528 THE EVOLUTION OF LIFE. 



lengths. There is not the slightest evidence of this. The 

 metatarsals of a bird have to bear no appreciable strains but 

 those due to the superincumbent weight. Standing in the 

 water does not appreciably alter such strains; and even if 

 it did, an increase in the lengths of these bones would not 

 fit them any better to meet the altered strains. 



§ 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 inci- 

 dent 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 struc- 

 tures fitted to the modified conditions. But, at the same 

 time, we see that the modified conditions to which organ- 

 isms may be adapted by direct equilibration, are conditions 

 of certain classes only. That a new external action may be 

 met by a new internal action, it is needful that it shall either 

 continuously or frequently be borne by the individuals of 

 the species, without killing or seriously injuring them; and 

 shall act in such way as to affect their functions. And we 

 find that many of the environing agencies — evil or good — ^to 

 which organisms have to be adjusted, are not of these kinds: 

 being agencies 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 acting on them; and as their environments 

 change, the changes which plants and animals undergo must 

 necessarily be changes towards re-establishment of the 

 balance. Besides direct equilibration, there must therefore 

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

 to inquire. 



