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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 will mingle with the palms and pines over the land,
our flowers will begin to brighten the landscape, and the forms
of our familiar birds and mammals, even the form of man, will be
discernible in the crowds of animals. At its close another mighty
period of selection will clear the stage for its modern actors.
A curious reflection is prompted in connection with this division
of the earth's story into periods of relative prosperity and
quiescence, separated by periods of disturbance. 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. Every geologist, indeed, admits "oscillations of
climate," as Professor Chamberlin puts it. But amidst all these
oscillations we trace a steady lowering of the temperature.
Unless we put a strained and somewhat arbitrary interpretation on
the facts of the geological record, earlier ages knew nothing of
our division of the year into pronounced seasons and of the globe
into very different climatic zones. It might plausibly be
suggested that we are still living in the last days of the
Ice-Age, and that the earth may be slowly returning to a warmer
condition. Shackleton, it might be observed, found that there has
been a considerable shrinkage of the south polar ice within the
period of exploration. But we shall find that a difference of
climate, as compared with earlier ages, was already evident in
the middle of the Tertiary Era, and it is far more noticeable
to-day.
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