PHYSICAL CONDITIONS IN GENESIS OB' SPECIES. B85 



markable in this respect, and in conseqnence mainly of this fact the 

 southern types have been varietally separated from the shorter-tailed 

 northern forms of the same species. 



Variations in color with locality are still more obvious, ])articu- 

 larly among birds, in which the colors are more positive, the con- 

 trasts of tint greater, and the markings consequently better defined 

 than is usually 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 char- 

 acterize birds. Mammals, however, present many striking instances 

 of geographical variation in color. 



As already stated, geographical variation in color may be con- 

 veniently considered under two heads. "Wliile the variation with 

 latitude consists mainly in a nearly uniform increase in one direc- 

 tion, the variation observed in passing from the Atlantic coast Avest- 

 ward is more complex. In either case, however, the variation 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 restricted. 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 climate and the absence of forests, 

 the greatest pallor occurring over the almost rainless and semiclesert 

 regions of the Great Basin and Colorado Desert. On the Pacific 

 slope north of California the color again increases, with a tendency 

 to heavy, somber tints over the rainy, heavily wooded region of the 

 northwest coast. 



Geographical variation in color among mammals, for reasons 

 already stated, is generally, but not always, manifested merely 

 through the varying intensity or de})th of the tints. It is, hoAvever, 

 often strongly marked. The connnon chickaree, or red squirreJ 

 (Sciurus hudsonhis) , for examjile, Avhich ranges from high northern 

 latitudes southward over the northern portion of the United States, 

 shows an increase in the color over the middle of the dorsal surface 

 from pale yellowish or fulvous to rufous. The fox squirrel of the 

 Mississijipi Valley {Sciurus niger^ Ivdomciarvus), which ranges from 

 Dakota southward to the Gulf of Mexico, has the lower parts, at 

 the northward, very pale yellowish white, wdiich tint gradually in- 



