In an account of the geographical distribution of fishes the _Freshwater_ forms are to be kept separate from the _Marine_. 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 accommodate 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, Retropinna, 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 _brackish_ 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 _Serranus_, _Sciænidæ_, _Pleuronectes_, _Clupeidæ_, _Tetrodon_, _Carcharias_, _Trygonidæ_), ascending for hundreds of miles of their course; and not periodically, or from any apparent 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-tseKiang, the Amazons, the Congo, etc. This is evidently the commencement of a change in a fish’s habits, and, indeed, not a few of such fishes have actually taken up their permanent residence in fresh waters (as species of Ambassis, Apogon Dules, Therapon, Sciæna, Blennius, Gobius, Atherina, Mugil, Myxus, Hemirhamphus, Clupea, Anguilla, Tetrodon, Trygon): all forms _originally marine_.