78 CIRCUS TANCES FAVOURABLE TO THE [CHAP. IV. 



and less extermination. Hence, we can understand how it is that 

 the flora of Madeira, according to Oswald Heer, resembles to a 

 certain extent the extinct tertiary flora of Europe. All fresh- 

 water basins, taken together, make a small area compared with 

 that of the sea or of the land. Consequently, the competition 

 between fresh-water productions will have been less severe than 

 elsewhere ; new forms will have been then more slowly produced, 

 and old forms more slowly exterminated. And it is in fresh- 

 water basins that we find seven genera of Ganoid fishes, remnants 

 of a once preponderant order : and in fresh water we find some 

 of the most anomalous forms now known in the world as the 

 Ornithorhynchus and Lepidosiren, which, like fossils, connect to 

 a certain extent orders at present widely sundered in the natural 

 scale. These anomalous 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 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 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 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 

 porportional 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 th? 



