282 THE EVOLUTION THEORY 



colonies which will graJualh^ spread all over the isolated area as far 

 as it affords favourable conditions of life. 



But oceanic islands are not the only cases of isolated regions ; 

 mountains and mountain-ranges which rise in the midst of a plain 

 also form isolation-areas for mountain- dwelling plants or animals 

 which have not much power of migrating. In the same wa}^ marine 

 animals may be completely isolated from each other by land-barriers, 

 as the inhabitants of the Red Sea, for instance, are from those of the 

 Mediterranean, as has been clearly expounded by Darwin. The 

 idea of an isolated region is always a relative one, and the region 

 which seems absolutely insular for a terrestrial snail is not so at all 

 for a strong-flying sea-bird. There is no such thing as absolute 

 isolation of any existing colony, for otherwise the colony could never 

 have reached the region; but the degree of isolation may be absolute 

 as far as the time of our observation is concerned, if the trans- 

 portation of the species concerned occurs so rarely that we cannot 

 observe it in centuries, or perhaps in thousands of years, or if the 

 extension of its range couhl only take place through climatic or 

 geological changes, such as a subsidence of land-barriers between 

 previously separated portions of the sea, or, in the case of land 

 animals such as snails, the elevation of the sea-floor and the filling 

 up of arms of the sea which had separated two land-areas. But 

 even the transportation of a species by the accidental means already 

 indicated will occur so rarely, if tlie isolated insular region is very 

 distant, that the isolation of a colony by such a chance may be 

 regarded as almost absolute as far as the members of the same species 

 in the original habitat are concerned. 



If we examine one of these insular reo-ions with reference to the 

 animal inhabitants which live upon it in isolation, we are confronted 

 Avith the surprising fact that it harbours numerous so-called endemic. 

 species, that is to say, species which occur nowhere else upon the 

 earth, and that these species are the more numerous the further 

 the island is removed from the nearest area of related species. It 

 looks at first sight quite as if isolation alone were a direct cause of 

 the transformation of species. 



The facts which seem to point in this direction are so numerous 

 that I can only select a few of them. The Sandwich Islands, to whicli 

 we have already referred, possess eighteen endemic land-birds, and no 

 fewer than 400 endemic terrestrial snails, all belonging to the family 

 group of Achatinellina^, which occurs there alone. 



The Galapagos Islands lie i,ooc kilometres distant from the coast 

 of South America, and tho\^ too harbour twenty-one endemic species of 



