NATURAL SELECTION 155 



production of any new forms of life, fitted to endure for 

 a long time and to spread widely. While the area ex- ; 

 isted as a continent, the inhabitants will have been \ 

 numerous in individuals and kinds, and will have been 

 subjected to severe competition. When converted by 

 subsidence into large separate islands, there will still 

 have existed many individuals of the same species on , 

 each island: intercrossing on the confines of the range of { 

 each new species will have been checked: after physical ' 

 changes of any kind, immigration will have been pre- 

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

 will have had to be filled up by the ,modification of the 

 old inhabitants; and time will have been allowed for 

 the varieties in each to become well modified and per- 

 fected. When, by renewed elevation, the islands were re- 

 converted into a continental area, there will again have 

 been very severe competition: the most favored or im- 

 proved varieties will have been enabled to spread: there 

 will have been much extinction of the less improved 

 forms, and the relative proportional numbers of the vari- 

 ous inhabitants of the reunited continent will again have 

 been changed; and again there will have been a fair 

 field for natural selection to improve still further the 

 inhabitants, and thus to produce new species. > 



That natural selection generally acts with extreme 

 slowness I fully admit. It can act only when there 

 are places in the natural polity of a district which can 

 be better occupied by the modification of some of its ex- 

 isting 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^ 



