GENERAL LAWS OF DISTRIBUTION. 36? 



even above the highest summits of the Andes, and disappears 

 from view where the cold is most intense. Nor can it be from 

 lack of prey. 



589. Again, the peculiar configuration of a country some- 

 times determines a peculiar grouping of animals into what 

 may be called local faunas. Such, for example, are the prai- 

 ries of the West, the pampas of South America, the steppes 

 of Asia, the deserts of Africa ; and for marine animals, the 

 basin of the Caspian. In all these localities, animals are met 

 with which exist only there, and are not found except under 

 those particular conditions. 



590. Finally, to obtain a true picture of the zoological 

 distribution of animals, not the terrestrial types alo-ne, but the 

 marine species must also be included. Notwithstanding the 

 uniform nature of the watery element, the animals which dwell 

 in it are not dispersed at random ; and though the limits of 

 the marine may be less easily denned than those of the terres- 

 trial fauna, still marked differences between the animals of great 

 basins are not less observable. Properly to apprehend how 

 marine animals may be distributed into local faunas, it must 

 be remembered that their residence is not in the high sea, but 

 along the coasts of continents and on soundings. It is on the 

 Banks of Newfoundland, and not in the deep sea, that the 

 great cod-fishery is carried on ; and it is well known that when 

 fishes migrate, they run along the shores. The range of 

 marine species being therefore confined to the vicinity of the 

 shores, their distribution must be subjected to laws similar 

 to those which regulate the terrestrial faunas. As to the 

 fresh-water fishes, not only do the species vary in the dif- 

 ferent zones, but even the different rivers of the same region 

 have species peculiar to them, and not found in neighbouring 

 streams. The gar-pikes, Lepidosteus, of the American rivers, 

 afford a striking example of this kind. 



591. A very influential cause in the distribution of aqua- 

 tic 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 dif- 

 ferent zones at different elevations in ascending mountains. 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 colouring, 



