436 THE EVOLUTION OF LIFE. 



constituting a species, there were a continuously- existing 

 individual, any such functional and structural divergence as 

 we see produced by a new incident force, would necessarily 

 go on increasing until the new incident force was counter- 

 poised; and that the replacing of a continuously- existing 

 individual by a succession of individuals, each formed out of 

 the modified substance of its predecessor, will not prevent the 

 like effect from being produced — -the persistence of force 

 negativing any other inference. Here we further find, that 

 this limit towards which any such organic change advances, 

 in the species as in the individual, is a new moving equi- 

 librium adjusted to the new arrangement of external forces. 



But now, what are the conditions under which alone, direct 

 equilibration can occur ? Are all the modifications that serve 

 to re-fit organisms to their environments, directly adaptive 

 modifications? And if otherwise, which are the directly 

 adaptive and which are not ? How are we to distinguish 

 between them ? . . 



Manifestly, for any moving equilibrium to be gradually 

 altered, it is needful, first, that some force shall operate upon 

 it ; and, second, that the force shall not be such as to over- 

 throw it. If in the environment there exists some agency 

 that would act advantageously on an organism were the or- 

 ganism a little modified, but which does not act on it in the 

 absence of the required modification ; it is clear that this 

 agency cannot itself tend to produce the modification. On 

 the other hand, if the external agency be of such kind, that 

 individuals of the species whenever affected by it, are either 

 killed or so injured that the production of vigorous offspring 

 is much interfered with, there cannot be directly wrought in 

 the species, any such alteration as will fit it to cope with 

 this external agency. The only new incident forces which 

 can work the changes of function and structure required to 

 bring any animal or plant into equilibrium with them, are 

 such incident forces as operate on this animal or plant, 

 either continuously or frequently. They must be capable 



