INDIRECT EQUIXIBKATION. 461 



endowed individuals will be continually killed off; and 

 eventually none but the highly-endowed individuals wiU be 

 produced — a new moving equilibrium, adapted to the new 

 environing conditions, wiU result. 



It may be objected that this mode of expressing the facts, 

 does not include the numerous cases in which a species be- 

 comes modified in relation to surrounding agencies that 

 do not actively influence it — cases like that of the 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 afiect 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 inter 

 pretation 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 oi 

 its members, there are others which facilitate the maiutenance 

 of their moving equilibria, and some which are capable of 

 giving their moving equilibria increased stability : instance 

 the spread into their habitat of some new kind of prey, which 

 is abundant at seasons when other prey is scarce. Now what 

 is the process by which the moving equilibrium in any 

 species, becomes adapted to some additional external factor 

 which furthers its maintenance ? Instead of an increased 

 resistance to be met and counter-balanced, there is here a 

 diminished resistance ; and the diminished resistance is 

 equiKbrated in the same way as the increased resistance. As, 

 in the one case, there is a more frequent survival of those in- 

 dividuals whose peculiarities of constitution enable them best 

 to resist the new adverse factor ; so, in the other case, there 

 is a more frequent survival of individuals whose peculiarities 

 of constitution enable them to take advantage of the new 

 favourable factor. In each member of the species, the balance 

 of functions and correlated arrangement of structures, difier 

 slightly from those existing in other members. To say that 



