Before doing so let us be quite clear that here we have nothing
to do with theories of the origin of the earth. The Permian
cold--which, however, is universally admitted--is more or less
entangled in that controversy; the Cretaceous cold has no
connection with it. Whatever excess of carbon-dioxide there may
have been in the early atmosphere was cleared by the
Coal-forests. We must set aside all these theories in explaining
the present facts.
It is also useful to note that the fact that there have been
great changes in the climate of the earth in past time is beyond
dispute. There is no denying the fact that the climate of the
earth was warm from the Arctic to the Antarctic in the Devonian
and Carboniferous periods: that it fell considerably in the
Permian: that it again became at least "warm temperate"
(Chamberlin) from the Arctic to the Antarctic in the Jurassic,
and again in the Eocene: that some millions of square miles of
Europe and North America were covered with ice and snow in the
Pleistocene, so that the reindeer wandered where palms had
previously flourished and the vine flourishes to-day; and that
the pronounced zones of climate which we find today have no
counterpart in any earlier age.
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