442 THE EVOLUTION OF LTFE. 



§ 163. The conclusion at which we arrive is, then, that 

 there go on in all organisms, certain changes of function and 

 structure that are directly consequent on changes in the 

 incident forces — inner changes by which the outer changes 

 are balanced, and the equilibrium restored. Such re- equi- 

 librations, which are often conspicuously exhibited in in- 

 dividuals, we have reason to believe continue in successive 

 generations ; until they are completed by the arrival at 

 structures fitted to the modified conditions. But, at the 

 same time, we see that the modified conditions to which or- 

 ganisms may be adapted by direct equilibration, are con- 

 ditions of certain classes only. That a new external actior. 

 may be met by a new internal action, it is needful that i'i 

 shall either continuously or frequently be borne by the in- 

 dividuals of the species, without killing or seriously injuring 

 them; and shall act in such way as to affect their functions. 

 And we find on examination, that many of the environing 

 changes to which organisms have to be adjusted, are not of 

 these kinds : being changes which either do not immediately 

 affect the functions at all, or else affect them in ways that 

 prove fatal. 



Hence there must be at work some other process, which 

 equilibrates the actions of organisms with the actions they 

 are exposed to. Plants and animals that continue to exist, 

 are necessarily plants and animals whose powers balance the 

 powers that act on them ; and as their environments 

 change, the changes which plants and animals undergo, must 

 necessarily be changes towards a re-establishment of the 

 balance. Besides direct equilibration, there must therefore 

 be an indirect equilibration. How this goes on we have now 

 to inquire. 



