Miocene forms of the mammalian
Fauna and those which exist at present are the results of gradual
modification; and, since such differences in distribution as obtain
are readily explained by the changes which have taken place in the
physical geography of the world since the Miocene epoch, it is clear
that the result of the comparison of the Miocene and present Fauna is
distinctly in favour of evolution. Indeed I may go further. I may
say that the hypothesis of evolution explains the facts of Miocene,
Pliocene, and Recent distribution, and that no other supposition even
pretends to account for them. It is, indeed, a conceivable supposition
that every species of Rhinoceros and every species of Hyaena, in the
long succession of forms between the Miocene and the present species,
was separately constructed out of dust, or out of nothing, by
supernatural power; but until I receive distinct evidence of the fact,
I refuse to run the risk of insulting any sane man by supposing that
he seriously holds such a notion.

Let us now take a step further back in time, and inquire into the
relations between the Miocene Fauna and its predecessor of the Upper
Eocene formation.

Here it is to be regretted that our materials for forming a judgment
are nothing to be compared in point of extent or variety with those
which are yielded by the Miocene strata. However, what we do know
of this Upper Eocene Fauna of Europe gives sufficient positive
information to enable us to draw some tolerably safe inferences. It
has yielded representatives of _Insectivora_, of _Cheiroptera_,
of _Rodentia_, of _Carnivora_, of artiodactyle and perissodactyle
_Ungulata_, and of opossum-like Marsupials. No Australian type of
Marsupial has been discovered in the Upper Eocene strata, nor any
Edentate mammal. The genera (except perhaps in the case of some of the
_Insectivora_, _Cheiroptera_, and _Rodentia_) are different from those
of the Miocene epoch, but present a remarkable general similarity to
the Miocene and recent genera. In several cases, as I have already
shown, it has now been clearly made out that the relation between
the Eocene and Miocene forms is such that the Eocene form is the less
specialized; while its Miocene ally is more so, and the specialization
reaches its maximum in the recent forms of the same type.

So far as the Upper Eocene and the Miocene Mammalian Faunae are
comparable, their relations are such as in no way to oppose the
hypothesis tha