The Evolution of the Horse. 
119 
more yellow colour, instead of pure grey, in wanting the dark 
streak across the shoulder, and in having smaller ears. It has 
a dark-coloured stripe along the middle of the back. There 
are several distinct varieties, which some naturalists consider 
species — the Syrian wild ass ; the Onager from Persia, the 
Punjab, Scinde, and the Desert of Cutch ; and the Kiang, or 
Dzeggetai of the high table-lands of Tibet, where it is usually 
met with at an elevation of 15,000 feet and upwards above the 
sea-level. They are all remarkably swift, having been known to 
outstrip the fleetest horse in speed. 
Lastly, there are three, or perhaps four, striped species, all in- 
habitants of Africa — the quagga (Equus quagga), the dauw, or 
Burchell's zebra (Equus Burchelli), the mountain zebra of the 
Cape Colony (Equus zebra), and another recently discovered in 
eastern Africa, called Equus grevyi. At the beginning of the 
present century these were all exceedingly abundant, especially 
Burchell's zebra and the quagga, which roamed in enormous 
herds over the great plains north of the Orange River ; but since 
the introduction of European firearms, their numbers are rapidly 
diminishing, and their complete extermination seems to be only 
a question of time. 
There are thus at least six modifications of the horse type at 
present existing sufficiently distinct to be reckoned as species 
by zoologists, and easily recognised by their external characters. 
They are, however, all so closely allied that each will, at least 
in a state of domestication or captivity, breed with perfect 
freedom with any of the others. Cases of fertile union are re- 
corded between the horse and the quagga ; the horse and 
dauw or Burchell's zebra ; the horse and the hemionus or Asiatic 
wild ass ; the common ass and the zebra, the common ass and 
the dauw, the common ass and the hemionus, the hemionus and 
the zebra, and the hemionus and the dauw. The two species 
which are, perhaps, the farthest removed in general structure — the 
horse and the ass — produce, as is well known, hybrids or mules, 
which, in some qualities useful to man, excel both their progeni- 
tors, and in some countries, and for certain kinds of work, are in 
greater requisition than either. Although occasional 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 mules breeding with one another. The different 
species of the group are, therefore, now in that degree of physio- 
logical separation which enables them to produce 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. 
