256 FISHES. 



independent of the variable local conditions which tie the 

 shore fish to its original home, and therefore roam freely 

 over a space which would take a freshwater or shore fish 

 thousands of years to cover in its gradual dispersal. Such 

 as are devoid of rapidity of motion are dispersed over simi- 

 larly large areas by the oceanic currents, more slowly than, 

 but as surely as, the strong swimmers. Therefore, an accu- 

 rate definition of their distribution within certain areas equi- 

 valent to the terrestrial regions is much less feasible than in 

 the case of shore fishes. 



3. Deep-sea Fishes — that is, fishes which inliabit such 

 depths of the ocean as to be but little or not infiuenced by 

 light or the surface temperature ; and which, by their organ- 

 isation are prevented from reaching the surface stratum in a 

 healthy condition. Living almost under identical tellurian 

 conditions, the same type, the same species, may inhabit an 

 abyssal depth under the equator as well as one near the 

 arctic or antarctic circle ; and all we know of these fishes 

 points to the conclusion that no separate horizontal regions 

 can be distinguished in the abyssal fauna, and that no 

 division into bathymetrical strata can be attempted on the 

 base of generic much less of family characters. 



It must not be imagined that these three categories are 

 more sharply defined than Freshwater and Marine Fishes. 

 They gradually pass into each other, and there are numerous 

 fishes about which uncertainty exists whether they should 

 be placed in the Shore or Pelagic series, or in the Pelagic or 

 Deep-sea series ; nay, many facts favour the view that changes 

 in the mode of life and distribution of fishes are still in 

 progress. 



The change in habitat of numerous fishes is regulated by 

 the distribution of their favourite food. At certain seasons 

 the surface of the sea in the vicinity of land swarms with 

 moUusks, larval Crustaceans, Medusae, attracting shoals of 



