84 CIRCUMSTANCES FAVOURABLE TO THE [Chap. IV. 



area, and from having been exposed 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 pro- 

 duction 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 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 become 

 well modified and perfected. When, by renewed elevation, the 

 islands were reconverted into a continental area, there will again 

 have been very severe compevition : 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 propor- 

 tional 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 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 will 

 often depend on physical changes, which generally take place very 

 slowly, and on the immigration of better adapted forms being pre- 

 vented. As some few of the old inhabitants become modified, 

 the mutual relations of others will often be disturbed ; and this 

 will create new places, ready to be filled up by better adapted forms; 

 but all this will take place very slowly. Although all the indi- 

 viduals of the same species differ in some slight degree from each 

 other, it would often be long before differences of the right nature 

 in various parts of the organisation might occur. The result would 

 often be greatly retarded by free intercrossing. Many will exclaim 

 that these several causes are amply sufficient to neutralise the power 

 of natural selection. I do not believe so. But I do believe that 



