464 COMPARATIVE ZOOLOGY 



four hundred species, and are all confined to the Western 

 Hemisphere. The explanation of this fact is that they 

 originated in this part of the world, and are too small 

 and weak to make the long flight necessary to reach 

 other regions. 



Islands are usually populated by forms brought from 

 the nearest mainland, unless the ocean currents are such 

 as to bring animals from places more remote. Such 

 islands as are separated from a neighboring continent 

 by deep channels have a fauna more archaic and primi- 

 tive than that found on the mainland. Australia and New 

 Zealand are thought to have been separated from the 

 nearest larger bodies of land for long geological periods, 

 and possibly since the time of their formation. Their 

 faunas are of a very primitive type, including the mar- 

 supials, one of the oldest and least highly developed 

 orders of mammals; the monotremes, Ornithorhynchus 

 and Echidna, the lowest representatives of the same 

 class, and Apteryx, the lowest of living birds. Isolated 

 on their island continents and free from the competition 

 of higher forms, and especially from the attacks of car- 

 nivores, these lowly organized and almost defenseless 

 species have retained to a marked degree the character- 

 istics of their remote ancestors. Where the separation 

 of island and continent has taken place more recently, 

 or where the channel is shallow or narrow, there is 

 greater resemblance between their faunas. Thus, wild 

 animals of Great Britain are quite the same as those 

 of western Europe. The number of species found on 

 islands is usually small as compared with those inhabit- 

 ing an equal continental area, because the number of 

 ancestral forms which have been carried to the island by 

 currents, wind, and man is likely to be small. 



The animals found on high mountain peaks and ranges 

 are distinctly allied to arctic forms. On the northward 



