42 THE LAW OF BIRTHS AND DEATHS 



to the capacity for survival, it must bear a similar relation 

 to the factors which make for survival. As these factors 

 wax, fertility must wane, and vice versa. What, then, 

 are the factors which affect survival-capacity and which 

 vary with the environment ? 



There are two parties to this question : the organism 

 and the environment. There are factors in the environ- 

 ment which act on the organism and make for survival 

 or elimination ; and there are factors within the organism 

 which react to the environmental factors. The factors 

 in the environment which act on the organism may be 

 broadly stated as follows : 



Food : both quantity and quality. 

 Climate : hot or cold ; moist or dry. 

 All those varied factors which make for greater 

 or lesser mental and physical activity. 



If we can find within the organism a factor which 

 reacts to all these environmental factors, and is at the 

 same time the factor above all others which makes for 

 survival, that factor we may be sure is the one we seek. 

 Where shall we look for it ? 



We have seen that the development of the nervous 

 system is the factor above all others which makes for 

 survival in the struggle between species. So we may 

 first direct our search there. The mass of the nervous 

 system, though it may be largely developed by the action 

 of the environment, will bear a fairly constant ratio to 

 the rest of the organism throughout life after maturity 

 is reached. Is there any factor within or developed by 

 the nervous system capable of reacting to all the varied 

 factors of the environment a factor which will vary 

 with the environmental factors, forming an approxi- 

 mately accurate measure of the survival-value of the 



