Origin of Species Variorum

To sum up the circumstances favourable and unfavourable for the production of new species through 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, 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 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 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: