of the Lower Eocene formation we have the perissodactyle
_Ungulata_ represented by _Coryphodon, Hyra-cotherium_, and
_Pliolophus_. Suppose for a moment, for the sake of following out
the argument, that _Pliolophus_ represents the primary stock of the
Perissodactyles, and _Dichobune_ that of the Artiodactyles (though
I am far from saying that such is the case), then we find, in the
earliest fauna of the Eocene epoch to which our investigations carry
us, the two divisions of the _Ungulata_ completely differentiated, and
no trace of any common stock of both, or of five-toed predecessors to
either. 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.
But if this be the case, how much further back must we go to find the
common stock of the monodelphous _Mammalia_? As to the _Didelphia_,
if we may trust the evidence which seems to be afforded by their
very scanty remains, a Hypsiprymnoid form existed at the epoch of the
Trias, contemporaneously with a Carnivorous form. At the epoch of the
Trias, therefore, the _Marsupialia_ must have, already existed long
enough to have become differentiated into carnivorous and herbivorous
forms. But the _Monotremata_ are lower fo