wait the day when man will break the seals and put flesh once more on
the petrified bones.

CHAPTER XI. 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 th