reas 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.

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 intermigration, the feebler will yield to the more
dominant forms, and there will be nothing immutable in the distribution
of organic beings.

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