132 EXTINCTION BY NATUEAL SELECTION. [Chap. IV. 



inhabitants. The occurrence of such places will often 

 depend on physical changes, which generally take place 

 very slowly, and on the immigration of better adapted 

 forms being prevented. As some few of the old inhabi- 

 tants 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 organisation might 

 occur. The result would often be greatly retarded by 

 free intercrossing. Many will exclaim that these several 

 causes are amply sufficient to neutralise the power of 

 natural selection. I do not believe 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, intermittent results accord well with what 

 geology tells us of the rate and 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 com- 

 plexity of the coadaptations between all organic beings, 

 one with another and with their physical conditions of 

 life, which may have been affected in the long course of 

 time through nature's power of selection, that is by the 

 survival of the fittest. 



Extinction caused by Natural Selection. 



This subject will be more fully discussed in our 

 chapter on Geology ; but it must here be alluded to from 



