DISTRIBUTION.] On the other hand, we find fishes belonging to fresh water genera descending rivers and sojourning in the sea for a more or less limited period ; but these instances are much less in number than those in which the reverse obtains. We may mention species of Salmo (the common trout, the northern charr), and Siluroids (as Arius, Plotosus), Coregonus, a genus so characteristic of the inland lakes of Europe, Northern Asia, and North America, nevertheless offers some instances of species wandering by the effluents into the sea, and taking up their residence in salt water, apparently by preference, as Coregonus oxy- rhynchus. But of all the freshwater families none exhibit so great a capability of surviving the change from fresh into salt water as the Gastrosteidoe (sticklebacks) of the northern hemisphere, and the equally diminutive Cyprino- dontidce of the tropics ; not only do they enter into, and live freely in, the sea, but many species of the latter family inhabit inland waters, which, not having an outlet, have become briny, or impregnated with a larger proportion of salts than pure sea- water. During the voyage of the " Challenger " a species of Funduius (F. nigrofasciatus) which inhabits the fresh and the brackish waters of the Atlantic States of North America was obtained, with Scopelids and other pelagic forms, in the tow-net, midway between St Thomas and Teneriffe. Some fishes annually or periodically ascend rivers for the purpose of spawning, passing the rest of the year in the sea, as sturgeons, many Salmonoids, some Clupeoids, lampreys, &c. The first two evidently belonged originally to the freshwater series, and it was only in the course of their existence that they acquired the habit of descending to the sea, perhaps because their freshwater home did not furnish a sufficient supply of food. These migrations of freshwater fishes have been compared with the migrations of birds ; but they are much more limited in extent, and do not, as is the case with birds, impart an additional element to the fauna of the place to which the fishes migrate. The distinction between freshwater and marine fishes is further obscured by geological changes, in consequence of which the salt water is gradually being changed into fresh, or vice versa. These changes are so gradual, and spread over so long a time, that many of the fishes inhabiting such localities accommodate themselves to the new con ditions. One of the most remarkable and best studied instances of such an alteration is the Baltic, which, during the second half of the Glacial period, was in open and wide communication with the Arctic Ocean, and evidently had the same marine fauna as the White Sea. Since then, by the rising of the land of Northern Scandinavia and Finland, this great gulf of the Arctic Ocean has become an inland sea, with a narrow outlet into the North Sea, and, in consequence of the excess of fresh water pouring into it over the loss by evaporation, it has been so much diluted as to be nearly fresh at its northern extremities ; and yet nine species, the origin of which from the Arctic Ocean can be proved, have survived the changes, propa gating their species, and agreeing with their brethren in the Arctic Ocean in every point, but remaining comparatively smaller. On the other hand, fishes which we must regard as true freshwater fishes, like the rudd, roach, pike, perch, enter freely the brackish watex of the Baltic. In stances of marine fishes being permanently retained in fresh water in consequence of geological changes are well known : as Coitus quadricornis in the large lakes of Scandinavia ; species of Gobius, Blennius, and Atherina in the lakes of northern Italy ; ComephorMs, which seems to be a dwarfed Gadoid, in the depths of Lake Baikal. Car- charias gangeticus in inland lakes of the Fiji Islands, is another instance of a marine fish which has permanently established itself in fresh water. 669 Thus there is a constant interchange of species in pro gress between the freshwater and marine faunae, and in not a few cases it would seem almost arbitrary to refer a genus or even a larger group of fishes to the one or the other ; yet there are certain groups of fishes which entirely, or with but few exceptions, are,, and apparently during the whole period of their existence have been, inhabitants either of the sea or of fresh water; and, as the agencies operating upon the distribution of marine fishes differ greatly from those influencing the dispersal of freshwater fishes, the two series must be treated separately. The most obvious fact that dry land, which intervenes between river systems, pre sents to the rapid spreading of a freshwater fish an obstacle j which can be surmounted only exceptionally or by a most i circuitous route, whilst marine fishes may readily and voluntarily extend their original limits, could be illustrated I by a great number of instances. Without entering into I details, it may suffice to state, as the general result, that no species or genus of freshwater fishes has anything like the immense range of the corresponding categories of marine fishes, and that, with the exception of the Siluroids, no freshwater family is so widely spread 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 tbe distribution of marine fishes, but in a less degree, and only in the case of those which live near the shore or the surface of the ocean ; it ceases to exercise its influence in proper, tion to the depth, the true deep-sea forms being entirely exempt from its operation. Light, which is pretty equally distributed over the localities inhabited by freshwater fishes, cannot be considered as an important factor in their I distribution, but it contributes to the formation of the impassable barrier between the surface 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 conditions of minor and local importance, under which freshwater fishes live, and by which their dispersal is regulated, are more complicated than similar ones of the ocean, is probable, though perhaps less so than is generally supposed ; for the fact is that the former are more acces sible to observation than the latter, and are therefore more generally and more readily comprehended and acknowledged It will thus be necessary to treat of the two series separately, not only because mqny of the most characteristic forms of the marine and the freshwater series < are found, on taking a broader view of the subject, to be j sufficiently distinct, but also because their distribution j depends on causes different in their nature as well as in the degree of their action. Whether the oceanic areas^corre- spund in any way to the terrestrial will be seen in the sequel. FRESHWATER FISHES. Having shown above that numerous marine fishes enter fresh waters, and that some of them have permanently established themselves therein, we have to eliminate from the category of freshwater fishes all such adventitious elements. They are derived from forms the distribution of which is regulated by other agencies, and which therefore would obscure the relations of the faunce of terrestrial
regions if they were included in them. They will bo
