If the same pain recurs frequently, the animal soon remarks that some among these vague movements will contribute more directly than others to assuage it, and will give the preference to them. The habit of resisting a particular suffering by a special movement, becoming hereditary, forms a veritable instinct. In conformity with the general laws of evolution, there is established a selection between injurious and useful reflex actions, and the latter will gradually predominate.
Even when we are not suffering from any accidental uneasiness provocative of special muscular reaction, we are impelled to move by the simple need of motion. Every animal has to expend daily a more or less considerable sum of energy to procure food for itself. The oyster, fixed on its rock, imbibes, without effort and almost passively, the vegetable matter which the waves bring to it. A snail, drawing itself slowly along on its belly, easily reaches the leaves which are in its way. The ox marches, step by step, in the field for hours, feeding upon the grass-leaves with which its lips come in contact. A wolf has to make journeys of leagues every day in search of its prey. The swallow has to keep in incessant motion to procure enough insects to satisfy its appetite. To the necessity for eating is added that of escaping enemies, and this exacts an increase of activity from the animal. Thus, each one, according to its kind, is obliged to be in motion more or less every day, and is organized for it. If, through accidental circumstances, its activity ceases to be useful, it is nevertheless obligatory upon it, for its physical constitution, having become adapted by heredity to the normal life of the species, can not abruptly bend itself to other conditions of existence. Its organism continues to furnish it the same quantity of energy, which it has to expend in some way. Hence the movements of the captive animals—of the lion which paces its cage, and of the canary-bird that leaps from bar to bar. Hence the physical exercises with which persons whose occupation condemns them to a too sedentary life relax themselves. This necessity for motion is especially great in youth, because the young animal must train itself in all the movements it will have to perform at a later age, and must also exercise its muscles and joints to develop them. Thus every animal has a tendency daily to expend a certain quantity of force, which is determined, not by the accidental wants of the individual, but by the general wants of the species.
How is this expenditure regulated? By what criterion do we know when we need exercise? A matter so indispensable to the good working of our organization can not be the product of reflex action. It is evident that animals can not take exercise by rule, after the manner of a gentleman who imposes upon himself the