TWO OBJECTIONS CONSIDERED 

 )Ochs of elevation. For since the only varia- 



I 



^Kons preserved by natural selection are those 

 which bring the organism into closer adaptation 

 to its environment ; and since in most cases 

 the organic environment of any group of organ- 

 isms, comprising its enemies, competitors, and 

 prey, is a much more important factor of change 

 than its inorganic environment, comprising cli- 

 mate and soil ; it follows that those periods dur- 

 ing which groups of organisms, hitherto isolated, 

 are gradually brought into contact with one an- 

 other must be the periods most favourable for 

 specific change. The most rapid variation, at- 

 tended by the greatest frequency of transitional 

 forms, will therefore occur during those epochs 

 of elevation when archipelagoes are being con- 

 verted into continents, and when shallow parts 

 of the sea, hitherto divided by deep channels, 

 are getting practically united together by the 

 diminishing depth of the channel. During such 

 periods it is not only the inorganic agencies of 

 climate and soil which will be altered ; the or- 

 ganic environment of each group of organisms 

 will be immensely increased in extent and het- 

 erogeneity. The struggle for existence will in- 

 crease in violence, and there will be an increased 

 amount both of variation and of extinction. 



We are thus driven to the remarkable con- 

 clusion, not only that each system of fossilifer- 

 ous strata now remaining has been preceded 

 57 



