evident. The exertion of muscular force, unless exhaustively employed, seems not injurious to the reproductive functions. Mental exertion, on the contrary, seems to restrict reproductive energy, even when not employed exhaustively.
But the animals below man do not employ mentality to any great extent. Their principal exertion is muscular, and this hinders reproduction only in case of the whole vigor of the animal being exhausted. If, for instance, the food-supplies of any animal tribe be diminished or its numbers increased, a greater exercise of agility is required to satisfy its appetite. And if it depend on cunning or shrewdness to obtain food, its mental faculties must become very actively exercised. If these efforts become exhaustive, reproduction is necessarily restricted; while the young born under such circumstances are apt to be constitutionally weak, and unable to bear the strain of an excessive effort in food-getting. There is thus in this effect a strong check on population from strictly physiological causes.
The conclusion here reached applies equally to the lower orders of mankind. A diminution of food-supply must have an effect upon savages similar to its influence upon the lower animals. Excessive muscular exertion, extensive migratory movements, warlike efforts, and exercise of mental vigor in food-getting, which become more physically exhaustive the greater the difficulty in obtaining food, must act to greatly restrict reproductive energy, and to enfeeble the children who may be born during such an exhausted condition of their parents. The lack of sufficient nutriment is a correlative agency under the same conditions.
The physiological check, therefore, in this phase of its action, tends to prevent the Malthusian law from being other than an abstract possibility. Decrease in food-supply causes a decrease in food-consumers, through the exhaustion of organic energy in other directions than that of reproduction. And the new generation of consumers is constitutionally enfeebled, and unsuited to bear the sharp struggle of life, so that the population becomes diminished during the continuance of such conditions.
But the physiological check, in this form of its application, brings mankind too near the starvation limit to be at all desirable. There is, however, another mode in which it exercises itself, yielding far more promising results. For there is reason to believe that active mental labor is far more exhaustive of reproductive energy than is equally vigorous muscular exertion. Just what is the organic cause of this we shall not attempt to guess. It is possible that the brain, in its action, may exhaust some material necessary to germ-formation—perhaps phosphorus, which seems to be an element both of the sperm-cells and of the brain. But it is the visible results, rather than the organic causes, with which we are just now concerned.
It is an undoubted fact that the families of the poor are, as a rule,
