FOREST, AND ITS FAUNA 43 
common British squirrel (S. vulgaris), widely distributed 
in the more northerly parts of Asia, as is well known, 
rarely descends to the ground, and has great powers of 
leaping. It shows interesting colour variations, tend- 
ing to be darker in colour in the colder parts of its 
range in summer, and turning white in winter where 
the climate is severe. It will eat almost any forest 
product, and hibernates during winter. The final stage 
in the mastery over the forest is represented by the 
flying squirrels, which are furnished with a parachute 
by means of which they can take long leaps from one 
tree to another, and thus obviate the risks due to 
a descent to terra firma. Though chiefly inhabitants of 
the tropical forests of Asia, one species occurs in tem- 
perate Asia and one in temperate North America. 
The beaver, which haunts the streams flowing through 
wooded regions, was once widely distributed over the 
cooler parts of the northern hemisphere. The American 
species, if not identical with the Eurasiatic one, is at 
least very nearly allied. Beavers feed upon bark and 
twigs, and also upon the roots, &c., of such plants as 
water-lilies. 
While the porcupine of the warmer parts of Asia 
inhabits rocky country, the porcupine of Canada is 
a purely arboreal animal in habits, though it does not 
possess the same adaptations to this mode of life as 
are exhibited by its southern allies, the tree-porcupines 
of South America, Mexico, &c. The Canadian form 
(Erethizon dorsatus) follows the taiga to its northern 
limit, and also follows the western coniferous forest to ° 
its southern limit. It spends the greater part of its 
time in the trees, though the nest is usually made on the 
ground among stones. The food consists of leaves, 
bark, twigs, beech mast, and other nuts, and the 
