64 THE LAW OF BIRTHS AND DEATHS 



nervous system. Lombard has found that the power to 

 perform voluntary muscular work is reduced by a decrease 

 in the atmospheric pressure, and that several successive 

 days of high temperature, particularly if accompanied 

 by great humidity, are followed by a considerable loss 

 of strength. 1 



Physiologists find that if a given point in a nerve be 

 repeatedly stimulated, the response of the nerve becomes 

 weaker after a time until it finally ceases. But if it be 

 allowed to rest for a period new pulsations can be elicited. 

 The nerve, while resting, becomes charged with energy. 

 Evidently the intensity of this charge will at all times 

 depend on the relation between production and expendi- 

 ture. Now, the term " development " in our formula is 

 hardly an adequate expression of the principle. The 

 potential degree of fertility, at any given moment, will 

 bear an inverse proportion, not to the amount of nervous 

 energy developed or produced, but to the intensity of 

 the nervous charge at that moment. The term " nervous 

 charge " must not be taken literally in the sense in which 

 a wire is charged with electricity. What goes on in the 

 nerve is, as yet, an almost complete mystery. The phrase 

 must be taken to apply to the nutritive condition which 

 favours the production of energy, and is the best hypo- 

 thetical statement of the case which can be given. 



The nervous charge will be an approximately accurate 

 reflection of the circumstances of the moment. " A " 

 is a Society woman of means, and with little need for 

 active exertion. She will produce a moderate amount of 

 nervous energy, but will also expend moderately. Being 

 highly fed, she will have a nervous system always highly 

 charged. " B " is, say, a successful professional woman. 

 She owes her success to a great natural fund of nervous 

 1 The Growth of the Brain, H. H. Donaldson, chap. xiv. 



