A PRIORI PRINCIPLE. 419 



ation. Let us consider how, in the case before us, such a 

 stable equilibrium must be constituted. 



321. When a season favourable to it, or a diminution of 

 creatures detrimental to it, causes any species to become 

 more numerous than usual, an immediate increase of certain 

 destructive influences takes place. If it be a plant, the sup 

 posed greater abundance itself implies fuller occupation of 

 the places available for growth an occupation which, leav 

 ing fewer such places as the multiplication goes on, becomes 

 a check on further multiplication itself causes a greater 

 mortality of seeds that fail to root themselves. And after 

 wards, in addition to this passive resistance to continued 

 increase, there comes an active resistance : the creatures which 

 thrive at the expense of the species the larvae, the birds, the 

 herbivores increase too. If it be an animal that has grown 

 more numerous, then, unless by some exceptional coincidence 

 a simultaneous and proportionate addition to the animals or 

 plants serving for food has occurred, there must result a 

 relative scarcity of food. Enemies, too, be they beasts of 

 prey or be they parasites, must quickly begin to multiply. 

 Hence, each kind of organism, previously existing in some 

 thing like its normal number, cannot have its number raised 

 without a rise of the destructive forces, negative and positive, 

 quickly commencing. Both negative and posi 



tive destructive forces must augment until this increase of 

 the species is arrested. The competition for places on which 

 to grow, if the species be vegetal, or for food if it be animal, 

 must become more intense as the over-peopling of the habitat 

 progresses ; until there is reached the limit at which the 

 mortality equals the reproduction. And as, at the same 

 time, enemies will multiply with a rapidity which soon 

 brings them abreast of the augmented supply of prey, the 

 positive restraint they exert will help to bring about an 

 earlie^ arrest of the expansion than pressure of population 

 alone would cause. One more inference mav be 



