The suggestion that the disk of the sun is appreciably smaller since
Tertiary days is absurd; and the idea that the earth has only recently
ceased to allow its internal heat to leak through the crust is hardly
more plausible. The cause remains to be discovered.

We turn now to consider the effect of the great Ice-Age, and the
relation of man to it. The Permian revolution, to which the Pleistocene
Ice-Age comes nearest in importance, wrought such devastation that the
overwhelming majority of living things perished. Do we find a
similar destruction of life, and selection of higher types, after the
Pleistocene perturbation? In particular, had it any appreciable effect
upon the human species?

A full description of the effect of the great Ice-Age would occupy
a volume. The modern landscape in Europe and North America was very
largely carved and modelled by the ice-sheet and the floods that ensued
upon its melting. Hills were rounded, valleys carved, lakes formed,
gravels and soils distributed, as we find them to-day. In its vegetal
aspect, also, as we saw, the modern landscape was determined by the
Pleistocene revolution. A great scythe slowly passed over the land. When
the ice and snow had ended, and the trees and flowers, crowded in the
southern area, slowly spread once more over the virgin soil, it was only
the temperate species that could pass the zone guarded by the Alps and
the Pyrenees. On the Alps themselves the Pleistocene population still
lingers, their successful adaptation to the cold now preventing them
from descending to the plains.

The animal world in turn was winnowed by the Pleistocene episode. The
hippopotamus, crocodile, turtle, flamingo, and other warm-loving animals
were banished to the warm zone. The mammoth and the rhinoceros met the
cold by developing woolly coats, but the disappearance of the ice, which
had tempted them to this departure, seems to have ended their fitness.
Other animals which became adapted to the cold--arctic bears, foxes,
seals, etc.--have retreated north with the ice, as the sheet melted.
For hundreds of thousands of years Europe and North America, with their
alternating glacial and interglacial periods, witnessed extraordinary
changes and minglings of their animal population. At one time
the reindeer, the mammoth, and the glutton penetrate down to the
Mediterranean, in the next phase the elephant and hippopotamus again
advance nearly to Central Europe. It is impossible here t