Large jelly-fishes (of which casts are preserved)
swim in the water; coral-animals lay their rocky foundations, but
do not as yet form reefs; coarse sponges rise from the floor; and
myriads of tiny Radiolaria and Thalamophores, with shells of
flint and lime, float at the surface or at various depths.
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.
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