GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



antiquity. It is customary to group islands into two classes, 

 suggested by their mode of formation — the continental and 

 the oceanic. Continental islands are detached portions of 

 continents, from which most of them are separated by shal- 

 low seas. They have the geological structure of the con- 

 tinents and are composed of the familiar stratified rocks, such 

 as sandstone, slate, and limestone, and some contain granite- 

 like igneous rocks and volcanic material. The faunas of 

 these continental islands are determined by many factors, of 

 which we may disregard several, such as area and climate, 

 and consider only the distance of the island from the main- 

 land and the geological date of its separation. 



Great Britain, Ireland, and the islands of the East Indies 

 or Malay Archipelago are continental islands that lie near the 

 parent continents, are surrounded by comparatively shoal 

 water, and although they were detached from the mainland 

 at different times, they were yet, on the whole, of relatively 

 recent origin as islands. Great Britain is, 2o51ogically speak- 

 ing, indistinguishable from an equal area of the European 

 continent; the species are so generally identical that the 

 islands must have been separated from the continent during 

 the present geological epoch, the Recent. The great Asiatic 

 islands, Borneo, Sumatra, Java, etc., were detached somewhat 

 earlier and contain more peculiar and more characteristic 

 species, but the difference from those of Asia is not great. 



Oceanic islands lie far from any continent and rise 

 abruptly, with steep submarine slopes, from profound depths 

 of the sea. Most oceanic islands are composed of volcanic 

 rocks and coral reefs. Nearly all of them seem to have been 

 submarine volcanoes, which have built up their cones from 

 the sea-floor; and the cones that lie in warm seas are gen- 

 erally capped with coral reefs, which may or may not bury 

 the volcanic pedestal out of sight. The rocks that form the 



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