CHAPTEE XVI. 



THE DISTRIBUTION OF EXISTING FISHES OVER THE 

 earth's surface — GENERAL REMARKS. 



In an account of tlie geographical distribution of fishes the 

 Freshwater forms are to be kept separate from the Marme. 

 However, when we attempt to draw a line between these two 

 kinds of fishes, we meet with a great number of species and 

 of facts which would seem to render that distinction very 

 vague. There are not only species which can gradually accom- 

 modate themselves to a sojourn in either salt or fresh water, but 

 there are also such as seem to be quite indifferent to a rapid 

 change from one into the other : so that individuals of one 

 and the same species (Gastrosteus, Gobius, Blennius, Osmerus, 

 Eetropinna, Clupea, Syngnathus, etc.), may be found at some 

 distance out at sea, whilst others live in rivers far beyond the 

 influence of the tide, or even in inland fresh waters without 

 outlet to the sea. The majority of these fishes belong to 

 forms of the fauna of the hracldsh water, and as they are not 

 an insignificant portion of the fauna of almost every coast, 

 we shall have to treat of them in a separate chapter. 



Almost every large river offers instances of truly marine 

 fishes (such as Scrranus, Sciccnidcv, Pleuronedes, Clupeidce, 

 Tetrodon, Carcharias, Trygonidm), ascending for hundreds of 

 miles of their course ; and not periodically, or from any appa- 

 rent physiological necessity, but sporadically throughout the 

 year, just like the various kinds of marine Porpoises which 

 are found all along the lower course of the Ganges, Yang-tse- 



