Origin of Species Variorum

To sum up the circumstances favourable and unfavourable to natural selection, as far as the extreme intricacy of the subject permits. I conclude ... that for terrestrial productions a large continental area, which has undergone many oscillations of level, and which consequently has existed for long periods in a broken condition, has been the most favourable for the production of many new forms of life, fitted to endure long and to spread widely. For the area first existed as a continent, and the inhabitants, at this period numerous in individuals and kinds, 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 species will thus 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 modifications 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 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 proportional numbers of the various inhabitants of the renewed 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 produce new species.

That natural selection always acts with extreme slowness I fully admit. Its action depends on there being places in the polity of nature, which can be better filled through some of the inhabitants of the country under- going