GENERAL LAWS OF DISTRIBUTION. 193 



411. The migration of animals might at first seem to pre- 

 sent a serious difficulty in determining the character or the 

 limits of a fauna ; but this difficulty ceases, if we regard the 

 country of an animal to be the place where it makes its 

 habitual abode. As to Birds, which of all animals wander 

 farthest, it may be laid down as a rule, that they 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, notwith- 

 standing their migration during winter to the confines of the 

 torrid zone. This rule does not apply to the fishes who an- 

 nually leave their proper home, and migrate to a distant 

 region merely for the purpose of spawning. The Salmon, 

 for example, comes down from the North, to spawn on the 

 coast 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. 



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