can
only follow the development by broad analogies. The lowest flat-worms
of to-day may represent some of these early types, and as we ascend
the scale of what is loosely called "worm" organisation, we get some
instructive suggestions of the way in which the various organs develop.
Division of labour continues among the colony of cells which make up
the body, and we get distinct nerve-cells, muscle-cells, and digestive
cells. The nerve-cells are most useful at the head of an organism which
moves through the water, just as the look-out peers from the head of the
ship, and there they develop most thickly. By a fresh division of labour
some of these cells become especially sensitive to light, some to the
chemical qualities of matter, some to movements of the water; we have
the beginning of the eyes, the nose, and the ears, as simple little
depressions in the skin of the head, lined with these sensitive cells. A
muscular gullet arises to protect the digestive tube; a simple drainage
channel for waste matter forms under the skin; other channels permit
the passage of the fluid food, become (in the higher worms) muscular
blood-vessels, and begin to contract--somewhat erratically at first--and
drive the blood through the system.

Here, perhaps, are millions of years of development compressed into
a paragraph. But the purpose of this work is chiefly to describe the
material record of the advance of life in the earth's strata, and show
how it is related to great geological changes. We must therefore abstain
from endeavouring to trace the genealogy of the innumerable types of
animals which were, until recently, collected in zoology under the
heading "Worms." It is more pertinent to inquire how the higher classes
of animals, which we found in the Cambrian seas, can have arisen from
this primitive worm-like population.

The struggle for life in the Archaean ocean would become keener and more
exacting with the appearance of each new and more effective type. That
is a familiar principle in our industrial world to-day, and we shall
find it illustrated throughout our story. We therefore find the various
processes of evolution, which we have already seen, now actively at
work among the swarming Archaean population, and producing several
very distinct types. In some of these struggling organisms speed is
developed, together with offensive and defensive weapons, and a line
slowly ascends toward the fish, which we will consider later. In o