er, and the nail upon it
is spreading around it and is growing firmer. The creature, too, is
standing more nearly upon his toes; his legs are getting longer; he
stands higher from the ground, and now has come to be the size of a
hound.

We can only surmise why this creature should have undergone such a
change, but the presence of flesh-eating animals having the size of a
fox, and presumably of the fox's swiftness, probably tells the story.
The little bands of early horses, pursued by their carnivorous foes,
were slowly modified into swifter creatures. It is not so much that
running made them fast, as that the slow ones were continually being
caught. If this process of constant elimination of the slow members of
any herd is kept up long enough, the group will necessarily develop
speed. As time goes on, of these early horses those which happened to
have longer legs and stood higher upon their toes won in the race, and
handed on their qualities to their long-legged descendants. As the
animal rose upon his toes, the inner toe, corresponding to our thumb,
was first raised off the ground and rendered useless, while a similar
change came over the corresponding toe on the hind foot. The hard work
of running being done on the latter, this superfluous toe was more
detrimental there than on the front foot, and disappeared,
consequently, more rapidly. In time, however, it also disappeared from
the front foot. Gradually the further elevation of the foot lifted the
toe, which corresponds to our little finger, off the ground, and this
now disappears also.

With increasing toughness of the grasses, as the climate becomes drier
and the region more elevated, the teeth of the horse are given harder
work. The points begin to spread into ridges and to unite with each
other in such way as to form the crescents, which are later to be so
characteristic of the teeth of the modern horse.

By the middle of the Tertiary this ancestral horse has risen in height
until he is taller and heavier than a setter dog. Three toes are found
on each front foot. The middle toe is getting constantly more
developed, though the smaller toes are evidently still of use. The
ridges of the teeth are quite crescentic now on the outer side, and
becoming better adapted to the evidently firmer food which the
creature is obliged to eat.

As we come toward the end of the Tertiary, the development which had
been all pointing in one direction has advanced very much furt