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.
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