Evolution of the Horses 191 
more than a thread of bone. The skull has a longer face and a nearly 
enclosed orbit, and the brain-case is fuller and more capacious, the 
internal cast of which shows that the brain was richly convoluted. 
The teeth are still very short-crowned, but the upper incisors plainly 
show the beginning of the “mark”; the premolars have assumed the 
molar form, and the upper molars, though plainly derived from those 
of Eohippus, have made a long stride toward the horse pattern, in 
that the separate cusps have united to form a continuous outer wall 
and two transverse crests. 
In the lower Miocene the interesting genus Desmatippus shows 
a further advance in the development of the teeth, which are beginning 
to assume the long-crowned shape, delaying the formation of roots ; 
a thin layer of cement covers the crowns, and the transverse crests 
of the upper grinding teeth display an incipient degree of their 
modern complexity. This tooth-pattern is strictly intermediate 
between the recent type and the ancient type seen in Mesohippus 
and its predecessors. The upper Miocene genera, Protohippus and 
Hipparion are, to all intents and purposes, modern in character, but 
their smaller size, tridactyl feet and somewhat shorter-crowned teeth 
are reminiscences of their ancestry. 
From time to time, when a land-connection between North 
America and Eurasia was established, some of the successive equine 
genera migrated to the Old World, but they do not seem to have 
gained a permanent footing there until the end of the Miocene or 
beginning of the Pliocene, eventually diversifying into the horses, 
asses, and zebras of Africa, Asia and Europe. At about the same 
period, the family extended its range to South America and there 
gave rise to a number of species and genera, some of them extremely 
peculiar. For some unknown reason, all the horse tribe had become 
extinct in the western hemisphere before the European discovery, but 
not until after the native race of man had peopled the continents. 
In addition to the main stem of equine descent, briefly considered 
in the foregoing paragraphs, several side-branches were given off at 
successive levels of the stem. Most of these branches were short- 
lived, but some of them flourished for a considerable period and 
ramified into many species. 
Apparently related to the horses and derived from the same 
root-stock is the family of the Palaeotheres, confined to the Eocene 
and Oligocene of Europe, dying out without descendants. In the 
earlier attempts to work out the history of the horses, as in the 
famous essay of Kowalevsky1, the Palaeotheres were placed in the 
direct line, because the number of adequately known Eocene mam- 
1 “Sur lAnchitherium aurelianense Cuv. et sur Vhistoire paléontologique des Chevaux,”’ 
Mém. de V Acad. Imp. des Sc. de St Pétersbourg, xx. no. 5, 1873. 
