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 ahnost non- 

 migratory, hving 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 crisfata) 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 

 mysticelus), 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 c 



