ITS NEAREST EXISTING BELATIONS. 99 



half -breeds are recorded between the horse and the 

 quagga, the horse and BurchelPs zebra, the horse 

 and the hemionus or Asiatic wild ass, the common 

 ass and the zebra, the common ass and BurchelPs 

 zebra, the common ass and the hemionus, the hemi- 

 onus and the zebra, the hemionus and BurchelPs ze- 

 bra. 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 oc- 

 casional 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 



