368 GENERAL LAWS OF DISTRIBUTION. 



in particular, varies, according to the quantity of light they re- 

 ceive, as has also been shown to be the case with marine plants. 



592. It is sometimes the case that one or more animals 

 are found upon a certain chain of mountains, and not else- 

 where ; as, for instance, the mountain sheep (Ovis montana), 

 upon the Rocky Mountains, or the chamois and the ibex upon 

 the Alps. The same is also the case on some of the wide 

 plains or prairies. This, however, does not entitle such regions 

 to be considered as having an independent fauna, any more 

 than a lake is to be regarded as having a peculiar fauna, ex- 

 clusive of the animals of the surrounding country, merely be- 

 cause some of the species found in the lake may not ascend 

 the rivers emptying into it. It is only when the whole group 

 of animals inhabiting such a region has such peculiarities as 

 to give it a distinct character, when contrasted with animals 

 found in surrounding regions, that it is to be regarded as a 

 separate fauna. Such, for example, is the fauna of the great 

 steppe or plain of Gobi, in Asia ; and such indeed that of the 

 chain of the Rocky Mountains may prove to be, when the 

 animals inhabiting them shall be better known. 



593. 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 habi- 

 tual abode. As to birds, which of all animals wander the 

 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 be- 

 long to the temperate faunas, notwithstanding their migration 

 during winter to the confines of the torrid zone. This rule 

 does not apply to the fishes, who annually 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 coasts of Maine, Nova Scotia, and 

 the British isles. 



594. 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 



