and general as that of the Permian period. The warmth of the
Jurassic period is generally attributed to the low relief of the land,
and the very large proportion of water-surface. The effect of this would
be to increase the moisture in the atmosphere. Whether this was assisted
by any abnormal proportion of carbon-dioxide, as in the Carboniferous,
we cannot confidently say. Professor Chamberlin observes that, since
the absorbing rock-surface was greatly reduced in the Jurassic, the
carbon-dioxide would tend to accumulate in its atmosphere, and help to
explain the high temperature. But the great spread of vegetation and the
rise of land in the later Jurassic and the Cretaceous would reduce this
density of the atmosphere, and help to lower the temperature.

It is clear that the cold would at first be local. In fact, it must be
carefully realised that, when we speak of the Jurassic period as a time
of uniform warmth, we mean uniform at the same altitude. Everybody knows
the effect of rising from the warm, moist sea-level to the top of even
a small inland elevation. There would be such cooler regions throughout
the Jurassic, and we saw that there were considerable upheavals of
land towards its close. To these elevated lands we may look for the
development of the Angiosperms, the birds, and the mammals. When
the more massive rise of land came at the end of the Cretaceous, the
temperature would fall over larger areas, and connecting ridges would
be established between one area and another. The Mesozoic plants and
animals would succumb to this advancing cold. What precise degree of
cold was necessary to kill the reptiles and Cephalopods, yet allow
certain of the more delicate flowering plants to live, is yet to be
determined. The vast majority of the new plants, with their winter
sleep, would thrive in the cooler air, and, occupying the ground of
the retreating cycads and ginkgoes would prepare a rich harvest for the
coming birds and mammals.

CHAPTER XV. THE TERTIARY ERA

We have already traversed nearly nine-tenths of the story of terrestrial
life, without counting the long and obscure Archaean period, and still
find ourselves in a strange and unfamiliar earth. With the close of the
Chalk period, however, we take a long stride in the direction of the
modern world. The Tertiary Era will, in the main, prove a fresh period
of genial warmth and fertile low-lying regions. During its course our
deciduous trees and grasses wil