46 THE INTERNATIONAL MONTHLY. 
third digit of each foot, and the modern horses are thus function- 
ally monodactyl, the splints remaining as evidence that these 
animals are descended from ancestors which had at least three 
toes. 
The geographical relations of the horses are of much interest. 
It is not possible to say in just what part of the world the line 
took its origin, for the early genera of the series are found both 
in North America and Europe, and they evidently migrated back- 
ward and forward, for at that period Bering Sea was a land bridge 
connecting America and Asia, and the mild climate was no bar- 
rier to a northward range. The principal development of the 
series, however, took place in North America, for here the line is 
unbroken, each geological stage having its own characteristic 
genus, while in the European upper Eocene, Oligocene, lower 
and middle Miocene, no representative of the direct line has yet 
been discovered. In the Pliocene and Pleistocene the family had 
acquired an immense geographical range, extending into all the 
continents except Australia. The true horse, in the restricted 
sense of the term (species Equus caballus), was not developed in 
North America and appears never to have reached this continent, 
until brought here by the European settlers. The Pleistocene 
horses of North America, some of which were exceedingly large, 
had, so far as the skeleton is concerned, a greater resemblance to 
the asses, zebras, etc., than to the true horses. South America, 
which received its horses very late, when the great Pliocene 
migration came in from the north, developed some extremely 
peculiar types of horses, which have not yet been found in any 
other continent. For reasons which can hardly be even con- 
jectured, the horses disappeared entirely from the western hemi- 
sphere before the discoveries of Columbus, and continued to exist 
only in Eurasia and Africa. 
We have followed only the main line of equine descent, which 
leads up to the modern types. In addition to this main series or 
trunk, branches were given off from time to time, which, after a 
longer or shorter career, died out without leaving any successors 
behind them. One of these branches is that of the peculiar 
