Thus there is a constant exchange of species in progress between the freshwater and marine faunæ, and in not a few cases it would seem almost arbitrary to refer a genus or even larger group of fishes to 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, offers to the rapid spreading 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. Without entering into 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 other 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 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; whilst it ceases to exercise its influence in proportion 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 distribution, but it contributes towards constituting 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 fresh water 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 accessible to observation than the latter, and are, therefore, more generally and more readily comprehended and acknowledged. Thus, not only because many