70 THE LAW OF BIRTHS AND DEATHS 



needs of the species, it follows that the qualification will 

 come into play when a further increase of the birthrate 

 is useless. In point of fact, it is known that both famine 

 and illness cause sterility. 1 



All this, of course, is merely another illustration of 

 the principle of the vital optimum, and we may apply 

 that principle to the variation of the degree of animal 

 fertility thus : 



The variation of the degree of animal fertility in response 

 to the direct action of the environment will be directly 

 proportional to the intensity of the nervous charge until 

 the optimum point for fertility is reached : it will thence- 

 forward bear an inverse proportion to the intensity of 

 the nervous charge. 



1 Principles of Heredity, Dr. Archdall Reid. 



