It is my belief, founded upon the tropical character of the Fauna, that
a much milder climate then prevailed over the whole northern hemisphere
than is now known to it. Some naturalists have supposed that the
presence of the tropical Mammalia in the Northern Temperate Zone might
be otherwise accounted for,--that they might have been endowed with
warmer covering, with thicker hair or fur. But I think the simpler and
more natural reason for their existence throughout the North is to be
found in the difference of climate; and I am the more inclined to this
opinion because the Tertiary animals generally, the Fishes, Shells,
etc., in the same regions, are more closely allied in character to those
now living in the Tropics than to those of the Temperate Zones. The
Tertiary age may be called the geological summer; we shall see,
hereafter, how abruptly it was brought to a close.
One word more as to the relation of the Tertiary Mammalia to the
creation which preceded them. I can only repeat here the argument used
before: the huge quadrupeds characteristic of these epochs make their
appearance suddenly, and the deposits containing them follow as
immediately upon those of the Cretaceous epoch, in which no trace of
them occurs, as do those of the Cretaceous upon those of the Jurassic
epoch.
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