AMERICAN PTARMIGANS 
63 
enter a belt of more or less stunted timber before reaching the 
real forest. Yet even here quite a number of new forms of 
animal life are met with that are quite absent in the more 
northerly districts. They are mostly species occurring also 
in the forest zone of western Labrador. In referring to them 
in the last chapter, I alluded more fully to the geological 
history of the moose (pp. 32—33), but I dwelt also on the fact 
that the flying squirrel, musk rat, .chipmunk, woodchuck and 
others made their appearance as soon as we entered the 
forest from the east. And it is precisely these forms of animal 
life that make such a change in the appearance of the fauna 
as we leave the Mackenzie region barren-grounds on our 
way to the great Canadian forest region. 
The flying squirrel (Sciuropterus sabrinus) of northern 
Canada and arctic America is also found further south. Other 
species of the same genus inhabit the United States. The 
skin at the sides of its body extends between the limbs in 
such] a manner as to act like a kind of parachute, but nothing 
in the nature of actual flight takes place. The flying 
squirrel is rarely seen,’ being an entirely nocturnal crea¬ 
ture. The distribution of these flying squirrels is decidedly 
interesting. They are found from Florida to Alaska, and 
are only known as fossils from the Potter Creek Cave in 
California. They have almost certainly been in the country 
since Pliocene times. Hence Sciuropterus yukonensis, which 
is peculiar to Alaska, h.as, in all probability, survived the 
Glacial Epoch in that country. On the western side of Bering 
Strait, in Kamchatka, another species is found whose range 
extends westward as far as Scandinavia. Southward, as 
we cross China into Burma and India, we still encounter 
species of flying squirrels, and even on the islands of 
Java, Sumatra and Borneo. And all these belong f to the same 
genus Sciuropterus. Such an extensive distribution implies 
that the genus must be an ancient one, and, indeed, we are 
acquainted with several species of Sciuropterus from the 
Miocene and Pliocene deposits of France. This in itself is 
no proof that the flying squirrels originated in Europe. They 
may have spread there from Asia in later Tertiary times and 
have subsequently died out in Europe, only to be re-estab¬ 
lished more recently from a later Asiatic invasion. In any 
