ditions already
described (p. 109, and fig. 5, p. 111) as capable of producing the
extreme of heat were ever combined at any geological period of which we
have yet obtained information. It is more probable, from what has been
stated in the preceding chapters, that a slight approximation to such an
extreme state of things would be sufficient; in other words, if most of
the dry land were tropical, and scarcely any of it arctic or antarctic,
a prodigious elevation of temperature must ensue, even though a part of
some continents should penetrate far into the temperate zones.

_Changes during the tertiary periods._--The secondary and tertiary
formations of Europe, when considered separately, may be contrasted as
having very different characters; the secondary appearing to have been
deposited in open seas, the tertiary in regions where dry land, lakes,
bays, and perhaps inland seas, abounded. The secondary series is almost
exclusively marine; the tertiary, even the oldest part, contains
lacustrine strata, and not unfrequently freshwater and marine beds
alternating. In fact there is evidence of important geographical changes
having occurred between the deposition of the cretaceous system, or
uppermost of the secondary series, and that of the oldest tertiary
group, and still more between the era of the latter and that of the
newer tertiary formations. This change in the physical geography of
Europe and North America was accompanied by an alteration no less
remarkable in organic life, scarcely any _species_ being common both to
the secondary and tertiary rocks, and the fossils of the latter
affording evidence of a different climate.

On the other hand, when we compare the tertiary formations of successive
ages, we trace a gradual approximation in the imbedded fossils, from an
assemblage in which extinct species predominate, to one where the
species agree for the most part with those now existing. In other words,
we find a gradual increase of animals and plants fitted for our present
climates, in proportion as the strata which we examine are more modern.
Now, during all these successive tertiary periods, there are signs of a
great increase of land in European and North American latitudes. By
reference to the map (Pl. 1), and its description, p. 121, the reader
will see that about two-thirds of the present European lands have
emerged since the earliest tertiary group originated. Nor is this the
only revolution which the same regi