Microbic Cannibalism
Now it is plain that in an age of increasing Evolution
the toughening of the skin would be one of the first advantages
to secure survival, and this is, in point of fact, almost the
second leading principle in early development. Naturally, as the
skin becomes firmer, the animal can no longer, like the Amoeba,
take food at, or make limbs of, any part of it. There must be
permanent pores in the membrane to receive food or let out rays
of the living substance to act as oars or arms. Thus we get an
immense variety amongst these Protozoa, as the one-celled animals
are called. Some (the Flagellates) have one or two stout oars;
some (the Ciliates) have numbers of fine hairs (or cilia). Some
have a definite mouth-funnel, but no stomach, and cilia drawing
the water into it. Some (Vorticella, etc.), shrinking from the
open battlefield, return to the plant-principle, live on stalks,
and have wreaths of cilia round the open mouth drawing the water
to them. Some (the Heliozoa) remain almost motionless, shooting
out sticky rays of their matter on every side to catch the food.
Some form tubes to live in; some (Coleps) develop horny plates
for armour; and others develop projectiles to pierce their prey
(stinging threads).
This miniature world is full of evolutionary interest, but it is
too vast for detailed study here. We will take one group, which
we know to have been already developed in the Cambrian, and let a
study of its development stand for all. In every lecture or book
on "the beauties of the microscope" we find, and are generally
greatly puzzled by, minute shells of remarkable grace and beauty
that are formed by some of these very elementary animals They are
the Radiolaria (with flinty shells, as a rule) and the
Thalamophora (with chalk frames). Evolution furnishes a simple
key to their remarkable structure.
As we saw, one of the early requirements to be fostered by
natural selection in the Archaean struggle for life was a "thick
skin," and the thick skin had to be porous to let the animal
shoot out its viscid substance in rays and earn its living. This
stage above the Amoeba is beautifully illustrated in the
sun-animalcules (Heliozoa). Now the lowest types of Radiolaria
are of this character. They have no shell or framework at all.
The next stage is for the little animal to develop fine irregular
threads of flint in its skin, a much better security against the
animal-eater. These animalcules, it must be recollected, are bits
of almost pure plasm, and, as they live in crowds, dividing and
subdividing, but never dying, make excellent mouthfuls for a
small feeder. Those with the more flint in their skins were the
more apt to survive and "breed."
The threads of flint increase
until they form a sort of thorn-thicket round a little social
group, or a complete lattice round an individual body. Next,
spikes or spines jut out from the lattice, partly for additional
protection, partly to keep the little body afloat at the surface
of the sea. In this way we get a bewildering variety and
increasing complexity of forms, ascending in four divergent lines
from the naked ancestral type to the extreme grace and intricacy
of the Calocyclas monumentum or the Lychnaspis miranda. These,
however, are rare specimens in the 4000 species of Radiolaria. I
have hundreds of them, on microscopic slides, which have no
beauty and little regularity of form. We see a gradual evolution,
on utilitarian principles, as we run over the thousands of forms;
and, when we recollect the inconceivable numbers in which these
little animals have lived and struggled for
life--passively--during tens of millions of years, we are not
surprised at the elaborate protective frames of the higher types.
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