Seite - 119 - in The Origin of Species

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