THE PROBLEM OF POPULATION. 675 



evident. The exertion of muscular force, unless exhaustively em- 

 ployed, 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 repro- 

 duction 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 restrict- 

 ed ; while the young born under such circumstances are apt to be con- 

 stitutionally weak, and unable to bear the strain of an excessive effort 

 in food-getting. There is thus in this effect a strong check on popu- 

 lation 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 pos- 

 sibility. Decrease in food-supply causes a decrease in food-consum- 

 ers, through the exhaustion of organic energy in other directions than 

 that of reproduction. And the new generation of consumers is con- 

 stitutionally 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, 

 brinsrs 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 — per- 

 haps 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, 



