th might have been
expected more readily than the forms of animals, to have afforded
conflicting proofs, had the popular theory been without foundation. When
the examination of fossil remains was extended to rocks in the most
northern parts of Europe and North America, and even to the Arctic
regions, indications of the same revolution in climate were discovered.

It cannot be said, that in this, as in many other departments of
geology, we have investigated the phenomena of former eras, and
neglected those of the present state of things. On the contrary, since
the first agitation of this interesting question, the accessions to our
knowledge of living animals and plants have been immense, and have far
surpassed all the data previously obtained for generalizing on the
relation of certain types of organization to particular climates. The
tropical and temperate zones of South America and of Australia have been
explored; and, on close comparison, it has been found that scarcely any
of the species of the animate creation in these extensive continents are
identical with those inhabiting the old world. Yet the zoologist and
botanist, well acquainted with the geographical distribution of organic
beings in other parts of the globe, would have been able, if distinct
groups of species had been presented to them from these regions, to
recognize those which had been collected from latitudes within, and
those which were brought from without the tropics.

Before I attempt to explain the probable causes of great vicissitudes of
temperature on the earth's surface, I shall take a rapid view of some of
the principal data which appear to support the popular opinions now
entertained on the subject. To insist on the soundness of these
inferences, is the more necessary, because some zoologists have
undertaken to vindicate the uniformity of the laws of nature, not by
accounting for former fluctuations in climate, but by denying the value
of the evidence in their favor.[126]

_Proofs from fossil shells in tertiary strata._--In Sicily, Calabria,
and in the neighborhood of Naples, the fossil testacea of the most
modern tertiary formations belong almost entirely to species now
inhabiting the Mediterranean; but as we proceed northwards in the
Italian peninsula we find in the strata called Subapennine an assemblage
of fossil shells departing somewhat more widely from the type of the
neighboring seas. The proportion of species identifiable with those