With the case of the Horses before us, justifying a belief in
the production of new animal forms by modification of old ones, I see
no escape from the necessity of seeking for these ancestors of the
_Ungulata_ beyond the limits of the Tertiary formations.
I could as soon admit special creation, at once, as suppose that the
Perissodactyles and Artiodactyles had no five-toed ancestors. And when
we consider how large a portion of the Tertiary period elapsed before
_Anchitherium_ was converted into _Equus_, it is difficult to escape
the conclusion that a large proportion of time anterior to the
Tertiary period must have been expended in converting the common stock
of the _Ungulata_ into Perissodactyles and Artiodactyles.
The same moral is inculcated by the study of every other order of
Tertiary monodelphous _Mammalia_. Each of these orders is represented
in the Miocene epoch: the Eocene formation, as I have already said,
contains _Cheiroptera, Insectivora, Rodentia, Ungulata, Carnivora,_
and _Cetacea_. But the _Cheiroptera_ are extreme modifications of the
_Insectivora_, just as the _Cetacea_ are extreme modifications of
the Carnivorous type; and therefore it is to my mind incredible
that monodelphous _Insectivora_ and _Carnivora_ should not have been
abundantly developed, along with _Ungulata_, in the Mesozoic epoch.
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