ON VARIATION. 57 



"Variations in color with locality are still more ob- 

 vious, particularly among birds, in which the colors 

 are more positive, the contrasts of tints greater, and 

 the markings consequently better defined than is usu- 

 ally the case in mammals. The soft finely-divided 

 covering of the latter is poorly fitted for the display 

 of the delicate pencilings and the lustrous, prismatic 

 hues that so often characterize birds. Mammals, how- 

 ever, present many striking instances of geographical 

 variation in color. 



"As already stated, geographical variations in color 

 may be conveniently considered under two heads. 

 While the variation with latitude consists mainly in a 

 nearly uniform increase in one direction, the variation 

 observed in passing from the Atlantic Coast westward 

 is more complex. In either case, however, the varia- 

 tion results primarily from nearly the same causes, 

 which are obviously climatic, and depend mainly upon 

 the relative humidity, or the hygrometric conditions 

 of the different climatal areas of the continent. In re- 

 spect to the first, or latitudinal variation, the tendency 

 is always toward an increase in intensity of coloration 

 southward. Not only do the primary colors become 

 deepened in this direction, but dusky and blackish 

 tints become stronger or more intense, iridescent hues 

 become more lustrous, and dark markings, as spots 

 and streaks or transverse bars, acquire greater area. 

 Conversely, white or light markings become more re- 

 stricted. In passing westward a general and gradual 

 blanching of the colors is met with on leaving the 

 wooded regions east of the Mississippi, the loss of 

 color increasing with the increasing aridity of the cli- 

 mate and the absence of forests, the greatest pallor 

 occurring over the almost rainless and semi-desert re- 



