GEOGRAPHICAL DISTRIBUTION. 25 



The annual rain-fall is generally below 15 inches, but ranges at different 

 localities from 3 to 20 inches ; this increase or decrease depending 

 greatly upon the position (if such localities in immediate or close proximity 

 to hills or mountains. Here a general paleness of color is the distinctive 

 feature " It is in this region that nearly all the natural history work of the 

 survey has been performed. 



" The fifth region begins in the Pacific coast, at about the fortieth par- 

 allel, embracing a comparatively narrow belt along the coast from North- 

 era California to Sitka. Its peculiarities are most strongly developed west 

 of the Cascade range, north of 45°; they also prevail eastward nearly or 

 quite to the main chain of the Rocky Mountains. It may hence be termed 

 the Columbian Region.. With an average annual rain-fall of 55 to 65 inches, 

 the prevalent tendency in color is to dusky and fuscous, rather than rufous 

 tints. The district between the Cascade range and the main chain of the 

 Rocky Mountains presents features that may almost entitle it to rank as a 

 distinct region, as might also the region of maximum rain-fall in the Missis- 

 sippi Region. The southern half of Florida is also, perhaps, entitled to 

 recognition as a distinct region, being characterized by excessive humidity 

 and a sub-tropical intensity of color. It may also be necessary to eventually 

 recognize as distinct districts the almost rainless portion of the Campestrian 

 Region. 



"In respect to the correlation of intensity of color in animals with the 

 degree of humidity, it would perhaps be more in accordance with cause and 

 effect to express this law of correlation as a decrease of intensity of color 

 with a decrease of humidity ; the paleness evidently resulting from exposure 

 and the blanching effect of intense sunlight, and a dry, often intensely heated, 

 atmosphere." This theory is strongly corroborated by the fact that, in the 

 human race, many instances are known of persons, having dark hair and 

 beards, losing their depth of color under the same condition and circumstances 

 as affect other mammals as well as birds, and transmitting the lighter colors 

 to the hah of their progeny. Professor Allen continues : "With the decrease 

 of aqueous precipitation, the forest-growth and the protection afforded by 

 arborescent vegetation gradually also decreases as of course does also the 

 protection afforded by clouds, the excessively humid regions being also 



