CHAPTER X 



THE EARTH'S SURFACE-CHANGES AS THE CONDITION 

 AND MOTIVE-POWER OF ORGANIC EVOLUTION 



Having now sketched in outline the main factors on which 

 organic evolution depends — heredity, variation, and rapid 

 powers of increase — and having shown by a sufficient 

 number of examples that these factors are omnipresent 

 features of organic life, only varying somewhat in the pro- 

 portions of their occurrence in different species, we are now 

 prepared to indicate the conditions under which they have 

 acted in the production of those numerous changes of form 

 and structure which we observe in the various forms of life. 



We have seen (in Chapter VI.) that so long as no con- 

 siderable changes occur in the inorganic world, the effect 

 produced by the constant interaction between species and 

 species, or between plants and animals, results in changes of 

 local distribution of the various species rather than in any 

 important modification of the species themselves. And 

 there really seems no reason why such changes should 

 occur ; because when once complete or sufficiently complete 

 adaptation to conditions is brought about, the whole of the 

 organic world will be in a state of stable equilibrium, with 

 sufficient elasticity in all its parts to become adjusted to all 

 minor periodical changes of climate, etc., by temporary 

 :hanges in numbers, and by the local distribution of the 

 slightly altered numbers. Once such an equilibrium is 

 ittained, there seems no reason why it should not be 

 permanent. Natural selection would keep up the sufficient 

 idaptation of each species, but would not tend to change 

 :hem. 



Geology proves that the inorganic environment — the 



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