The teeth,
and the bones and joints of the limbs, are also developing in the
direction of the horse.
In the succeeding geological period, the Oligocene, we find
several horse-types in which the adaptation of the limbs to
running on the firm grassy plains and of the teeth to eating the
grass continues. Mesohippus has lost the fourth toe of the fore
foot, which is now reduced to a splintbone, and the lateral toes
of its hind foot are shrinking. In the Miocene period there is a
great development of the horse-like mammals. We have the remains
of more than forty species, some continuing the main line of
development on the firm and growing prairies of the Miocene, some
branching into the softer meadows or the forests, and giving rise
to types which will not outlive the Tertiary. They have three
toes on each foot, and have generally lost even the rudimentary
trace of the fourth toe. In most of them, moreover, the lateral
toes--except in the marsh-dwelling species, with spreading
feet--scarcely touch the ground, while the central toe is
developing a strong hoof. The leg-bones are longer, and have a
new type of joint; the muscles are concentrated near the body.
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