516 LAND ANIMALS 



in a series, from islands whose faunae are scarcely different from those 

 of the adjacent continent to groups in which insular characteristics are 

 especially pronounced. These two extremes are recognized in the group- 

 ing of islands as continental, i.e., islands which have been connected 

 with the mainland, and oceanic, those which have never had a land 

 connection. 1 Continental islands are for the most part near the main- 

 land, and resemble the mainland in geologic structure, in being com- 

 posed of sedimentary deposits, while the sea which separates them 

 from the continent is usually shallow, rarely deeper than 1800 m. and 

 usually much shallower. Oceanic islands are of volcanic or secondarily 

 of coralline origin, are often far from the nearest continental mainland, 

 and are usually separated from it by greater depths. Both continental 

 and oceanic islands may be classified according to their age. 



The different mode of origin of continental and oceanic islands 

 naturally has an important effect on their faunal character. A conti- 

 nental island first received its fauna from the mainland and, in general, 

 if not too small in area, it will consist of the same groups as that of 

 the mainland. On ancient continental islands, some animal groups may 

 have become extinct; or forms which have become extinct on the 

 mainland on account of competition with more modern species may be 

 preserved in the insular habitat; or forms not represented on the main- 

 land may have entered from other sources. An oceanic island, however, 

 must have originally been without air-breathing animals, and its land 

 fauna must be composed of animals which have been able to cross the 

 ocean, either by active flight or swimming, or by some means of passive 

 flotation (see p. 69) . The capacity for such dispersal, however, varies 

 for the different groups of animals. Animals which are incapable of 

 flight or other aerial transport, and to which sea water is fatal at all 

 stages of development, are excluded from oceanic islands. Land turbel- 

 larians, many fresh-water forms, and amphibians tend to be so ex- 

 cluded. Land mammals do not appear to be able to survive long 

 journeys on driftwood, and are also characteristically wanting on 

 oceanic islands. If introduced they may flourish there, like the rabbits 

 of Kerguelen or the cattle of New Amsterdam, but they rarely reach 

 the islands without the intervention of man. The animals which have 

 thus accumulated on oceanic islands have reached these havens acci- 

 dentally, and neighboring islands, on account of the rarity of such 

 transportation, may have radically different animal life. Oceanic island 

 faunae appear to be accidental assemblages, with some animal groups 

 conspicuously absent, and a haphazard composition as compared with 

 the more ordered faunae of continental islands. 



