,°S 93 J Allen, Origin and Distribution of N. A. Birds. T 20 



locally restricted types, including families as well as genera and 

 species, to say nothing of merely geographical races or sub- 

 species. Consequently it is divisible into a larger number of 

 faunal areas, varying to a much greater extent as regards their 

 relative taxonomic rank, and much more strongly contrasted in 

 respect to their faunal characteristics. 



The Warm Temperate Subregion may be considered as prim- 

 arily divisible along a nearly north and south line into two 

 Provinces, namely (i) an eastern or Humid Province and (2) 

 a western or Arid Province. The Humid Province extends 

 from the Atlantic coast westward to about the 100th meridian, or 

 to the eastern border of the Great Plains, exclusive, of course, of 

 the Appalachian Highlands; the Arid Province extends thence 

 westward to the Pacific coast, exclusive, of course, of the south- 

 ward extensions of the Cold Temperate along the mountain 

 systems. These regions thus correspond respectivelv with Dr. 

 Merriam's 'Humid Sonoran' and his 'Arid Sonoran,' as he origi- 

 nally defined them. The Humid Province, as the name implies, 

 has a rather moist climate and is (or was a century ago) in most 

 parts heavily forested ; the Arid Province is, on the other hand, a 

 region of open plains and deserts, with, in general, an excessively 

 arid climate. 



These two provinces thus coincide with the two strongly 

 marked regions of the middle portion of North America in 

 respect to climate as modified by rainfall. The line of separation 

 is thus meridional instead of transcontinental as are the faunal 

 boundaries determined by temperature. In this case temperature, 

 as a controling influence in the distribution of animals and plants, 

 is subordinated to the usually less powerful agent, humidity. 



The transition between the Humid and Arid Provinces is no- 

 where abrupt ; they gradually merge into each other everywhere 

 along their line of junction, as the prairies of the Mississippi 

 Valley gradually become more arid and take on the characteristic 

 aspect of the plains. There is thus here the usual 'transition' 

 belt occurring between contiguous faunal areas. It is, however, 

 rather broader than between regions where temperature is prim- 

 arily the limiting influence, as in the case of boundaries trending 

 in a nearly east and west direction, the transition being first from 

 a forested region to one of fertile prairies, and thence to arid 

 17 



