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THE MIDDLE AGES OF THE EARTH
The story of the earth from the beginning of the Cambrian period
to the present day was long ago divided by geologists into four
great eras. The periods we have already covered--the Cambrian,
Ordovician, Silurian, Devonian, Carboniferous, and Permian--form
the Primary or Palaeozoic Era, to which the earlier Archaean
rocks were prefixed as a barren and less interesting
introduction. The stretch of time on which we now enter, at the
close of the Permian, is the Secondary or Mesozoic Era. It will
be closed by a fresh upheaval of the earth and disturbance of
life-conditions in the Chalk period, and followed by a Tertiary
Era, in which the earth will approach its modern aspect. At its
close there will be another series of upheavals, culminating in a
great Ice-age, and the remaining stretch of the earth's story, in
which we live, will form the Quaternary Era.
In point of duration these four eras differ enormously from each
other. If the first be conceived as comprising sixteen million
years--a very moderate estimate--the second will be found to
cover less than eight million years, the third less than three
million years, and the fourth, the Age of Man, much less than one
million years; while the Archaean Age was probably as long as all
these put together. But the division is rather based on certain
gaps, or "unconformities," in the geological record; and,
although the breaches are now partially filled, we saw that they
correspond to certain profound and revolutionary disturbances in
the face of the earth. We retain them, therefore, as convenient
and logical divisions of the biological as well as the geological
chronicle, and, instead of passing from one geological period to
another, we may, for the rest of the story, take these three eras
as wholes, and devote a few chapters to the chief advances made
by living things in each era. The Mesozoic Era will be a
protracted reaction between two revolutions: a period of
low-lying land, great sea-invasions, and genial climate, between
two upheavals of the earth. The Tertiary Era will represent a
less sharply defined depression, with genial climate and
luxuriant life, between two such upheavals.
The Mesozaic ("middle life") Era may very fitly be described as
the Middle Ages of life on the earth. It by no means occupies a
central position in the chronicle of life from the point of view
of time or antiquity, just as the Middle Ages of Europe are by no
means the centre of the chronicle of mankind, but its types of
animals and plants are singularly transitional between the
extinct ancient and the actual modern types. Life has been lifted
to a higher level by the Permian revolution. Then, for some
millions of years, the sterner process of selection relaxes, the
warm bosom of the earth swarms again with a teeming and varied
population, and a rich material is provided for the next great
application of drastic selective agencies. To a poet it might
seem that nature indulges each succeeding and imperfect type of
living thing with a golden age before it is dismissed to make
place for the higher.
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