I ?0 Allen, Origin and Distribution of N. A. Birds. April 



plains and deserts. The dividing line may be considered as coin- 

 cident with the isohyetal curve marking an annual rainfall of 20 

 inches or less, as shown on the rainfall charts of the United States, 

 — in other words, as already stated, near the 100th meridian. 



The contrasting climatic conditions of these two regions are 

 due to peculiar physiographic and geographic conditions which 

 extend back to a comparatively remote period — so remote as to 

 have given time for the development of many generic forms 

 limited almost entirely to one or, the other of these two regions. 

 Thus, in respect to birds, we have 17 genera in the Humid 

 Province which do not range to any great extent into the Arid 

 Province, and 2S genera in the Arid Province which are not 

 found to any extent in the Humid. The distinctively eastern or 

 Humid Province genera are : — 



Philohela Ictinia Protonotaria 



Bartramia Dolichonyx Helinaia 



Colinus Quiscalus Helmitherus 



Tympanuchus Ammodramus 1 Compsothlypis 



Catharista Spiza Galeoscoptes 



Elanoides Mniotilta 



A few of these genera range into the eastern border of the 

 Plains, and sometimes, as in the case of Galeoscoptes, occur 

 locally westward to the Pacific coast. They are nevertheless to be 

 regarded as distinctively characteristic of the eastern side of the 

 continent. Colinus is more difficult to locate, as southern forms 

 of the genus occur in the Sonoran Subprovince of the Arid, while 

 its northern form, C. virginianus, is distinctively an eastern 

 form characteristic of the Humid Province. 



On the other hand, the following genera may in like manner 

 be considered as distinctively characteristic of the Arid Province, 

 although some of them range northward into the Cold Tem- 

 perate : — 



^Echmophorus Pediocsetes Micropallas 



Oreortyx Centrocercus Geococcyx 



Callipepla Pseudogryphus Xenopicus 



Cvrtonyx Glaucidium Phalaenoptilus 



1 Subgenus. 



