I I A. Allen, Origin and Distribution of N. A. Birds. \ April 



that of the rest of the world, we will divide the non-tropical part 

 of the continent into thi-ee belts, namely : (i) An Arctic, includ- 

 ing that part of the continent and its adjacent islands north of 

 about the limit of forest vegetation. (2) The Cold Temperate, 

 limited on the north by the Arctic, and on the south by what is 

 commonly recognized as the Canadian Fauna (see PI. III). Its 

 southern border thus coincides in a genei - al way with the north- 

 ern limit of the successful cultivation of the staple grains and 

 fruits of the temperate zone. (3) The Warm Temperate, extend- 

 ing from the southern border of the Cold Temperate to about the 

 edge of the palm belt, or to the hot lowlands of Mexico. 



The Arctic belt is inhabited during the breeding season by 

 about 65 genera of birds, of which only 5 are exclusively Ameri- 

 can ; the remaining 60 have either a general circumarctic distri- 

 bution or occur in Eastern Siberia or on the Arctic coast of 

 Europe as well as in North America. None of the 5 American 

 genera is str-ictly Arctic ; they merely extend into the Arctic 

 from the Cold Temperate, while quite a proportion of the cir- 

 cumpolar genera is strictly Arctic in their breeding range. 



In the Cold Temperate belt 120 genera are represented, of 

 which 98 are circumpolar and 22 American. Of these 46, or 

 nearly half of the circumpolar genera, range also into the 

 Warm Temperate belt, as do also 14 of the strictly American 

 genera. 



In the Warm Temperate belt 95 genera occur which do not 

 range into the Cold Temperate, of which only 12 are Old 

 World, 83 being exclusively American. In addition 60 genera 

 are common to both the Cold Temperate and Warm Temperate, 

 of which 46 are Old World and 14 American. This gives a 

 total of 155 genera in the Warm Temperate, of which 58 are 

 Old World and 97 exclusively American. Besides these, 50 

 essentially tropical genera reach or extend somewhat into the 

 Warm Temperate, of which 43 are American and 7 tropicopoli- 

 tan, increasing the total number of genera occurring in the 

 Warm Temperate to about 205. 



These statistics illustrate a number of important points: (1) 

 the rapid increase of bird life in North America from the Arctic 

 regions southward, notwithstanding the fact that the continent 

 steadily and greatly decreases in breadth from the north south- 

 ward, the number of genera in the Arctic belt being 65, in the 



