Evolutionary Transition
Here we have a very clear Evolution from the
solitary microbe to a higher level, but, unfortunately, it does
not take us far. The Sponges are a side-issue, or cul de sac,
from the Protozoic world, and do not lead on to the higher. Each
one-celled unit remains an animal; it is a colony of
unicellulars, not a many-celled body. We may admire it as an
instructive approach toward the formation of a many-celled body,
but we must look elsewhere for the true upward advance.
The next stage is best illustrated in certain spherical colonies
of cells like the tiny green Volvox (now generally regarded as
vegetal) of our ponds, or Magosphoera. Here the constituent cells
merge their individuality in the common action. We have the first
definite many-celled body. It is the type to which a moving close
colony of one-celled microbes would soon come. The round surface
is well adapted for rolling or spinning along in the water, and,
as each little cell earns its own living, it must be at the
surface, in contact with the water. Thus a hollow, or
fluid-filled, little sphere, like the Volvox, is the natural
connecting-link between the microbe and the many-celled body, and
may be taken to represent the first important stage in its
development.
The next important stage is also very clearly exhibited in
nature, and is more or less clearly reproduced in the embryonic
development of all animals. We may imagine that the age of
microbes was succeeded by an age of these many-celled larger
bodies, and the struggle for life entered upon a new phase. The
great principle we have already recognised came into play once
more. Large numbers of the many-celled bodies shrank from the
field of battle, and adopted the method of the plant. They rooted
themselves to the floor of the ocean, and developed long arms or
lashes for creating a whirlpool movement in the water, and thus
bringing the food into their open mouths. Forfeiting mobility,
they have, like the plant, forfeited the greater possibilities of
progress, and they remain flowering to-day on the floors of our
waters, recalling the next phase in the evolution of early life.
Such are the hydra, the polyp, the coral, and the sea-anemone. It
is not singular that earlier observers could not detect that they
were animals, and they were long known in science as
"animal-plants" (Zoophytes).
When we look to the common structure of these animals, to find
the ancestral type, we must ignore the nerve and muscle-cells
which they have developed in some degree. Fundamentally, their
body consists of a pouch, with an open mouth, the sides of the
pouch consisting of a double layer of cells. In this we have a
clue to the next stage of animal development. Take a soft
india-rubber ball to represent the first many-celled animal.
Press in one half of the ball close upon the other, narrow the
mouth, and you have something like the body-structure of the
coral and hydra. As this is the course of embryonic development,
and as it is so well retained in the lowest groups of the
many-celled animals, we take it to be the next stage. The reason
for it will become clear on reflection. Division of labour
naturally takes place in a colony, and in that way certain cells
in the primitive body were confined to the work of digestion. It
would be an obvious advantage for these to retire into the
interior, leaving the whole external surface free for the
adjustment of the animal's relations to the outer world.
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