PRODUCTION OF NEW FORMS U9 



living fossils ; they have endured to the present day, from 

 having inhabited a confined area, and from having been ex- 

 posed to less varied, and therefore less severe, competition. 



To sum up, as far as the extreme intricacy of the subject 

 permits, the circumstances favourable and unfavourable for 

 the production of new species through natural selection. I 

 conclude that for terrestrial productions 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 in- 

 habitants 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 prevented, 

 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 be- 

 come well modified and perfected. When, by renewed eleva- 

 tion, 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 im- 

 proved forms, and the relative proportional numbers 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 further the inhabitants, and 

 thus to produce new species. 



That natural selection generally acts with extreme slow- 

 ness 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 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 old inhabitants become modified, the mutual 

 relations of others will often be disturbed; and this will 



