158 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



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

 passing 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. 



405. 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 being 

 once removed, it is difficult for it to return ; and more- 

 over, being fixed, and consequently unable to choose posi- 

 tions 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. 



406. 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 con- 

 fined in their range than herbivorous ones ; because their 

 food is almost everywhere to be found. The herbivora, on 

 the other hand, are restricted to the more limited regions 

 corresponding 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 



