THE CONVERGENCE OF THE EVIDENCES. 473 



173. But the conclusion deductively reached, is in har- 

 mony with the inductive conclusion. Passing from the evi- 

 dence that evolution has taken place, to the question How 

 has it taken place ? we find in known agencies and known 

 processes, adequate causes of its phenomena. 



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 gener- 

 ally, all members of the Earth's Flora and Fauna are con- 

 tinually passing into new environments 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-exist- 

 ing differentiations, new differentiations are added ; and thus 

 that lapse from a more homogeneous 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. Meanwhile, that growing com- 

 plexity of structure thus produced, must, in the average of 

 cases, be accompanied by an increasing definiteness of struc- 

 ture ; since only those organisms can survive which subject 

 themselves to aggregates of forces that are not, in their essen- 

 tials, greatly unlike those with which their structures cor- 

 respond. And at the same time that progression is thus 

 necessitated as a general result ; yet, as change of structure 

 arises only where there is change in the distribution of forces, 

 it will not take place in organisms which elude changes 

 in the distribution of forces, by migration or otherwise. 



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- 



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