3o6 GENERAL LAWS OF DISTRIBUTION. 



but. it is doubtful if animals could sustain the pressure of so 

 great a column of water, although many of them are provided 

 with a system of pores ( 403), which enables them to sustain 

 * much greater pressure than terrestrial animals. 



586. When there is no great natural limit, the transition 

 from one fauna to another is made insensibly. Thus, in pass- 

 ing from the arctic to the temperate regions of North America, 

 one species takes the place of another, a third succeeds the 

 second, and so on, until finally the fauna is found to be an 

 entirely new one, without its being always possible to mark 

 the precise limit between the two. 



587. The range of species does not at all depend upon 

 their powers of locomotion ; if it were so, animals which 

 move slowly and with difficulty would have a narrow range, 

 whilst those which are very active would be widely diffused. 

 Precisely the reverse of this is actually the case. The com- 

 mon oyster extends at least from Cape Cod to the Carolinas ; 

 its range is consequently very great ; much more so than that 

 of some of the fleet animals, as, for instance, the moose. It 

 is even probable that the very inability of the oyster to travel, 

 really contributes to its diffusion, inasmuch as having once 

 spread over extensive grounds, their is no chance of its return 

 to a former limitation, being fixed, and consequently unable . 

 to choose positions for its eggs, they must be left to the mercy 

 of currents ; while fishes, by depositing their eggs in the 

 bays and inlets of the shore, undisturbed by currents and 

 winds, secure them from too wide a dispersion. 



588. The nature of their food has an important bearing 

 upon the grouping of animals, and upon the extent of their 

 distribution. Carnivorous animals are generally less confined 

 in their range than herbivorous ones ; because their food is 

 almost everywhere to be found. The herbivora, on the othei 

 hand, are restricted to the more limited regions correspond- 

 ing to the different zones of vegetation. The same remark 

 may be made with respect to birds. Birds of prey, like 

 the eagle and vulture, have a much wider range than the 

 granivorous and gallinaceous birds. Still, notwithstanding 

 the facilities they have for change of place, even the birds 

 that wander widest recognize limits which they do not over- 

 pass. The condor of the Cordilleras does not descend into 

 the temperate regions of the United States ; and yet it is not 

 that he fears the cold, since he is frequently known to ascend 



