There was--on
the most modest estimate--a stretch of some fifteen million years
between the Cambrian and the Permian upheavals. On the same
chronological scale the interval between the Permian and
Cretaceous revolutions was only about seven million years, and
the Tertiary Era will comprise only about three million years.
One wonders if the Fourth (Quaternary) Era in which we live will
be similarly shortened. Further, whereas the earth returned after
each of the earlier upheavals to what seems to have been its
primitive condition of equable and warm climate, it has now
entirely departed from that condition, and exhibits very
different zones of climate and a succession of seasons in the
year. One wonders what the climate of the earth will become long
before the expiration of those ten million years which are
usually assigned as the minimum period during which the globe
will remain habitable.
It is premature to glance at the future, when we are still some
millions of years from the present, but it will be useful to look
more closely at the facts which inspire this reflection. From
what we have seen, and shall further see, it is clear that, in
spite of all the recent controversy about climate among our
geologists, there has undeniably been a progressive refrigeration
of the globe.
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