552 THE EVOLUTION OP LIFE. 



only causes of the successive modifications; and these 

 changes have continued to be causes. But as, through the 

 diffusion of organisms and consequent differential actions of 

 inorganic forces, there arose unlikenesses among them, pro- 

 ducing varieties, species, genera, orders, classes, the actions 

 of organisms on one another became new sources of organic 

 modifications. And as fast as types have multiplied and 

 become more complex, so fast have the mutual actions of 

 organisms come to be more influential factors in their re- 

 spective evolutions : eventually becoming the chief factors. 



Passing from the external causes of change to the internal 

 processes of change entailed by them, we see that these, too, 

 have varied in their proportions: that which was originally 

 the most important and almost the sole process, becoming 

 gradually less important, if not at last the least important. 

 Always there must have been, and always there must con- 

 tinue to be, a survival of the fittest; natural selection must 

 have been in operation at the outset, and can never cease to 

 operate. While yet organisms had small abilities to co- 

 ordinate their actions, and adjust them to environing actions, 

 natural selection worked almost alone in moulding and re- 

 moulding organisms into fitness for their changing environ- 

 ments; and natural selection has remained almost the sole 

 agency by which plants and inferior orders of animals 

 have been modified and developed. The equilibration of 

 organisms that are almost passive, is necessarily effected in- 

 directly, by the action of incident forces on the species as a 

 whole. But along with the evolution of organisms having 

 some activity, there grows up a kind of equilibration which 

 is in part direct. In proportion as the activity increases 

 direct equilibration plays a more important part. Until, 

 when the nervo-muscular apparatus becomes greatly deve- 

 loped, and the power of varying the actions to fit the varying 

 requirements becomes considerable, the share taken by direct 

 equilibration rises into co-ordinate importance or greater 

 importance. As fast as essential faculties multiply, and as 



