CHAPTER X 



THE EAETIl's SURFACE-CHANGES AS THE CONDITION AND 

 MOTIVE-POWER OE 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 nuni- 

 ber of examples that these factors are omnipresent features of 

 organic life, only varying somewhat in the proportions of their 

 occurrence in different species, we are now prepared to indi- 

 cate the conditions under which they have acted in the produc- 

 tion 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 consid- 

 erable changes occur in the inorganic world, the effect pro- 

 duced by the constant interaction between species and species, 

 or between plants and animals, results in changes of local dis- 

 tribution 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 bo 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 changes in numbers, and by the 

 local distribution of the sliffhtlv altered numbers. Once such 

 an efpiilibrium is attained, there seems no reason why it should 

 not be permanent. Xatural selection would keep up the suffi- 

 cient adaptation of each species, but would not tend to change 

 them. 



Geology proves that the inorganic environment — the 



187 



