The protozoa are mostly, though not purely, nutritive and
reproductive. These functions are essential to the existence of the
species. Naturally in the early protozoan colonies, and in forms
like hydra, these functions predominated. But mere digestive tissue
is not enough for digestion. Muscles are needed to draw the food to
the mouth, to keep the digestive sack in contact with it, and for
other purposes. A little higher they are used to enable the animal
to go in search of its food. They are still, however, more or less
entirely subservient to digestion. But in the highest worms we are
beginning to see signs that muscles are predominating in the body;
and we feel that, while mutually helpful, the digestive system
exists for the muscles, and these latter are becoming the aim of
development. From worms upward there is a marked advance in physical
activity and strength. The muscles thicken and are arranged in
heavier bands. Skeleton and locomotive appendages and jaws follow in
insects and vertebrates. The direct battle of animal against animal,
and of strength opposed to strength or activity, becomes ever
sharper. The strongest and most active are selected and survive.
And yet this is not the whole truth. Some power of perception is
possessed by every animal. But until muscles had developed the
nervous system could be of but little practical value.
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