108 NATURAL SELECTION. Chap. IV. 



vented, so that new places in the polity of each island 

 will have to be filled up by modifications of the old in- 

 habitants ; 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 competi- 

 tion : 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 con- 

 tinent will again be changed ; and again there will be 

 a fair field for natural selection to improve 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 de- 

 pend on some of the inhabitants becoming slowly modi- 

 fied ; the mutual relations of many of the other inha- 

 bitants being thus disturbed. Nothing can be effected, 

 unless favourable variations occur, and variation itself is 

 apparently always a very slow process. The process will 

 often be greatly retarded by free intercrossing. Many 

 will exclaim that these several causes are amply suffi- 

 cient wholly to stop the action of natural selection. I do 

 not believe so. On the other hand, I do believe that 

 natural selection will always act very slowly, often only 

 at long intervals of time, and generally on only a very 

 few of the inhabitants of the same region at the same 

 time. I further believe, that this very slow, intermit- 



