98 ON THE ORIGIN OF SPECIES 



converted by subsidence into large separate islands, 

 there will still exist many individuals of the same 

 species on each island : intercrossing on the confines 

 of the range of each species will thus be checked : 

 after physical changes of any kind, immigration will 

 be prevented, so that new places in the polity of each 

 island will have to be filled up by modifications of the 

 old inhabitants ; and time will be allowed for the 

 varieties in each to become well modified and perfected. 

 When, by renewed elevation, the islands shall be re- 

 converted into a continental area, there will again be 

 severe competition : the most favoured or improved 

 varieties will be enabled to spread : there will be much 

 extinction of the less improved forms, and the relative 

 proportional numbers of the various inhabitants of the 

 renewed continent will again be changed ; and again 

 there will be a fair field for natural selection to im- 

 prove still further the inhabitants, and thus produce 

 new species. 



That natural selection will always act with extreme 

 slowness, I fully admit. Its action depends on there 

 being places in the polity of nature, which can be 

 better occupied by some of the inhabitants of the country 

 undergoing modification of some kind. The existence 

 of such places will often depend on physical changes, 

 which are generally very slow, and on the immigration 

 of better adapted forms having been checked. But the 

 action of natural selection will probably still oftener 

 depend on some of the inhabitants becoming slowly 

 modified ; the mutual relations of many of the other 

 inhabitants being thus disturbed. Nothing can be 

 eifected, unless favourable variations occur, and varia- 

 tion itself is apparently always a very slow process. 

 The process will often be greatly retarded by free inter- 

 crossing. Many will exclaim that these several causes 

 are amply sufficient wholly to stop the action of 

 natural selection. I do not believe so. On the other 

 hand, I do believe that natural selection always acts 

 very slowly, often only at long intervals of time, and 

 generally on only a very few of the inhabitants of the 



