546 THE EVOLUTION OF LIFE. 



great that it leaves alive only the few individuals possessing 

 extreme endowments of the power required to antagonize it; 

 then, in succeeding generations, there is a rapid multiplica- 

 tion of individuals similarly possessing extreme endowments 

 of this power — the force impressed calls out an equivalent 

 conflicting force. Moreover, the change is temporary where 

 the cause is temporary, and permanent where the cause is 

 permanent. All that are deficient in the needful attribute 

 having been killed off, and the survivors having the needful 

 attribute in a comparatively high degree, there will descend 

 from them, not only some possessing equal amounts of this 

 attribute with themselves, but also some possessing less 

 amounts of it. If the destructive agency has not continued 

 in action, such less-endowed individuals will multiply; and 

 the species, after sundry oscillations, will return to its pre- 

 vious mean state. But if this agency be a persistent one, 

 such less endowed individuals will be continually killed off, 

 and eventually none but highly-endowed individuals will be 

 produced — a new moving equilibrium, adapted to the new 

 environing conditions, will result. 



It may be objected that this mode of expressing the facts 

 does not include the cases in which a species becomes modified 

 in relation to surrounding agencies of a passive kind — cases 

 like that of a plant which acquires hooked seed-vessels, by 

 which it lays hold of the skins of passing animals, and makes 

 them the distributors of its seeds — cases in which the outer 

 agency has no direct tendency at first to affect the species, 

 but in which the species so alters itself as to take advantage 

 of the outer agency. To cases of this kind, however, the 

 same mode of interpretation applies on simply changing the 

 terms. While, in the aggregate of influences amid which a 

 species exists, there are some which tend to overthrow the 

 moving equilibria of its members, there are others which 

 facilitate the maintenance of their moving equilibria, and 

 some which are capable of giving their moving equilibria 

 increased stability: instance the spread into their habitat of 



