Cambrian Animals
This slight sketch of the Cambrian population shows us that
living things had already reached a high level of development.
Their story evidently goes back, for millions of years, deep into
those mists of the Archaean age which we were unable to
penetrate. We turn therefore to the zoologist to learn what he
can tell us of the origin and family-relations of these Cambrian
animals, and will afterwards see how they are climbing to higher
levels under the eye of the geologist.
At the basis of the living world of to-day is a vast population
of minute, generally microscopic, animals and plants, which are
popularly known as "microbes." Each consists, in scientific
language, of one cell. It is now well known that the bodies of
the larger animals and plants are made up of millions of these
units of living matter, or cells--the atoms of the organic
world--and I need not enlarge on it. But even a single cell lends
itself to infinite variety of shape, and we have to penetrate to
the very lowest level of this luxuriant world of one-celled
organisms to obtain some idea of the most primitive living
things. Properly speaking, there were no "first living things."
It cannot be doubted by any student of nature that the microbe
developed so gradually that it is as impossible to fix a precise
term for the beginning of life as it is to say when the night
ends and the day begins. In the course of time little one-celled
living units appeared in the waters of the earth, whether in the
shallow shore waters or on the surface of the deep is a matter of
conjecture.
We are justified in concluding that they were at least as
rudimentary in structure and life as the lowest inhabitants of
nature to-day. The distinction of being the lowest known living
organisms should, I think, be awarded to certain one-celled
vegetal organisms which are very common in nature. Minute simple
specks of living matter, sometimes less than the five-thousandth
of an inch in diameter, these lowly Algae are so numerous that it
is they, in their millions, which cover moist surfaces with the
familiar greenish or bluish coat. They have no visible
organisation, though, naturally, they must have some kind of
structure below the range of the microscope. Their life consists
in the absorption of food-particles, at any point of their
surface, and in dividing into two living microbes, instead of
dying, when their bulk increases. A very lowly branch of the
Bacteria (Nitrobacteria) sometimes dispute their claim to the
lowest position in the hierarchy of living nature, but there is
reason to suspect that these Bacteria may have degenerated from a
higher level.
Here we have a convenient starting-point for the story of life,
and may now trace the general lines of upward development. The
first great principle to be recognised is the early division of
these primitive organisms into two great classes, the moving and
the stationary. The clue to this important divergence is found in
diet. With exceptions on both sides, we find that the non-moving
microbes generally feed on inorganic matter, which they convert
into plasm; the moving microbes generally feed on ready-made
plasm--on the living non-movers, on each other, or on particles
of dead organic matter. Now, inorganic food is generally diffused
in the waters, so that the vegetal feeders have no incentive to
develop mobility. On the other hand, the power to move in search
of their food, which is not equally diffused, becomes a most
important advantage to the feeders on other organisms. They
therefore develop various means of locomotion. Some flow or roll
slowly along like tiny drops of oil on an inclined surface;
others develop minute outgrowths of their substance, like fine
hairs, which beat the water as oars do. Some of them have one
strong oar, like the gondolier (but in front of the boat); others
have two or more oars; while some have their little flanks
bristling with fine lashes, like the flanks of a Roman galley.
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