644 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
possible intermediate shades ; these varieties are sometimes more or less con- 
stant in particular localities, sometimes changing with every litter. I am not 
aware that there is any material difference of color at different seasons or ages 
in the same animal. 
‘Another source of perplexity to the naturalist is the alteration in aver- 
age size with the latitude. Many of our animals become smaller as we pro- 
ceed southward, until, on the seacoast of Georgia, Florida, and the Gulf, they 
reach their minimum. This is very strikingly seen in the common Deer 
[Cariacus virginianus], which on the sea islands of Georgia is so small as to 
be readily lifted and thrown across a horse with perfect ease by a man of 
ordinary strength. It is in the Sciuride that, next to the Deer, we find this 
law to prevail most decidedly. Nearly all the species of extensive north and 
south range will be found, on careful examination, to substantiate this position. 
‘CA similar variation in color to that of the Squirrels is seen in the Foxes 
and Wolves,[*] most strikingly in the former. It is now well known that the 
Red, Cross, and Black Foxes are identical in species, the same litter fre- 
quently embracing all the colors, and that some of these varieties again are 
more or Jess permanent, while the more boreal the locality the greater the 
tendency to black. This is the case also with the Squirrels, where the smaller 
species assume the black pelage to the greatest extent in the more northern 
portions of the United States. 
“As a general rule it may be stated that when a Squirrel exhibits any 
annulations of the fur on the throat or belly it is a variety of some species, 
typical specimens of which have the under parts either uniformly white or 
reddish to the roots, which, however, are sometimes plumbeous. In every 
such instance that has come under my examination I have had no difficulty in 
tracing it to its proper type. Such annulation is usually accompanied by a 
duskier color of the pelage. The tendency to annulation below is strongest 
in the Squirrels of the Mississippi Valley, and applies both to gray and fox- 
colored: species. No such instances of annulation have yet come under my 
notice among the Squirrels west of the Rocky Mountains. As a general 
rule the bones of the entire skeleton of the Fox Squirrels, or those with rusty 
bellies, are red, while the white-bellied varieties have them white. 
“After carefully examining a large number of American Squirrels I 
*See also on this point Bull. U. S. Geol. and Geogr. Surveys of the Territories, vol. ii, pp. 313-321, 
July, 1876. 
