The ways in which the dispersal of Freshwater fishes has been effected were various; they are probably all still in operation, but most work so slowly and imperceptibly as to escape direct observation; perhaps, they will be more conspicuous, after science and scientific inquiry shall have reached to a somewhat greater age. From the great number of freshwater forms which we see at this present day acclimatised in, gradually acclimatising themselves in, or periodically or sporadically migrating into, the sea, we must conclude that, under certain circumstances, salt water may cease to be an impassable barrier at some period of the existence of freshwater species, and that many of them have passed from one river through salt water into another. Secondly, the headwaters of some of the grandest rivers, the mouths of which are at opposite ends of the continents which they drain, are sometimes distant from each other a few miles only; the intervening space may have been easily bridged over for the passage of fishes by a slight geological change affecting the level of the watershed, or even by temporary floods; and a communication of this kind, if existing for a limited period only, would afford the ready means of an exchange of a number of species previously peculiar to one or the other of those river or lake systems. Some fishes, provided with gill-openings so narrow that the water moistening the gills cannot readily evaporate; and endowed, besides, with an extraordinary degree of vitality, like many Siluroids (Clarias, Callichthys), Eels, etc., are enabled to wander for some distance over land, and may thus reach a water-course leading them thousands of miles from their original home. Finally, fishes or their ova may be accidentally carried by waterspouts, by aquatic birds or insects, to considerable distances.
Freshwater fishes of the present fauna were already in existence when the great changes of the distribution of land and water took place in the tertiary epoch; and having stated that salt water is not an absolute barrier to the spreading of Freshwater fishes, we can now more easily account for those instances of singular disconnection of certain families or genera. It is not necessary to assume that there was a continuity of land stretching from the present coast of Africa to South America, or from South America to New Zealand and Australia, to explain the presence of identical forms at so distant localities; it suffices to assume that the distances were lessened by intervening archipelagoes, or that an oscillation has taken place in the level of the land area.
Dispersal of a type over several distant continental areas may be evidence of its great antiquity, but it does not prove that it is of greater antiquity than another limited to one region only. Geological evidence is the only proof of the antiquity of a type. Thus, although the Dipnoi occur on the continents of Africa, South America, and Australia, and their present distribution is evidently the consequence of their wide range in palæozoic and secondary epochs; the proof of their high antiquity can be found in their fossil remains only. For, though the Siluroids have a still greater range, their wide distribution is of comparatively recent date, as the few fossil remains that have been found belong to the tertiary epoch. The rapidity of dispersal of a type depends entirely on its facility to accommodate itself to a variety of physical conditions, and on the degree of vitality by which it is enabled to survive more or less sudden changes under unfavourable conditions; proof of this is afforded by the family of Siluroids, many of which can suspend for some time the energy of their respiratory functions, and readily survive a change of water.
To trace the geological sequence of the distribution of an ichthyic type, and to recognise the various laws which have governed, and are still governing its dispersal, is one of the ultimate tasks of Ichthyology. But the endeavour to establish by means of our present fragmentary geological knowledge the divisions of the fauna of the globe, leads us into a maze of conflicting evidence; or, as Mr. Wallace truly observes, “any attempt to exhibit the regions of former geological ages in combination with those of our own period must lead to confusion.” Nevertheless, as the different types of animals found at the present day within a particular area have made their appearance therein at distant periods, we should endeavour to decide as far as we can, in an account of the several zoo-geographical divisions, the following questions:—
1. Which of the fishes of an area should be considered to be the remnants of ancient types, probably spread over much larger areas in preceding epochs?
2. Which of them are to be considered to be autochthont species, that is, forms which came in the tertiary epoch or later into existence within the area to which they are still limited, or from which they have since spread?
3. Which are the forms which must be considered to be immigrants from some other region?
The mode of division of the earth’s surface into zoological regions or areas now generally adopted, is that proposed by Mr. Sclater, which recommends itself as most nearly agreeing with the geographical divisions. These regions are as follows:—
I. Palæogæa.