e_, has only two, or, at
most, three toes. Among the scanty mammals of the Lower Eocene formation
we have the perissodactyle _Ungulata_ represented by _Coryphodon,
Hyracotherium_, 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
car