The front teeth are now chopping incisors, and the grinding teeth
approach those of the modern horse in the distribution of the
enamel, dentine, and cement. They are now about the size of a
donkey, and must have had a distinctly horsy appearance, with
their long necks and heads and tapering limbs. One of them,
Merychippus, was probably in the direct line of the evolution of
the horse. From Hipparion some of the authorities believe that
the zebras may have been developed. Miohippus, Protohippus, and
Hypohippus, varying in size from that of a sheep to that of a
donkey, are other branches of this spreading family.
In the Pliocene period the evolution of the main stem culminates
in the appearance of the horse, and the collateral branches are
destroyed. Pliohippus is a further intermediate form. It has only
one toe on each foot, with two large splint bones, but its hoof
is less round than that of the horse, and it differs in the shape
of the skull and the length of the teeth. The true horse (Equus)
at length appears, in Europe and America, before the close of the
Tertiary period. As is well known, it still has the rudimentary
traces of its second and fourth toes in the shape of splint
bones, and these bones are not only more definitely toe-shaped in
the foal before birth, but are occasionally developed and give us
a three-toed horse.
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