132 EXTINCTION BY NATURAL SELECTION. [Chap. IV. 



ing 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 

 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 gome slight de- 

 gree from each other, it would often be long before dif- 

 ferences of the right nature in various parts of the or- 

 ganisation might occur. The result would often he 

 greatly retarded by free intercrossing. Many will ex- 

 claim that these several causes are amply sufficient to 

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

 lieve so. But I do believe that natural selection will gen- 

 erally 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 ac- 

 cord well with what geology tells us of the rate and man- 

 ner 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 effected 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 



