282 THE EVOLUTION TMEOKY 



colonics wliieh will grailually spreail all ovor tho isolated aiva as I'av 

 as it aftbi'ds favonrable coiulitions oi' life. 



But oceanic islands aro not the only cases ol' isolated reo;it)ns ; 

 mountains and mountain-raTig-os which risi' in the midst ol' a plain 

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

 which have not much power of migratino-. In the same way marine 

 animals may be completely isolate<l from eneli other by land-baj-riers, 

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

 Mediteri-anean, as has lu'en clearly expounded by Darwin. Tlu^ 

 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 ahs<i/iilc 

 isolation of any existing colony, f(n' otlierwise the colony could ni'xcr 

 haA-e i-eached the region; but the degree of isolation may be ahtsohUe 

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

 portation of the species concerned occurs so raanily that we cannot 

 observe it in centxiries, or perhaps in thousands of years, or if tlu^ 

 extension of its range could 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 tlie sea-floor and the tilling 

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

 even the transportation of a species by tho accidental means alrea,dy 

 indicated will occur so rarely, if the 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 )nenibers ol' the same s])i'cies 

 in the original habitat are concerned. 



If we examine one of those insular regions with reference to the 

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

 with the surprising fact that it harbours numei'ous so-called endemic 

 species, that is to say, species which occur nowhere (^Ise upon tho 

 earth, and that these species are the more numerous the furthei' 

 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 ol' 

 tho transformation of species. 



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

 that I can only select a few of tliem. The Sandwich Islands, to which 

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

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

 group of Achatinellinas, which occurs there aJone. 



TJie (Jalapagos Islands lie 1,000 kilometres distant from the coast 

 of South Ame}'ica, and they too harbour twenty-one endemic species of 



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