52 CLASSIFICATIOX OF FISHES. 



many facts that could be mentioned^ we believe tbat the 

 geographic range of the certain genera and species is 

 much more definite than has hitherto been supposed. 

 Several of the Mediterranean species, which are uni- 

 versally believed to inhabit the seas of Tropical America, 

 we consider to be truly distinct ; and similar differ- 

 ences maybe detected even between the fish of Northern 

 and of Southern Europe. One great cause of the sup- 

 position that the same species so frequently inhabits 

 widely separated shores, is the fact that this class is less 

 affected by temperature than any other vertebrated ani- 

 mals ; and it is therefore inferred that the similar species 

 may exist both in temperate and tropical latitudes : this 

 may seem to be true ; but then the question arises, whether 

 their peculiar food is also found in the same seas ? The 

 majority of fishes are carnivorous ; and it may be said, 

 that as small fish are to be found every where, the larger 

 can prey upon them ; but such is not precisely the fact. 

 We know that every family, nay, almost every species, 

 of insectivorous birds, feeds only upon certain genera of 

 insects ; and all we know, both from fact and analogy, 

 favours the idea that carnivorous fishes are limited in 

 their choice of food by similar laws : indeed, this behef 

 almost amounts to absolute certaintv, when we consider 

 that different tribes are generally found restricted to dif- 

 ferent depths and descriptions of sub-marine soils. This 

 fact has been so ably illustrated by colonel Smith, that 

 it need not be insisted upon in this place. Now, it is 

 quite evident that this allotment of particular depths or 

 localities is an instinct given to them for frequenting 

 those situations, and those only, where they are sure of 

 finding their congenial food. We may even suppose 

 that such as live upon testaceous Mollusca and crusta- 

 ceous insects are more limited in their range than those 

 which live upon young fish, because the former animals 

 are more limited in their distribution than the latter: 

 again, the soft pelagic 3IoUusca are more widely dis- 

 tributed than shells or Crustacea ; and, therefore, those fish 

 which fed upon them would enjoy a greater range than 



