Nor is it less probable that the characteristic types of Australian
Mammalia were already developed in that region in Miocene times.

But Austro-Columbia presents difficulties from which Australia is
free; _Camelidae_ and _Tapiridae_ are now indigenous in South America
as they are in Arctogaea; and, among the Pliocene Austro-Columbian
mammals, the Austro-Columbian genera _Equus_, _Mastodon_, and
_Machairodus_ are numbered. Are these Postmiocene immigrants, or
Praemiocene natives?

Still more perplexing are the strange and interesting forms _Toxodon_,
_Macrauchenia_, _Typotherium_, and a new Anoplotherioid mammal
(_Homalodotherium_) which Dr. Cunningham sent over to me some time ago
from Patagonia. I confess I am strongly inclined to surmise that these
last, at any rate, are remnants of the population of Austro-Columbia
before the Miocene epoch, and were not derived from Arctogaea by way
of the north and east.

The fact that this immense fauna of Miocene Arctogaea is now fully
and richly represented only in India and in South Africa, while it
is shrunk and depauperized in North Asia, Europe, and North America,
becomes at once intelligible, if we suppose that India and South
Africa had but a scanty mammalian population before the Miocene
immigration, while the conditions were highly favourable to the new
comers. It is to be supposed that these new regions offered themselves
to the Miocene Ungulates, as South America and Australia offered
themselves to the cattle, sheep, and horses of modern colonists. But,
after these great areas were thus peopled, came the Glacial epoch,
during which the excessive cold, to say nothing of depression and
ice-covering, must have almost depopulated all the northern parts of
Arctogaea, destroying all the higher mammalian forms, except those
which, like the Elephant and Rhinoceros, could adjust their coats to
the altered conditions. Even these must have been driven away from the
greater part of the area; only those Miocene mammals which had passed
into Hindostan and into South Africa would escape decimation by such
changes in the physical geography of Arctogaea. And when the northern
hemisphere passed into its present condition, these lost tribes of the
Miocene Fauna were hemmed by the Himalayas, the Sahara, the Red Sea,
and the Arabian deserts, within their present boundaries. Now, on the
hypothesis of evolution, there is no sort of difficulty in admitting
that the differences between the