No. 392.] AMERICAN ARBOREAL SQUIRRELS. 637 
occur from the hot coast country to the region of oaks and 
pines close to timber line, but the ranges of different species or 
subspecies are never coincident and overlap only in a few 
instances, as in the case of S. colliei nuchalis and S. poliopus 
colimensts on the coast of Colima, and S. apache and S. durangi 
in the Sierra Madre. 
“Many species change their environment by periodical migra- 
tions in search of food, moving from one locality to another 
with the ripening of fruits or seeds upon which they subsist. 
This is most marked on high mountains, where a species may 
have a vertical range of many thousand feet.” Mr. Nelson also 
says: “ The effect of climate on the character of the pelage is 
so marked that it is possible to tell with considerable certainty 
whether a species belongs to the tropics or to the high moun- 
tains. Tropical species have thin pelage, short, thin under fur, 
and coarse, stiff, or almost bristly, dorsal hairs. ... These 
differences are sometimes strikingly shown in subspecies of the 
same squirrel.” Seasonal differences, however, are slight, but 
individual variation, on the other hand, is often excessive, 
rendering some species extremely difficult to describe. 
“« The extraordinary amount of geographical variation in trop- 
ical North-American squirrels is due mainly to an unusual 
plasticity of the organization which allows slight climatic differ- 
ences to produce visible effects, The most obvious of these 
- influences are differences in temperature and rainfall, with their 
distribution through the year and consequent effect on vegeta- 
tion.” Thus in the drier interior mountains certain subspecies 
of a group will be characterized by dull grayish upper parts and 
white under parts, while other subspecies of the same group 
inhabiting humid mountains near the coast will have nape and 
rump patches sharply contrasting with the rest of the dorsal 
surface.and bright ferruginous under parts, increased humidity 
within the tropics being usually eo by increased 
intensity of coloration. 
In this admirable paper Mr. Nelson attempted a most difficult 
task, which consisted in not only recognizing and defining the 
various forms of tree squirrels inhabiting tropical North Amer- 
ica, but in clearing up the tangle of synonymy involved in the 
