1^2 Allen, Origin and Distribution of ' N. A. Birds. ["April 



Temperate rather than Warm Temperate forms. The southern 

 list would include a large number of tropical genera which range 

 into the southern part of the Warm Temperate — such for instance 

 as Columbigallina, Scar da fella , Ur-7ibitinga* Asturina, Geo- 

 coccjx, Co7t?ir?/s, Nyctidromus , various genera of Hummingbirds, 

 Milvulus, Pyrocephalus, Campylorhynchus , Cat/ierpes, etc. ; 

 but they are nearly all limited to the Arid division of the Warm 

 Temperate, and thus serve to emphasize the differences distin- 

 guishing the Arid from the Humid Province quite as much as a 

 northern and southern division along a transcontinental line. 

 The differentiation of the Warm Temperate into two transconti- 

 nental belts — a northern and a southern — is thus due to the in- 

 crease of tropical forms near the southern border, in accordance 

 with the law of general increase of the forms of life from the 

 north southward. 



In respect to species and subspecies, the following lists may 

 serve to indicate the forms characteristic respectively of the 

 Humid and Arid Provinces. As the transition between the 

 two is gradual, with many interdigitations, through the extension 

 of forest or tree-loving species up the partially wooded river 

 valleys into the Plains, the lists are to some degree open to the 

 criticism that many species classified as eastern are not sharply 

 limited at the eastern edge of the Plains, and that a few of the 

 properly Arid Province species range eastward to the prairies 

 east of the Mississippi River, yet in general terms the distribution 

 is as implied in the headings of the two lists. The straggling of 

 Catbirds and Kingbirds to the Great Basin, and even to the Pacific 

 coast, does not weigh heavily against the propriety of including 

 them in the Humid Province list since they are here such charac- 

 teristic elements of the fauna. The lists, furthermore, are not 

 intended to be complete ; to save space subspecies are in many 

 instances omitted; the more tropical forms, and generally also 

 those of local distribution, as well as the maritime water birds, are 

 excluded. 



Humid Province. 



Ajaja ajaja Ardea ccerulea 



Guara alba Nycticorax violaceus 



Ardea rufe^cens Rallus elegans 



Ardea tricolor ruficollis Ionornis martinica 



