THE CONVERGENCE OP THE EVIDENCES. 557 



In astronomic, geologic, and meteorologic changes, ever in 

 progress, ever combining in new and more involved ways, 

 we have a set of inorganic factors to which all organisms are 

 exposed; and in the varying and complicating actions of 

 organisms on one another, we have a set of organic factors 

 that alter with increasing rapidity. Thus, speaking generally, 

 all members of the Earth's Flora and Fauna experience per- 

 petual re-arrangements of external forces. 



Each organic aggregate, whether considered individually 

 or as a continuously-existing species, is modified afresh by 

 each fresh distribution of external forces. To its pre-existing 

 differentiations new differentiations are added ; and thus that 

 lapse to a more heterogeneous state, which would have a fixed 

 limit were the circumstances fixed, has its limit perpetually 

 removed by the perpetual change of the circumstances. 



These modifications upon modifications which result in 

 evolution structurally considered, are the accompaniments 

 of those functional alterations continually required to re- 

 equilibrate inner with outer actions. That moving equi- 

 librium of inner actions corresponding with outer actions, 

 which constitutes the life of an organism, must either be 

 overthrown by a change in the outer actions, or must undergo 

 perturbations that cannot end until there is a re-adjusted 

 balance of functions and correlative adaptation of structures. 



But where the external changes are either such as are 

 fatal when experienced by the individuals, or such as act on 

 the individuals in ways that do not affect the equilibrium of 

 their functions; then the re-adjustment results through the 

 effects produced on the species as a whole — there is indirect 

 equilibration. By the preservation in successive generations 

 of those whose moving equilibria are least at variance with 

 the requirements, there is produced a changed equilibrium 

 completely in harmony with the requirements. 



§ 174. Even were this the whole of the evidence assign- 

 able for the belief that organisms have been gradually evolved. 



