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.  Knowledge of even a great emergency is of little use, if I can do nothing about it.  But when the muscles appeared, nerves and ganglion cells were necessary to stimulate and control them.  And this highest system holds for a long time a position subordinate to that of the lower muscular organ.  Its development seems at first sight extraordinarily slow.  Only in insects and vertebrates has it become a centre of instinct and thought.  Through the sense-organs it is gaining an ever clearer, deeper, and wider knowledge of its environment.  First it is affected only by the lower stimuli of touch, taste, and smell.  Then with the development of ear and eye it takes cognizance of ever subtler forces and movements.  Memory comes into activity very early.  The animal begins to learn by experience.  The brain is becoming not merely a steering but a thinking organ.  More and more nervous material is crowded into it and detailed for its work.  Wits and shrewdness are beginning to count for something in the battle.  Not only the animal with the strongest muscles, but the one with the best brain survives.  And thus at last the brain began to develop with a rapidity as remarkable as its long delay.  Thus each higher function is called into activity by the next lower, serves this at first, and only later attains its supremacy.