24 ZOOLOGY. 



Professor Allen defines these areas as follows : 



" The first region is that of the Atlantic slope, which will include not 

 only the countries east of the Alleghanies, but a large part of the British 

 Possessions, extending westward at least as far as Fort Simpson, and thence 

 northward and westward to Alaska, including apparently all of that terri- 

 tory north of the Alaskan Mountains, with an annual rain-fall throughout 

 the whole of this extended region of about 35 to 45 inches. Over this 

 region (to which may be given the general term Atlantic Region), the colors 

 may be regarded as of the average or normal type ; those of other regions 

 being either of a diminished or increased intensity. 



" The second region will embrace the Mississippi Valley, or more prop- 

 erly the Mississippi Basin, and may hence be termed the Mississippi Rq/ioit. 

 Here the annual rain-fall reaches 45 to 55 inches, and over a small area 

 east of the Lower Mississippi even exceeds 60 inches. The tendency here is 

 so often to an increase of fulvous and rufous tints, that we may regard it as 

 the distinctive chromatic peculiarity of the region; these tints reaching their 

 maximum in the limited area of greatest humidity, but a general increase in 

 intensity of color is also more or less characteristic of the region. 



"A third region embraces the central portion of the Rocky Mountains, 

 and, being developed most strongly within the present territory of Colorado, 

 and being also mainly included within that territory, may be termed the 

 Colorado Region. The tendency here again, as compared with the imme- 

 diately adjoining districts, is to a general increase of intensity of color, with 

 also a marked inclination to the development of rufous and fulvous tints ; 

 this region being also within the influence of a comparatively high tempera- 

 ture, at least in summer. The humidity is here less than in either of the 

 other regions already defined, the annual aqueous precipitation amounting 

 to only about 24 to 30 inches ; but it is yet greatly in excess of that of the 

 districts immediately surrounding it. 



" The fourth region may be regarded as made up of the arid plains and 

 deserts of the great central plateau of the continent; including not only the 

 'great plains,' usually so called, but the deserts and plains of Utah, Nevada, 

 Western Colorado, New Mexico, Arizona, and southwestward to Lower 

 California, and may hence be appropriately termed the Cai}>cxiri(t Ret/inn. 



