28 WILD NEIGHBORS CHAP, 
tense in regions of copious rainfall than in arid 
areas. 
The squirrels exemplify these rules. In this 
species (carolinensis), for instance, a steady grada- 
tion may be detected from the light pure gray of 
the upper parts, characteristic of New England 
specimens, to the yellowish dorsal fur of the Flor- 
ida type. In the fox-squirrels (Sczurus niger) of 
Wisconsin and Iowa the lower parts are only pale 
fulvous, in some specimens nearly white; about 
St. Louis they are strong, bright fulvous, and in 
lower Louisiana reddish fulvous or deep orange, 
while the back is far darker than northward. The 
same species fades westward from the bright speci- 
mens of the damp Mississippi Valley forests into a 
far paler variety along:the dry edges of the Great 
Plains. The red squirrel (S. Zudsonius) and the 
chipmunk ( Zamias striatus) are also excellent illus- 
trations of the action of climatic influences under 
this law — particularly the latter, whose color and 
stripes exhibited so many varieties between the 
Atlantic and Pacific coasts that early naturalists, 
having insufficient specimens, described confidently 
as several species what is now conceded to be only 
one. 
The relative amount of moisture and shade seem 
to be the determining causes of this diversity, 
drouth and the blanching power of the sun in 
the high dry plains fading the pigments in the 
hair, or perhaps checking their deposition. The 
