live and thrive in it. And yet how, 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.
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 of whose lapse
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 fossilized 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 error to suppose that these