iG6 THE EVOLUTION OF I.IJfB. 



balance of functions has not been oytrthrown. Inevitably, 

 therefore, survival through successive changes of conditions, 

 implies successive adjustments of the balance to the new con- 

 ditions. This deduction we find to be inductively verified, 

 What is ordinarily called adaptation, is, when translated into 

 mechanical terms, direct equilibration. And that procesB 

 which, under the name of natural selection, Mr Darwin has 

 shown to be an ever-acting means of fitting the structures of 

 organisms to their circumstances, we find, on analysis, to bo 

 expressible in mechanical terms as indirect equilibration. 



The actions that are here specified in succession, are in 

 reality simultaneous ; and they must be so conceived before 

 organic evolution can be rightly understood. Some aid 

 towards so conceiving them, will be given by the annexed 

 tabic, representing the co-operation of the factors. 



§ 170. Respecting this co-operation of these factors, it re- 

 mains only to point out their respective shares in prodiicing 

 the total result ; and the way in which the proportions of 

 their respective shares vary as evolution progresses. 



At first, changes in the amounts and combinations of ex- 

 ternal inorganic forces, astronomic, geologic, and meteoro- 

 logic, were the only causes of the successive modifications 

 undergone by organisms ; and these changes have continued, 

 and must still continue, to be causes of such modifications. 

 As, however, through the diffusion of organisms, and the 

 consequent differential actions of inorganic forces on them, 

 there arose unlikenesses among organisms, producing varieties, 

 species, genera, orders, classes, &c. ; the actions of organisms 

 on one another became new sources of organic modifications. 

 And as fast as types have multiplied, and become more com- 

 plex ; so fast have the mutual actions of organisms come to 

 be more influential factors in their respective evolutions. 

 Until, eventually, as w^e see exemplified in the human race, 

 (hey have come to be the chief factors. 



Passing from the external causes of change to the internal 



