ORGANIC EVOLUTION. 



continuously makes the various species of plants 

 and animals better and better suited to the 

 external conditions of their life. 



Now, if this process of continuously adapting 

 organisms to their environment takes place in 

 nature at all, there is no reason why we should 

 set any limits on the extent to which it is able 

 to go up to the point at which a complete and 

 perfect adaptation is achieved. Therefore we 

 might suppose that all species would attain to 

 this condition of perfect adjustment to their en- 

 vironment, and there remain fixed. And so 

 undoubtedly they would, if the environment were 

 itself unchanging. But forasmuch as the environ- 

 ment — or the sum total of the external condi- 

 tions of life — of almost every organic type alters 

 more or less from century to century (whether 

 from astronomical, geological, and geographical 

 changes, or from the immigrations and emi- 

 grations of other species living on contiguous 



