1^8 Allen, Origin and Distribution of N. A. Birds. 



Auk 

 April 



appear to be thus restricted. This boundary also forms about 

 the northern limit of many species and genera of the Sonoran Sub- 

 province. These two subprovinces are hence characterized 

 mainly by the presence of a large number of forms found in the 

 Sonoran which are absent from the Campestrian, and are thus 

 distinguished, like many northern divisions when compared with 

 adjoining southern ones of coordinate rank, from the Arctic 

 southward, by what they lack rather than by the posession of any 

 peculiar types. 



The Campestrian Subprovince itself may be divided into three 

 areas which may be termed districts, namely (i) the Great 

 Plains District, (2) the Great Basin District, and (3) the 

 Pacific Coast District. (See PI. IV.) The first two are 

 respectively the 'Great Plains subregion' and the 'Great Basin 

 subregion' ,of Dr. Merriam's first 'Biological Map of North 

 America.' 1 Although these two districts are separated by the 

 main chain of the Rocky Mountains, they arefaunally but slightly 

 differentiated. But few genera occur in the one that do not occur 

 in the other ;^a few species, and a larger number of subspecies 

 are restricted to one or the other during the breeding season, but 

 they often become more or less mixed during winter, when they 

 meet on common ground in the Sonoran Subprovince. Thus the 

 Rocky Mountains, while forming so imposing a feature in the 

 configuration of the continent, fail to be by any means an 

 impassable barrier to the dispersal of species, owing to their 

 numerous comparatively low depressions, and to their meridional 

 trend. 



The Pacific Coast District consists of a narrow belt situated 

 mainly west of the Sierra Nevada and Cascade Ranges, and is 

 characterized by the presence of a few species and a considerable 

 number of subspecies mainly restricted to it. 



The Sonoran Subprovince consists of Dr. Merriam's restricted 

 'Sonoran subregion,' 1 with the addition of Lower California, 

 which Dr. Merriam gave the rank of a 'subregion.' A careful 

 synopsis of the bird life of Lower California and its outlying 

 islands shows that it has 28 species and 24 subspecies which are 

 not found in the United States. Of the 29 species 10 are confined 

 exclusively to the outlying islands, and 9 others are either strictly 



1 N. Am. Fauna, No. 3, 1890, p. 25, and map 5. 



