From these successive remains we can confidently picture the
evolution, during two or three million years, of one of our most
familiar mammals. It must not, of course, be supposed that these
fossil remains all represent "ancestors of the horse." In some
cases they may very well do so; in others, as we saw, they
represent sidebranches of the family which have become extinct.
But even such successive forms as the Eohippus, Mesohippus,
Miohippus, and Pliohippus must not be arranged in a direct line
as the pedigree of the horse. The family became most extensive in
the Miocene, and we must regard the casual fossil specimens we
have discovered as illustrations of the various phases in the
development of the horse from the primitive Ungulate. When we
recollect what we saw in an earlier chapter about the evolution
of grassy plains and the successive rises of the land during the
Tertiary period, and when we reflect on the simultaneous advance
of the carnivores, we can without difficulty realise this
evolution of our familiar companion from a hyrax-like little
animal of two million years ago.
We have not in many cases so rich a collection of intermediate
forms as in the case of the horse, but our fossil mammals are
numerous enough to suggest a similar development of all the
mammals of to-day.
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