,g 93 X "J Allen, Origin and Distribution of N. A. Birds. 121 



as we proceed northward, as in North America for example, the 

 genera become in increasing ratio more and more those of wide 

 distribution, till in the extreme north we meet with few that are 

 not circumpolar. How gradually and completely this transition 

 is made has already been shown (antea, p. 114), 47 per cent of 

 the non-pelagic birds of North America occurring in corresponding 

 latitudes in the Old World. Extra-tropical North America has, 

 in fact, so much in common with extra-tropical Eurasia, that the 

 two areas constitute merely two regions of a single realm. 



To indicate my views in respect to the faunal relations of North 

 America to the world at large, I will recapitulate briefly what I 

 consider may be properly regarded as the primary life areas of the 

 earth's surface, namely : — 



(1) An Arctic Realm, occupying the region northward from 

 about the limit of forest vegetation, or from about the isotherm of 

 32 F. It is characterized by its paucity of life and its homo- 

 geneousness, nearly all its forms of both animal and vegetable life 

 ranging throughout its whole extent. 



(2) A North Temperate Realm, extending from the north- 

 ern limit of forest vegetation to the northern border of the palm 

 belt, or between the annual isotherms of 32 and 70 F. 



(3) An American Tropical Realm, consisting, as the name 

 implies, of tropical America. 



(4) An Indo-African Realm, consisting of Africa (except 

 the northern border), and tropical Asia and its outlying islands. 



(5) A South American Temperate Realm, embracing extra- 

 tropical South America. 



(6) An Australian Realm, including not only Australia, but 

 New Guinea, New Zealand, and the various groups of islands to 

 the northward and eastward. 



(7) A Lemurian Realm, consisting of Madagascar. 



An eighth or Antarctic Realm is also sometimes recognized, 

 as the Antarctic counterpart of the Arctic realm, though perhaps 

 less well characterized, its fauna consisting almost exclusively of 

 maritime and pelagic species. 



The North Temperate Realm is divisible into two regions, 

 (1) a North American Region, consisting of temperate North 

 America, and (2) a Eurasiatic Region, consisting of temperate 

 Eurasia. 



