206 FISHES. 



intervenes between river systems, offers to the rapid spread- 

 ing of a freshwater fish an obstacle which can be surmounted 

 only exceptionally or by a most circuitous route, whilst marine 

 fishes may readily and voluntarily extend their original limits, 

 could be illustrated by a great number of instances. With- 

 out entering into details, it may suffice to state as the general 

 result, that no species or genus of freshwater fishes has any- 

 thing like the immense range of the corresponding categories 

 of marine fishes ; and that, with the exception of the Siluroids, 

 no other freshwater family is so widely sj)read as the families 

 of marine fishes. Surface temperature or climate which is, if 

 not the most, one of the most important physical factors in the 

 limitation of freshwater fishes, similarly affects the distribution 

 of marine fishes, but in a less degree, and only those which 

 live near to the shore or the surface of the ocean ; wliilst it 

 ceases to exercise its influence in proportion to the depth, 

 the true deep-sea forms being entirely exempt from its opera- 

 tion. Light, which is pretty equally distributed over the 

 localities inhabited by freshwater fishes, cannot be considered 

 as an important factor in their distribution, but it contributes 

 towards constituting the impassable barrier between the sur- 

 face and abyssal forms of marine fishes. Altitude has stamped 

 the fishes of the various Alpine provinces of the globe with a 

 certain character, and limited their distribution; but the 

 number of these Alpine forms is comparatively small, ichthyic 

 life being extinguished at great elevations even before the 

 mean temperature equals that of the high latitudes of the 

 Arctic region, in which some freshwater fishes flourish. On 

 the other hand, the depths of the ocean, far exceeding the 

 altitude of the highest mountains, still swarm with forms 

 specially adapted for abyssal life. That other physical con- 

 ditions of minor and local importance, under which fresh 

 water fishes live, and by which their dispersal is regulated, 

 are more complicated than similar ones of the ocean, is 



