ancestral 

 intermediate type, with medium hoofs, ears, tail, mane, &c, 

 akin to the Quaggas, and with stripes and markings peculiar to 

 Zebras in general. That in the course of this evolution there 

 were independent strains from the ancestral type making 

 towards the same goal ; that is, that all the present tribe of 

 E. zebra are not absolutely descended from one original Adam 

 and Eve pair. This assumption would not be greatly in excess 

 (as probability goes) of the facts noted in regard to the Beetle 

 Leptinotarsa. 



We shall here admit a and /3 strains to be one and the same 

 species, for it is impossible to tell the difference between them, 

 or which individuals belong to which strain, or even if there are 

 two strains, or two hundred, or perhaps, in conformity with 

 general opinion, only one. It is also quite easy to admit the 

 probability of sterile hybrids between the specialized and ancestral 

 forms. 



Admitting, therefore, independent evolution of this descrip- 

 tion, we may proceed a stage further and suppose the presence, 

 contemporaneously with the ancestral Quagga-like type, of 

 another intermediate form, also with medium hoofs, ears, tail, 

 and mane, but of uniform coloration and akin to the Onagers ; 

 the above two ancestral types being sufficiently allied and 

 congenial to each other to produce fertile hybrids. 



* 'Field.' 



