on this hypothesis, are we to account for the absence of cattle in the
Pampas of South America, when those parts of the New World were
discovered? It is not that they were unfit for cattle, for millions of
cattle now run wild there; and the like holds good of Australia and
New Zealand. It is a curious circumstance, in fact, that the animals
and plants of the Northern Hemisphere are not only as well adapted to
live in the Southern Hemisphere as its own autochthones, but are, in
many cases, absolutely better adapted, and so overrun and extirpate
the aborigines. Clearly, therefore, the species which naturally
inhabit a country are not necessarily the best adapted to its climate
and other conditions. The inhabitants of islands are often distinct
from any other known species of animal or plants (witness our recent
examples from the work of Sir Emerson Tennent, on Ceylon), and yet
they have almost always a sort of general family resemblance to the
animals and plants of the nearest mainland. On the other hand, there
is hardly a species of fish, shell, or crab common to the opposite
sides of the narrow isthmus of Panama. [Footnote: See page 60
_Note_.] Wherever we look, then, living nature offers us riddles of
difficult solution, if we suppose that what we see is all that can be
known of it.

But our knowledge of life is not confined to the existing world. Whatever
their minor differences, geologists are agreed as to the vast thickness of
the accumulated strata which compose the visible part of our earth, and the
inconceivable immensity of the time the lapse of which they are the
imperfect but the only accessible witnesses. Now, throughout the greater
part of this long series of stratified rocks are scattered, sometimes very
abundantly, multitudes of organic remains, the fossilised exuviae of animals
and plants which lived and died while the mud of which the rocks are formed
was yet soft ooze, and could receive and bury them. It would be a great