Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/690

672 672 ICHTHYOLOGY [DISTRIBUTION. know that the Indian Discognathus lamta occurs in the reservoirs of Aden (having also found its way to the oppo site African coast), and that the ubiquitous Cyprinodonts thrive in the brackish pools of northern Arabia. In analysing the list of Indian fishes, we find that out of 40 families or groups of freshwater fishes 1 2 are represented in this region, and that 625 species are known to occur in it, or two-sevenths of the entire number of freshwater fishes known. This large proportion is principally due to the development of numerous local forms of Siluroids and Cyprinoids, of which the former show a contingent of about 200, and the latter of about 330 species. The combined development of those two families, therefore, and their undue preponderance over the other freshwater types, is the principal characteristic of the Indian region. The second important character of its fauna is the appar ently total absence of Ganoid and Cyclostomous fishes. Every other region has representatives of either Ganoids or Cyclostomes, some of both. Of the autochthonous freshwater fishes of the Indian region, some are still limited to it, viz., the Nandina, the Luciocephalidce (of which one species only exists in the archipelago), of Siluroids the Chacina and Bagariiua, of Cyprinoids the Semiplotina and Homalopterina ; others are very nearly so, such as the Labyrintkici, Opliioceplialidce, Mastacembelidce, of Siluroids the Silurina, of Cyprinoids the Kasborina and Danionina, and the /Symbranchidce. The regions to which the Indian has least similarity are the North American and the Antarctic, as they are the most distant. Its affinity to the other regions is of very different degrees : 1. Its affinity to the Europe-Asiatic region is veiy slight, and is indicated almost solely by three groups of Cyprinoids, viz. , the Cyprinina, Abramidina, and Cobitidina. The development of these groups north and south of the Himalayas is due to their common origin in the highlands of Asia ; but the forms which descended into the tropical climate of the south are now so different from their northern brethren that most of them are referred to dis tinct genera. The only genera which are still common to both regions are (1) the tme barbels (Sarbtcs), a genus which of all Cyprinoids has the largest range over the Old World, and of which some one hundred and sixty species have been described, and (2) the moun tain barbels (Schizothorax, &c. ), which, peculiar to the alpine waters of Central Asia, descend a short distance only towards the tropical plains, but extend farther into rivers within the northern temperate districts. The origin and the laws of the distribution of the Cobitidina appear to have been identical with those of Barbus, but they have not spread into Africa. 2. There exists a great affinity between the Indian and African regions ; seventeen out of the twenty-six families or groups found in the former are represented by one or more species in Africa, and many of the African species are not even generically different from the Indian. As the majority of these groups have many more representatives in India than in Africa, we may reasonably assume that the African species have been derived from the Indian stock ; but probably this is not the case with the Siluroid group of Clariina, which with regard to species is nearly equally distributed between the two regions, the African species being referable to three genera (Clarias, Heterobranchus, Gymnallabes, with the sub-genus Chan- nallabes), whilst the Indian species belong to two genera only, viz. , Clarias and Heterobranchus. On the other hand, the Indian region has derived from Africa one freshwater form only, viz. , Etroplus, a member of the family of Chromides, so well represented in tropical Africa and South America. Etroplus inhabits southern and western India and Ceylon, and has its nearest ally in a Madagascar freshwater fish, Parctroplics. Considering that other African Chromides have acclimatized themselves at the present day in saline water, we think it more probable that Etroplus should have found its way to India through the ocean than over the connecting land area, where, besides, it does not occur. 3. No closer affinity exists between the Indian and Tropical Ameri can regions than is indicated by the character of the equatorial zone generally. With two exceptions, no genus of freshwater fishes occurs in India and South America without being found in the in termediate African region. Four small Indian Siluroids (Sisor, Erethistcs, Pscudecheneis, and Ejcostomd) have been referred to the South American Hypostomatina ; bu-t it remains to be seen whether this combination is based upon a sufficient agreement of their internal structure, or whether it is not rather artificial. On the other hand, the occurrence and wide distribution in tropical America of a fish of the Indian family Symbranchidce (Symbranchus marmoratus), which is not only congeneric with, but also most closely allied to, the Indian Symbranchus bengalensis, furnishes one of those extraordinary anomalies in the distribution of animala of which no satisfactory explanation can at present be given. 4. The relation of the Indian to the Tropical Pacific region con sists only in its having contributed a few species to the poor fauna of the latter. This immigration must have taken place within a recent period, because some species now inhabit the fresh waters of tropical Australia and the South Sea Islands without having in any way changed their specific characters, as Lutes calca- rifer, species of Dules, Plotosus anguillaris ; others (species of A rius) differ but little from their Indian congeners. All these fishes must have migrated by the sea, a supposition which is supported by what we know of their habits. We need not add that India has not received a single addition to its freshwater fish fauna from the Pacific region. It may be mentioned, before concluding these remarks oh the Indian region, that peculiar genera of Cyprinoids and Siluroids inhabit the streams and lakes of its alpine ranges in the north. Some of them, like the Siluroid genera Glyptosternum, Euglyptostermim, Pseudeckeneis, have a folded disk on the thorax between their horizontally spread pectoral fins ; by means of this they adhere to stones at the bottom of the mountain torrents, and without it they would be swept away into the lower courses of the rivers, The Cyprinoid genera inhabiting similar localities and the lakes into which alpine rivers pass, such as Oreinus, Schizothorax, Ptyckobarbus, Schizopyyopsis, Diptychus, Gymnocypris, are distinguished by peculiarly enlarged scales near the vent, the physiological use of which has not yet been ascertained. These alpine genera extend far into the Europe-Asiatic region, where the climate is similar to that of their southern home. No observations have been made by which the altitudinal limits of fish life in the Himalayas can be fixed, but it is probable that it reaches the line of perpetual snow, as in the European Alps, which at that height are inhabited by Salmonoids. Griffith found an Oreinus and a loach, the former in abundance, in the Helmund at Griclun Dewar, altitude 10,500 feet, and another loach at Kaloo at 11,000 feet. The African Region comprises the whole of the African continent south of the Atlas and the Sahara. It might have been conjectured that the more temperate climate of its southern extremity would have been accompanied by a conspicuous difference in the fish fauna. But this is not the case ; the difference between the tropical and southern parts of Africa consists simply in the gradual disappearance of specifically tropical forms, whilst Siluroids, Cyprinoids, and even Labyrinthici penetrate to its southern coast ; no new form has entered to impart to South Africa a character distinct from the central portion of the continent. In the north-east the African fauna passes the Isthmus of Suez and penetrates into Syria ; the system of the Jordan pre sents so many African types that it has to be included in a description of the African region as well as of the Europo-Asiatic. This river is inhabited by three species of Chromis, one of Hemichromis, and Clarias macr acanthus^ a common fish of the upper Nile. Madagascar clearly belongs to this region. Besides some gobies and Dules, which are not true freshwater fishes, four Chromides are known. To judge from general accounts, its freshwater fauna is poorer than might be expected ; but, singular as it may appear, collectors have hitherto paid but little attention to the freshwater fishes of this island. The fishes found in the freshwaters of the Seychelles and Mascarenes are brackish-water fishes, such as Fundulus, Haplochilus, Elops, Mugil, &c. Out of the 40 families or groups of freshwater fishes 15 are represented in the African region, or three more than in the Indian region ; of two of them, however, viz., the Ophiocephalidce and Mastacembelidce, a few species only