160 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



409. A very influential cause in the distribution of aquatic 

 animals is the depth of the water. The Mollusks, and even 

 the Fishes found near the surface between high and low 

 water differ, in general, from those living at the depth of 

 twenty or thirty feet, and these again are found to be differ- 

 ent from those which are met with at a greater depth. Their 

 coloring, in particular, varies, according 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 ani- 

 mals are found upon a certain chain of mountains, and not 

 elsewhere ; 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 inde- 

 pendent 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 spe- 

 cies found in the lake may not ascend the rivers emptying 

 into it. It is only when the whole group of animals inhabit- 

 ing such a region has such peculiarities as to give it a dis- 

 tinct character, when contrasted with animals found in sur- 

 rounding 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 are better known. 



411. The migration' of animals might at first seem to 

 present 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 the farthest, it may be laid down as a rule, that they 



