192 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



not found in neighboring streams. The garpikes (Lepi- 

 dosteus) of the American rivers afford a striking example of 

 this kind. 



409. A very influential cause in the distribution of aquatic 

 animals is the depth of the water ; so that several zoological 

 zones, receding from the shore, may be defined, according 

 to the depth of water ; much in the same manner as we mark 

 different zones at different elevations in ascending moun- 

 tains, (398.) The Mollusks, and even the Fishes found near 

 the shore in shallow water, differ, in general, from those 

 living at the depth of twenty or thirty feet, and these again 

 are found to be different from those which are met with at 

 a greater depth. Their coloring, in particular, varies, ac- 

 cording to the quantity of light they receive, as has also 

 been shown to be the case with the marine plants. 



410. 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 ( Ouis 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, exclusive of the animals of the surrounding 

 country, merely because 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 exam- 

 ple, is the fauna of the great steppe, or plain of Gobi, in 

 Asia ; and such indeed that of the chain of the Rocky Moun- 

 tains may prove to be, when the animals inhabiting them shall 

 be better known. 



