THE PRINCIPLE DEDUCED 48 



whole ? There is such a factor. It is known as nervous 

 energy. 



If our food varies, whether in quantity or quality, it 

 will have an immediate effect on the sum of our nervous 

 energy. 



If the climate varies, hotter or colder, moister or dryer, 

 it will have an immediate effect on the sum of our nervous 

 energy. 



And any factor in the environment, no matter what, 

 which makes for greater or lesser mental or physical 

 exertion will have an immediate effect on the sum of our 

 nervous energy. 



Nervous energy, moreover, is the factor above all 

 others which makes for survival. 



As this factor answers all the necessities of the case, 

 and as it appears to be the only factor within the animal 

 organism which does answer all these necessities, it 

 follows that 



The variation of the degree of animal fertility in response 

 to the direct action of the environment will bear an 

 inverse proportion to the development of nervous energy. 



We shall see later that in human society birthrate 

 and deathrate rise and fall together with remarkable 

 regularity. The most obvious inference which we can 

 draw from this is that the same combination of causes 

 that produces a high deathrate produces also a high 

 birthrate, and that the same combination of causes that 

 produces a low deathrate produces also a low birthrate. 

 In order that this result may be, the factors which make 

 for death or survival must be co-ordinated and their 

 survival-value measured with approximate accuracy. The 

 great co-ordinating factor of the animal organism is the 

 nervous system, and nervous energy is its active agent. 



