1502 THE STORY OF THE UNIVERSE 



will often depend on physical changes, which gen- 

 erally take place very slowly, and on the immigra- 

 tion of better adapted forms being prevented. As 

 some few of the old inhabitants become modified, the 

 mutual relations of others will often be disturbed; 

 and this will create new places, ready to be filled 

 up by better adapted forms; but all this will take 

 place very slowly. Although all the individuals of 

 the same species differ in some slight degree from 

 each other, it would often be long before differences 

 of the right nature in various parts of the organiza- 

 tion might occur. The result would often be greatly 

 retarded by free intercrossing. Many will exclaim 

 that these several causes are amply sufficient to neu- 

 tralize the power of natural selection. I do not be- 

 lieve so. But I do believe that natural selection will 

 generally act very slowly, only at long intervals of 

 time, and only on a few of the inhabitants of the same 

 region. I further believe that these slow, intermit- 

 tent results accord well with what geology tells us 

 of the rate nd manner at which the inhabitants of 

 the world have changed. 



Slow though the process of selection may be, if 

 feeble man can do much by artificial selection, I can 

 see no limit to the amount of change, to the beauty 

 and complexity of the coadaptations between all or- 

 ganic beings, one with another and with their physi- 

 cal conditions of life, which may have been effected 

 in the long course of time through nature's power of 

 selection, that is, by the survival of the fittest. 



Natural selection acts solely through the preser- 

 vation of variations in some way advantageous, which 



