84 THE ORIGIN OF SPECIES 



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 per- 

 mits, the circumstances favorable and unfavorable for the produc- 

 tion of new species through natural selection. I conclude that for 

 terrestrial productions a large continental area, which has under- 

 gone many oscillations of level, will have been the most favorable 

 for the production of many new forms of life, fitted to endure for 

 a long time and to spread widely. While the area existed as a con- 

 tinent, the inhabitants will have been numerous in individuals and 

 kinds, and will have been subjected to severe competition. When 

 converted by subsistence 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, immigra- 

 tion 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 

 favored 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 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 the modifica- 

 tion of some of its existing inhabitants. The occurrence of such 

 places will often depend on physical changes, which generally take 

 place very slowly, and on the immigration of better adapted forms 

 being prevented. As some few of the old inhabitants become modi- 

 fied, the mutual relations of others will often be disturbed; and 

 this will create new places, ready to be filled up by better adapted 



