MIGRATION OF ANIMALS. 125 



north, as the summer heat parches it in the south. 

 The Esquimaux, on the other hand, move to the south 

 in summer, and support themselves by hunting ; while 

 they return northward to the sea in winter, to feed 

 upon seals and other breathing natives of the deep, 

 which must keep open holes in the ice to preserve their 

 existence. In like manner, the migratory flights of 

 birds appear to be chiefly influenced by the necessity 

 of seeking food, though partly also by the finding of 

 proper places for rearing their young. 



From the nature of their powers of motion, the sea- 

 sonal migrations of quadrupeds are necessarily limited. 

 If they be inhabitants of islands, they cannot pass over 

 the sea ; and upon continents, large rivers, mountains, 

 or desarts, limit their range. In Britain, the stag and 

 the roe, which are found only in the uplands in the 

 warm season, find their way to the warm and sheltered 

 plains in the winter; and on more extensive lands 

 some of the quadrupeds take longer journeys ; but 

 they are all comparatively limited, and extensive mi- 

 grations are performed only by those animals that can 

 make their pathways in the sea or the air. The seal, 

 which during summer is found in such numbers on 

 the dreary shores of Greenland, Jan Mayen, and Spitz- 

 bergen, finds its way to Iceland in the winter; but 

 its migration is limited; and numbers still remain in 

 the most northern regions that have been visited. 

 The inhabitants of the water have, indeed, less neces- 

 sity for seasonal changes of abode than those of the 

 land ; as the water undergoes less change of tempera- 

 ture, and as some of those sea animals which, like the 

 seal, require to come frequently to the surface to 



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