ace at the end of the Mesozoic epoch in Europe was the upheaval of
the eastern and northern regions of the Mesozoic sea-bottom into a
westward extension of the Mesozoic continent, over which the mammalian
fauna, by which it was already peopled, gradually spread. This
invasion of the land was prefaced by a previous invasion of the
Cretaceous sea by modern forms of mollusca and fish.

It is easy to imagine how an analogous change might come about in the
existing world. There is, at present, a great difference between the
fauna of the Polynesian Islands and that of the west coast of America.
The animals which are leaving their spoils in the deposits now forming
in these localities are widely different. Hence, if a gradual
shifting of the deep sea, which at present bars migration between the
easternmost of these islands and America, took place to the westward,
while the American side of the sea-bottom was gradually upheaved,
the palaeontologist of the future would find, over the Pacific area,
exactly such a change as I am supposing to have occurred in the
North-Atlantic area at the close of the Mesozoic period. An Australian
fauna would be found underlying an American fauna, and the transition
from the one to the other would be as abrupt as that between the Chalk
and lower Tertiaries; and as the drainage-area of the newly formed
extension of the American continent gave rise to rivers and lakes, the
mammals mired in their mud would differ from those of like deposits on
the Australian side, just as the Eocene mammals differ from those of
the Purbecks.

How do similar reasonings apply to the other great change of
life--that which took place at the end of the Palaeozoic period?

In the Triassic epoch, the distribution of the dry land and of
terrestrial vertebrate life appears to have been, generally, similar
to that which existed in the Mesozoic epoch; so that the Triassic
continents and their faunae seem to be related to the Mesozoic lands
and their faunae, just as those of the Miocene epoch are related to
those of the present day. In fact, as I have recently endeavoured
to prove to the Society, there was an Arctogaeal continent and an
Arctogaeal province of distribution in Triassic times as there is now;
and the _Sauropsida_ and _Marsupialia_ which constituted that
fauna were, I doubt not, the progenitors of the _Sauropsida_ and
_Marsupialia_ of the whole Mesozoic epoch.

Looking at the present terrestrial fauna of Austra