Chap. IV.] RESULTS OF NATURAL SELECTION. 131 



subject permits, the circumstances favourable and un- 

 favourable for the production of new species through 

 natural selection. I conclude that for terrestrial produc- 

 tions a large continental area, which has undergone many 

 oscillations of level, will have been the most favourable 

 for the production of many new forms of life, fitted to 

 endure for a long time and to spread widely. Whilst 

 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 con- 

 verted 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 prevented, so that new places in the polity of 

 each island will have had to be filled up by the modi- 

 fication of the old inhabitants ; and time will have been 

 allowed for the varieties in each to become well modified 

 and perfected. When, by renewed elevation, the islands 

 were reconverted into a continental area, there will 

 again have been very severe competition : the most 

 favoured or improved varieties will have been enabled 

 to spread : there will have been much extinction of the 

 less improved forms, and the relative porportional num- 

 bers of the various 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 

 farther 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 existing 



