Worm-like Organization
Probably myriads of different types of this worm-like
organisation were developed, but such animals leave no trace in
the rocks, and we 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 others
defensive armour is chiefly developed, and we get the lines of
the heavy sluggish shell-fish, the Molluscs and Brachiopods, and,
by a later compromise between speed and armour, the more active
tough-coated Arthropods. In others the plant-principle reappears;
the worm-like creature retires from the free-moving life,
attaches itself to a fixed base, and becomes the Bryozoan or the
Echinoderm. To trace the development of these types in any detail
is impossible. The early remains are not preserved. But some
clues are found in nature or in embryonic development, and, when
the types do begin to be preserved in the rocks, we find the
process of evolution plainly at work in them. We will therefore
say a few words about the general evolution of each type, and
then return to the geological record in the Cambrian rocks.
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