RESULTS OF NATURAL SELECTION. 93 



forms may be called living fossils ; they have endured to the 

 present day, from having inhabited a confined 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 favorable and unfavorable 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 favorable for the production of many 

 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 subsistence 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 competition ; 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 pro- 

 portional 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 ac*" only when there are places 

 in the natural polity of a district which can be better occu- 

 pied by the modification of some of its existing inhabitants. 

 The occurrence of such place* will often depend on physi- 

 cal changes, which generally take place ver}* slowly, and 

 on the immigration of better adapted forms being pre- 

 vented. As some few of the old inhabitants become modi- 

 fied, 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 individuals of the same specie J 



