visible labor of expenditure in walking, speaking, looking, hearing, etc., is doubtless, for the instant, a loss of motive force; but then, as we have just seen, there is in an adequately fed organism reparation of the nerve by nourishment in the measure that it is worn away by exercise. Simple rest is also a sufficient condition of reparation. There is, therefore, no absolutely definitive loss. Furthermore, exercise produces skill in reducing resistances and obstacles; and, when it is moderate and agreeable, it increases and feeds the organ instead of enfeebling it. Want of use, on the other hand, produces atrophy of an organ. Thus, normal exercise, expenditure proportioned to the force, is a necessary condition of reparation, conservation, and progress. Natural selection is therefore a law of work, of incessant expenditure; but the action fortifies, and the expenditure enriches. This means that life supposes incessant recomposition and decomposition, and consequently movements of disintegration as well as of integration. To feel life is to have an obscure perception of all the vital movements; to enjoy or suffer is to feel one's self living more or living less. The more intense the decomposition with an equally intense recomposition, the more precipitous is the vital movement, and the more we feel. It is not, therefore, to adopt the language of mechanics, the potential force, but its transformation into living force and into movement, that causes pleasure—provided the expenditure does not exceed the reparation necessary for the survival of the individual and the species.
Experiment confirms the deductions which are drawn from the laws of natural selection and of the struggle for life. Every normal and proportioned action of a well-fed nerve causes enjoyment; and the pleasure increases with the force of the stimulant to the point where the stimulation and the expenditure which it involves exceed the compensatory labor of reparation. Pain is due to the exhaustion, or the destruction, or the rupture, of the sensitive tissue; disorders which if prolonged would induce the death of the individual or of his posterity. The proportionate or disproportionate exercise of a particular nerve thus extends its effect, by diffusion and sympathy, so that it makes itself felt by the whole of the nervous system, and consequently of the organism. Hence, in the struggle for existence, four situations are possible when considered as to the relation of the expended to the accumulated energy, of the labor produced to the nutrition: 1. An excess of acquisition with insufficient expenditure produces the negative pain of want—the well-fed child suffers from immobility. 2. An increase of expenditure succeeding an increase of nutrition produces the positive pleasure of exercise—the child takes delight in running, jumping, and playing. 3. An increase of expenditure with insufficient reparation produces fatigue and positive pain—a too fast or too long race brings on weariness. 4. Absence of expenditure after exhaustion produces the negative pleasure of rest.
