ITS NEAREST EXISTING RELATIONS 



93 



ass, the common ass and the zebra, the common ass and 

 Burchell's zebra, the common ass and the hemionus, the 

 hemionus and the zebra, the hemionus and Burchell's 

 zebra. The two species which are, perhaps, the furthest 

 removed in general structure — the horse and the ass 

 — produce, as is well known, mules, which, in some 

 qualities useful to man, excel both their progenitors, 

 and in some countries, and for certain kinds of work, 

 are in greater requisition than either. Although occa- 

 sional instances have been recorded of female mules 

 breeding with the males of one or other of the pure 

 species, it is doubtful if any case has occurred of their 

 breeding inter se, although the opportunities of doing so 

 must have been great, as mules have been reared in 

 immense numbers for several thousands of years. We 

 may therefore consider it settled that the different 

 species of the group are now in that degree of physio- 

 logical differentiation which still enables them to pro- 

 duce offspring with each other, but does not permit the 

 progeny to continue the race, at all events unless 

 reinforced by the aid of one of the pure forms. 



The several members of the group show mental 

 differences quite as striking as those exhibited by their 

 external form, and more than, perhaps, might be expected 

 from the similarity of their cerebral organisation. The 

 patience of the ass, the high spirit of the horse, the 

 obstinacy of the mule, have long been proverbial. It 



