NATURAL SELECTION 1501 



gone many oscillations of level, will have been the 

 most favorable for the production of any new forms 

 of life, fitted to endure for a long time and to spread 

 widely. While the area existed 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 isl- 

 and : intercrossing on the confines of the range of 

 each new species will have been checked : after physi- 

 cal changes of any kind, immigration will have been 

 prevented, so that new places in the polity of each 

 island will have had to be filled up by the modifica- 

 tion of the old inhabitants; and time will have been 

 allowed for the varieties in each to become well 

 modified and perfected. When, by renewed eleva- 

 tion, the islands were reconverted into a continental 

 area, there will again have been very severe compe- 

 tition : the most favored or improved 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 various inhabi- 

 tants 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 in- 

 habitants, 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 

 existing inhabitants. The occurrence of such places 



