s at the present time, but rather a host of
islands lying where the great lands now are, the budding tops of the
continents just appearing above the sea.

Although the life of this time was far simpler than at the present
day, it had about as great variety as we would find on our present
sea-floors. There were as many different species living at the same
time on a given surface.

The Cambrian and Silurian time--the time before the coming of the
fishes--must have endured for many million years without any great
change in the world. Hosts of species lived and died; half a dozen
times or more the life of the earth was greatly changed. New species
came much like those that had gone before, and only a little gain here
and there was perceptible at any time. Still, at the end of the
Silurian, the life of the world had climbed some steps higher in
structure and in intelligence.

[Illustration: FIG. 1. NORTH AMERICA IN CAMBRIAN TIME.]

The next set of periods is known as the Devonian. It is marked by the
rapid extension of the fishes; for, although the fishes began in the
uppermost Silurian, they first became abundant in this time. These,
the first strong-jawed tyrants of the sea, came all at once, like a
rush of the old Norman pirates into the peaceful seas of Great
Britain. They made a lively time among the sluggish beings of that
olden sea. Creatures that were able to meet feebler enemies were swept
away or compelled to undergo great changes, and all the life of the
oceans seems to have a spur given to it by these quicker-formed and
quicker-willed animals. In this Devonian section of our rocks we have
proofs that the lands were extensively covered with forests of low
fern trees, and we find the first trace of air-breathing animals in
certain insects akin to our dragon-flies. In this stage of the earth's
history the fishes grew constantly more plentiful, and the seas had a
great abundance of corals and crinoids. Except for the fishes, there
were no very great changes in the character of the life from that
which existed in the earlier time of the Cambrian and Silurian. The
animals are constantly changing, but the general nature of the life
remains the same as in the earlier time.

[Illustration: FIG. 2. RANICEPS LYELLI--COAL TIME SALAMANDER.]

In the Carboniferous or coal-bearing age, we have the second great
change in the character of the life on the earth. Of the earlier
times, we have preserved only the rocks formed i