NORTHERN FROGS 
31 
enter the vast forest region, plant life as well as animal life 
become more and more abundant and differentiated. In¬ 
stead of the barren-ground caribou, we now meet with the 
woodland form accompanied by another large ungulate, the 
moose, while the flying squirrel, chipmunk, the ground 
squirrel, woodchuck, white-footed mouse, musk-rat, heaver, 
skunk, weasel, shrews, moles and many other beasts tenant 
the forests, meadows, and banks of rivers.* 
A few amphibians, even, have succeeded in surviving the 
rigours of the arctic winter of those regions, and have success¬ 
fully established the most northern outposts in eastern North 
America. The leopard frog (Rana pipiens), one of the 
commonest as well as one of the most brilliantly coloured of 
American frogs, is one of these. The pickerel (Rana palus- 
tris), also the northern wood frog (Rana cantabrigiensis) 
and the northern frog (Rana septentrionalis) have all been 
observed in the neighbourhood of Hudson Bay. The most 
interesting species is the swamp-tree frog (Chorophilus 
nigritus), whose northern variety has advanced into this in¬ 
hospitable region, though almost all of the other members 
of the tree frog family (Hylidae) are typically southern 
forms.f Whether newts occur in the Hudson Bay region is 
not definitely known, but the salamander (Plethodon 
cinereus), at any rate seems to have been met with. All these 
species are peculiar to America. 
No reptiles have been noticed. The distribution of the 
terrestrial mollusks, the snails and slugs, implies that an ad¬ 
vance in a northward direction, similar to that recorded in 
the case of mammals and amphibians, has taken place among 
some groups of invertebrates. The typically American snails, 
Polygyra monodon and Strobilops labyrinthica, have been 
collected near Hudson Bay. 
When we analyse the constitution of all these western and 
southern groups, and trace the relationship of the members 
more carefully, we notice that many of them are not of 
American ancestry. They all have lived, no doubt, long 
enough in America to have become thoroughly established, 
* Preble, E. A., “Hudson Bay Region.” 
f Dickerson, Mary C., “ The Frog Book,” p. 158, 
