6S4. THE POPULAR SCIENCE MONTHLY. 



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, exer- 

 cise 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 prog- 

 ress. Natural selection is therefore a law of work, of incessant ex- 

 penditure ; but the action fortifies, and the expenditure enriches. 

 This means that life supposes incessant recomposition and decompo- 

 sition, and consequently movements of disintegration as well as of in- 

 tegration. 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 in- 

 tense recomposition, the more precipitous is the vital movement, and 

 the more we feel. It is not, therefore, to adopt the language of me- 

 chanics, 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 indi- 

 vidual 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 particu- 

 lar nerve thus extends its effect, by diffusion and sympathy, so that it 

 makes itself felt by the whole of the nervous system, and consequent- 

 ly of the organism. Hence, in the struggle for existence, four situa- 

 tions 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 pro- 

 duces 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. 



