l 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 t