Lund and Clausen in the caves of Brazil. I was so
much impressed with these facts that I strongly insisted, in 1839 and 1845,
on this "law of the succession of types,"--on "this wonderful relationship
in the same continent between the dead and the living." Professor Owen has
subsequently extended the same generalisation to the mammals of the Old
World. We see the same law in this author's restorations of the extinct and
gigantic birds of New Zealand. We see it also in the birds of the caves of
Brazil. Mr. Woodward has shown that the same law holds good with
sea-shells, but from the wide distribution of most genera of molluscs, it
is not well displayed by them. Other cases could be added, as the relation
between the extinct and living land-shells of Madeira; and between the
extinct and living brackish-water shells of the Aralo-Caspian Sea.

Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after comparing the
present climate of Australia and of parts of South America under the same
latitude, would attempt to account, on the one hand, by dissimilar physical
conditions for the dissimilarity of the inhabitants of these two
continents, {340} and, on the other hand, by similarity of conditions, for
the uniformity of the same types in each during the later tertiary periods.
Nor can it be pretended that it is an immutable law that marsupials should
have been chiefly or solely produced in Australia; or that Edentata and
other American types should have been solely produced in South America. For
we know that Europe in ancient times was peopled by numerous marsupials;
and I have shown in the publications above alluded to, that in America the
law of distribution of terrestrial mammals was formerly different from what
it now is. North America formerly partook strongly of the present character
of the southern half of the continent; and the southern half was formerly
more closely allied, than it is at present, to the northern half. In a
similar manner we know from Falconer and Cautley's discoveries, that
northern India was formerly more closely related in its mammals to Africa
than it is at the present time. Analogous facts could be given in relation
to the distribution of marine animals.

On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained;