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 through dissimilar
physical conditions, for the dissimilarity of the inhabitants of these two
continents; and, on the other hand through similarity of conditions, for
the uniformity of the same types in each continent 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.
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