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,
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; for the inhabitants of each quarter
of the world will obviously tend to leave in that quarter, during the
next succeeding period of time, closely allied though in some degree
modified descendants. If the inhabitants of one continent formerly
differed greatly from those of another continent, so will their
modified descendants still differ in nearly the same manner and degree.
But after very long intervals of time and after great geographical
changes, permitting much inter-migration, the feebler will yield to the
more dominant forms, and there will be nothing immutable in the laws of
past and present distribution.
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