THE TUNDRA AND ITS FAUNA 33 
breed, and that the young must spend the first two or 
three weeks of their life on shore. Many of the seals 
are markedly migratory, going south in winter to avoid 
the intense cold of the north. This is especially true of 
the Greenland seal (Phoca groenlandica), which seems 
incapable of making a breathing-hole in the ice, and 
therefore must have relatively open water in its haunts. 
On the other hand, the ringed seal (Phoca hispida), 
which does make such breathing-holes, is almost non- 
migratory, living permanently in the coldest regions. 
These two species will thus not compete with each 
other, and all the species doubtless show similar 
minor adaptations, which fit them for different parts 
of the common environment; but it is not always 
possible to explain wherein these special adaptations 
consist. The large bearded seal (Phoca barbata) is 
another form which makes breathing-holes. The very 
curious crested seal (Cystophora cristata) is not only 
migratory but also pelagic in its habits, avoiding the 
neighbourhood of land, and bringing forth its young 
on ice-floes. It is a bold species, apparently a swift 
swimmer, and eats cuttles as well as fish. The common 
seal (Phoca vitulina) also occurs in Arctic waters. 
The cetaceans of the north are numerous, and in- 
clude some very large animals. Thus among the whale- 
bone forms we have the Greenland whale (Balaena 
mysticetus), with its enormous head, and its elaborate 
series of baleen plates, which filter the minute crus- 
taceans and molluscs upon which it feeds from the 
sea water. The huge rorquals (Balaenoptera), though 
found in practically all seas, often seek the vicinity of 
northern coast-lines in spring for breeding purposes. 
Among the toothed whales the narwhal (Monodon 
monoceros) is a true northern form, and lives largely 
1404 © 
