CHAPTER II. 



A PRIORI PRINCIPLE. 



319. THE number of a species must at any time be either 

 decreasing or stationary or increasing. If, generation after 

 generation, its members die faster than others are born, the 

 species must dwindle and finally disappear. If its rate of 

 multiplication is equal to its rate of mortality, there can be 

 no numerical change in it. And if the deductions by death 

 are fewer than the additions by birth, the species must 

 become more abundant. These we may safely set down as 

 necessities. The forces destructive of race must be either 

 greater than the forces preservative of race, or equal to them, 

 or less than them; and there cannot but result these effects 

 on number. 



We are here concerned only with races that continue to 

 exist; and may therefore leave out of consideration those 

 in which the destructive forces, remaining permanently in 

 excess of the preservative forces, cause extinction. Prac- 

 tically, too, we may exclude the stationary condition ; for the 

 chances are infinity to one against the maintenance of a per- 

 manent equality between the births and the deaths. Hence, 

 our inquiry resolves itself into this : In races that con- 

 tinue to exist, what laws of numerical variation result from 

 these variable conflicting forces, which are respectively de- 

 structive of race and preservative of race? 



320. Clearly if the forces destructive of race, when once 

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