e which took place 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