d America there is a great emergence of land.
The shore-tracts and the shallow water are narrowed, the struggle is
intensified in them, and we pass into the Silurian age with a greatly
reduced number but more advanced variety of animals. In the Silurian
age the sea advances once more, and the shore-waters expand. There is
another great "expansive evolution" of life. But the Silurian age closes
with a fresh and very extensive emergence of the land, and this time
it will have the most important consequences. For two new things have
meantime appeared on the earth. The fish has evolved in the waters, and
the plant, at least, has found a footing on the land.

These geological changes which we have summarised and which have been
too little noticed until recently in evolutionary studies, occupied
7,000,000 years, on the lowest estimate, and probably twice that period.
The impatient critic of evolutionary hypotheses is apt to forget the
length of these early periods. We shall see that in the last two or
three million years of the earth's story most extraordinary progress
has been made in plant and animal development, and can be very fairly
traced. How much advance should we allow for these seven or fourteen
million years of swarming life and changing environments?

We cannot nearly cover the whole ground of paleontology for the period,
and must be content to notice some of the more interesting advances, and
then deal more fully with the evolution of the fish, the forerunner of
the great land animals.

The Trilobite was the most arresting figure in the Cambrian sea, and its
fortunes deserve a paragraph. It reaches its climax in the Ordovician
sea, and then begins to decline, as more powerful animals come upon the
scene. At first (apparently) an eyeless organism, it gradually develops
compound eyes, and in some species the experts have calculated that
there were 15,000 facets to each eye. As time goes on, also, the eye
stands out from the head on a kind of stalk, giving a wider range of
vision. Some of the more sluggish species seem to have been able to
roll themselves up, like hedgehogs, in their shells, when an enemy
approached. But another branch of the same group (Crustacea) has
meantime advanced, and it gradually supersedes the dwindling Trilobites.
Toward the close of the Silurian great scorpion-like Crustaceans
(Pterygotus, Eurypterus, etc.) make their appearance. Their development
is obscure, but it must be remembered t