his 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.

It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the
sloth, armadillo, and anteater, as their degenerate descendants. This
cannot for an instant be admitted. Thes