Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/688

670 670 ICHTHYOLOGY [DISTRIBUTION. mentioned with greater propriety along with the fishes constituting the fauna of brackish water. The true freshwater fishes are all embraced in the fol lowing families and groups : Dipnoi, with 4 species ; Acipenseridce and Polyodontidfe, 26 ; AmiidiB, 1 ; Polypteridue, 2 ; Lepidosteidse, 3 ; Percina, 46 ; Grystina, 11 ; Aphredoderidse, 1 ; Centrarchina, 26 ; Dules, 10 ; Nandidse, 7 ; Polycentridse, 3 ; Labyrinthici, 30 ; Luciocephalitke, ] ; Gastrosteus, 10 ; Ophiocepluilidre, 31 ; Mastacembelidae, 13 ; Cliromides, 105 ; Comephoridse, 1 ; Gadopsidse, 1 ; Siluridse, 572 ; Characiuidse, 261 ; Haplochitonidte, 3 ; Salmonidaj (3 genera ex- cepted), 135 ; Percopsidae, 1 ; Galaxiidce, 15 ; MormyridiB (and Gymmirchidae), 52 ; Esocidse, 8 ; Umbridte, 2 ; Cyprinodontida?, 112 ; Heteropygii, 2 ; Cyprinidffi, 724 ; Kneriidse, 2 ; Hyodontidse, 1 ; Pantodontidse, 1; Osteoglossidse, 5 ; Notopteridse, 5 ; Gymno- tidte, 20 ; Symbraiichidse, 5 ; Petromyzoutidte, 12. Total, 2270 species. As in every other class of animals, these freshwater genera and families vary exceedingly with regard to the extent of their geographical range, some extending over more than the half of the continental areas, whilst others are limited to one continent only, or even to a very small portion of it. As a general rule, a genus or family of freshwater fishes is regularly dispersed and most developed within a certain district, the species and individuals becoming fewer towards the periphery as the type recedes more from its central home, some outposts, however, fre quently being pushed far beyond the outskirts of the area occupied by it. At the same time remarkable instances exist of closely allied forms occurring, almost isolated, at most distant points, without being connected by allied species in the intervening space, and of members of the same family, genus, or species inhabiting the opposite shores of an ocean, and separated by many degrees of abyssal depths. The dispersal of freshwater fishes has been effected in various ways ; probably all the causes are still in operation, most of them working so slowly and imperceptibly as to escape direct observation. From the great number of fresh water forms which we see at the present day already acclimatized or gradually becoming acclimatized in the sea, or periodically or sporadically migrating to it, 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 largest 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 easily have been 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 for an exchange of a number of species prtri- ously 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 Siluroicls (Clarias, Callichthys), eels, &c., are enabled to wander for some distance over land, and thus may reach a watercourse leading them thousands of miles from their original home. Finally, fishes or their ova may be accidentally carried by waterspouts, or by aquatic birds or insects, to considerable distances. Freshwater fishes of the present fauna were already in existence when the great changes in the distribution of land and water took place in the Tertiary epoch; and, having seen 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 disconnexion of certain families or genera. It is not necessary to assume that there was a continuity of laud 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 in localities so distant ; it suffices to assume that the distances were lessened by intervening archipelagoes, or that an alteration 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 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 in the continents of Africa, South America, and Australia, and their present distribution is evidently the consequence of their wide range in Palaeozoic and Secondary epochs, the proof of their high antiquity can be found in their fossil remains only. The Siluroids, for example, have a still grea J 3dr range, but their wide distribution is of compara tively recent date, as the few fossil remains that have been found belong to the Tertiary epoch. The rapidity of dis persal of a type depends entirely on its power to accommo date 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 re spiratory functions, and readily survive a change of water. To trace the geological sequence of the distribution of an ichthyic type, and to recognize 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 ; as Mr Wallace truly observes, &quot;any attempt to exhibit the regions of former geological ages in combination with those of our own period must lead to confusion.&quot; 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, in giving an account of the several zoo-geographical divisions, to decide, so far as we can, 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 1 2. Which are to be considered to be autochthonous species, that is, forms which in the Tertiary epoch or later came into existence within the area to which they are still limited, or from which they have since spread 1 ? 3. Which are the forms which must be considered to be immigrants from some other region 1 It is the aim of every philosophical classification to indicate the degrees of affinity which obtain between the various categories. In dividing the earth s surface into zoological regions, the two families, Cyprinidce and Siluridce, the former of which yields a contingent of one-third and the latter of one-fourth of all the known freshwater species of our period, afford most important guidance for the estima tion of those degrees of affinity. The Cyprinoids may be assumed to have originated in the alpine region dividing the temperate and tropical parts of Asia ; endowed with a greater capability than any other family of freshwater fishes of acclimatizing themselves in a temperate as well as in a tropical region, they spread north and south as well as east and west ; in the Pre-Glacial epoch they reached North America, but they have not had time to penetrate into South America, Australia, or the islands of the Pacific. The Siluroids, principally fishes of the sluggish waters of the plains, well adapted for surviving changes of the water in which they live, and for living either in mud or in sea- water, flourish most in the tropical climate in which this