Ix PROCEEDINGS OF THE GEOLOGICAL SOCIETY, 
demonstrate the necessity of the existence of all the great types of 
the Eocene epoch in some antecedent period. 
It is this Mesozoic continent (which may well have lain in the 
neighbourhood of what are now the shores of the North Pacific 
Ocean) which I suppose to have been occupied by the Mesozoic 
Monodelphia; and it is in this region that I conceive they must 
have gone through the long series of changes by which they were 
specialized into the forms which we refer to different orders. I 
think it very probable that what is now South America may have 
received the characteristic elements of its mammalian fauna during 
the Mesozoic epoch; and there can be little doubt that the general 
nature of the change 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 leaying 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 paleontologist 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 under- 
lying 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 gaye 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 Paleozoic 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 faunze seem to be related to the Mesozoic lands 
and their faunee, 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 Arctogeeal continent and an 
Arctogzeal 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 Marsu- 
pralia of the whole Mesozoic epoch. 
