Chap. IV. TO NATURAL SELECTION. 107 



ruination. Hence, perhaps, it comes that the flora of 

 Madeira, according to Oswald Heer, resembles 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 ; 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 exterminated. 

 And it is in fresh water 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 now widely separated in the natural scale. 

 These anomalous forms may almost be called living 

 fossils; they have endured to the present day, from 

 having inhabited a confined area, and from having thus 

 been exposed to less severe competition. 



To sum up the circumstances favourable and un- 

 favourable to natural selection, as far as the extreme 

 intricacy of the subject permits. I conclude, looking 

 to the future, that for terrestrial productions a large 

 continental area, which will probably undergo many 

 oscillations of level, and which consequently will exist 

 for long periods in a broken condition, will be the most 

 favourable for the production of many new forms of life, 

 likely to endure long and to spread widely. For the area 

 will first have existed as a continent, and the inhabitants, 

 at this period numerous in individuals and kinds, will 

 have been subjected to very severe competition. When 

 converted by subsidence into large separate islands, 

 there will still exist many individuals of the same 

 species on each island : intercrossing on the confines 

 of the range of each species will thus be checked : after 

 physical changes of any kind, immigration will be pre- 



