Chap. IV. KESULTS OF NATURAL SELECTION. iQ>j 



termination. Ilcncc, perhaps, it is liiat the flora of Madeira, 

 accordin<^ to Oswald Heer, resembles the extinct tertiary flora 

 of Europe. ^Vll fresh-water basins, taken together, make a 

 small area compared with that of the sea or of the land ; and, 

 consequently, the competition between fresh-water productions 

 will have been less severe than elsewhere ; new forms will have 

 been more slowly formed, and old forms more slowly extermi- 

 nated. And it is in fresh water that Ave find seven genera of 

 Ganoid fishes, remnants of a once preponderant order : and in 

 ii?sh 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 separated 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 having been 

 exposed to less varied and therefore less severe competition. 



To sum up the circumstances favorable and unfavorable for 

 the production of new species through natural selection, as far 

 as the extreme intricacy of the svibject permits. I conclude 

 that fur terrestrial productions a large continental area, which 

 has imdergone many oscillations of level, will have been the 

 most favorable for the production of many ncAV 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 Avill have been subjected 

 to severe competition. When converted by subsidence into 

 large separate islands, there will still have existed many indi- 

 viduals 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 conti- 

 nental area, there will again have been severe competition : 

 the most faxorcd or improved varieties will have been enabled 

 to spread : there will have been much extinction of the less 

 improved forms, and the relative proportional ninnbcrs of the 

 various inhabitants of tin; reunited continent will again have 

 been changed ; and, again, there will have been a fair lield for 

 natural selection to improve still further the inhabitants, and 

 thus to produce new species. 



