MUSEUM OF COMPARATIVE ZOOLOGY. 241 



most prolific of dark and light local forms witl rain-fall charts, — which 

 may be assumed as indicating relatively the liygrometric conditions of 

 different regions, — to find the distribution of the light-colored races so 

 strictly coincident with the regions of minimum mean annual rail-fall, 

 and the dark forms with those of maximum mean annual rain-fall, as 

 seems to be the case.» 



Humidity has hence apparently far more to do with climatic varia- 

 tion in color than solar intensity, though the latter has undoubtedly 

 an influence upon color. The occurrence of a light-colored race 

 of Arvicola riparius on Muskeget Island and the sandy sea-beaches 

 of the coast of Massachusetts shows clearly that the intense light 

 caused by reflection from a sandy surface tends to the diminution 

 rather than to an increase of color in animals, and even plants, 

 since the foliage of the latter in arid districts so commonly assumes 

 a dull grayish tint. The capture on Muskeget Island last season 

 (July, 1870), by Messrs. Maynard and Brewster, of two pairs of 

 the short-eared owl (Olus brachyotus) with the color of the plu- 

 mage so pale as at first to suggest their being albinos, is additional 

 evidence of the bleaching effect of strong light upon the colors of ani- 

 mals. Such facts render it doubtful whether the increased intensity of 

 the light in the tropics has really much to do with the brighter colors 

 of tropical birds and insects, and suggest that humidity alone may be 

 the principal agent in producing this accession of color. 



In regard to the cause of other climatic variations, certain other 

 facts are naturally recalled. In the remarks on. the climatic and faunal 

 peculiarities of East Florida,* attention was called to the less degree of 

 vivacity and energy exhibited by the southern as compared with the 

 northern members of the same species, and the general higher physio- 

 logical development of essentially extra-tropical species in the temperate 

 portions of their habitats. Is it hence improbable that the southward 

 deterioration in size seen in such species is directly related to the ener- 

 vating influence of increased heat ? And why is it that so large a pro- 

 portion of the birds pre-eminently singing-birds are found in temperate 

 latitudes ? 



In the increased size of the bill and tail to the southward, especially 

 of the former, we have a fact somewhat parallel to what is not unfre- 

 quently seen in mammals. The ears, for example, of the arctic repre- 



* See above, p. 166. 

 VOL. II. 16 



