THE PRINCIPLE DEDUCED 41 



should produce also a high birthrate, and that the same 

 combination of causes which produces a low deathrate 

 should produce also a low birthrate. As the purpose of 

 this work is to elucidate the law which governs the degree 

 of fertility in human society, and as in human society 

 both births and deaths are fairly regular from season 

 to season, it follows that this is the method with which 

 we are most concerned. Therefore we will deal with 

 this method first, leaving the alternative method for 

 later discussion. 



We may express the necessary condition of survival 

 and of progressive evolution in the following generali- 

 sation : 



The net result of the variations of the degree of fertility 

 under the direct action of the environment will bear an 

 inverse proportion to the variations of the capacity for 

 survival. 



And in order that birthrate and deathrate may rise 

 and fall together it is necessary that 



The variation of the degree of animal fertility in response 

 to the direct action of the environment shall bear an 

 inverse proportion to the variation of the survival-capacity 

 under that environment. 



This generalisation will need an important qualification 

 which can be considered later. But we need something 

 more definite than this. We need to know the factor 

 in the organism which controls the variation. Can this 

 be deduced from the above generalisation ? 



It is clear that such a phenomenon as the variation of 

 fertility must bear a definite relation to some factor 

 within the organism. It cannot be wholly " in the air." 

 Our task, then, is to find out to what factor or factors 

 it bears this relation and what the relation is. Clearly, 

 if the degree of fertility is to bear an inverse proportion 



