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 be- 

 come 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 

 cases in which the destructive forces, remaining permanently 

 in excess of the preservative forces, cause extinction. Prac- 

 tically, too, we may exclude the stationary condition of a 

 species ; for the chances are infinity to one against the main- 

 tenance of a permanent equality between the births and the 

 deaths. Hence, our inquiry resolves itself into this: In 

 races that continue to exist, what laws of numerical variation 

 result from these variable conflicting forces, that are respec- 

 tively destructive of race and preservative of race ? 



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



