DISTRIBUTION OF THE FAUNAS. 161 



belong to the zone in which they breed. Thus, the gulls, 

 many of the ducks, mergansers, and divers, belong to the 

 boreal regions, though they pass a portion of the year with 

 us. On the other hand, the swallows and martins, and 

 many of the gallinaceous birds belong to the temperate 

 faunas, notwithstanding they migrate during winter to the 

 confines of the torrid zone. This rule does not apply to 

 the fishes, who annually leave their proper home, and mi- 

 grate to a distant region merely for the purpose of spawn- 

 ing. The Salmon, for example, comes down from the 

 North to spawn on the coasts of Maine and Nova Scotia. 



412. Few of the Mammals, and these mostly of the tribe 

 of Rodents, make extensive migrations. Among the most 

 remarkable of these are the Kamtschatka rats. In Spring 

 they direct their course westward, in immense troops ; and 

 after a very long journey, return again in Autumn to their 

 quarters, where their approach is anxiously awaited by the 

 hunters, on account of the fine furs to be obtained from the 

 numerous carnivora which always follow in their train. 

 The migrations of the Lemmings are marked by the devas- 

 tations they commit along their course, as they come down 

 from the borders of the Frozen Ocean to the valleys of 

 Lapland and Norway ; but their migrations are not period- 

 ical. 



SECTION II. 



DISTRIBUTION OF THE FAUNAS. 



413. We have stated that all the faunas of the globe 

 may be divided into three departments, corresponding to as 

 many great climatal divisions, namely, the glacial or arctic, 

 the temperate and the tropical faunas. These three divisions 



14* 



