Of the autochthont freshwater fishes of the Indian region, some are still limited to it, viz., the Nandina, the Luciocephalidæ (of which one species only exists in the Archipelago), of Siluroids the Chacina and Bagariina, of Cyprinoids the Semiplotina and Homalopterina; others very nearly so, like the Labyrinthici, Ophiocephalidæ, Mastacembelidæ, of Siluroids the Silurina, of Cyprinoids the Rasborina and Danionina, and Symbranchidæ.
The regions with which the Indian has least similarity are the North American and Antarctic, as they are the most distant. Its affinity to the other regions is of a very different degree:—
1. Its affinity to the Europo-Asiatic region 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 distinct from their northern brethren that most of them are referred to distinct genera. The genera which are still common to both regions are only the true Barbels (Barbus), a genus which, of all Cyprinoids, has the largest range over the old world, and of which some 160 species have been described; and, secondly, the Mountain Barbels (Schizothorax, etc.), 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.
If, in determining the degree of affinity between two regions, we take into consideration the extent in which an exchange has taken place of the faunæ originally peculiar to each, we must estimate that obtaining between the freshwater fishes of the Europo-Asiatic and Indian regions as very slight indeed.
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 this is probably 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 subgenus Channallabes), 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 Madegasse Freshwater fish, Paretroplus. Considering that other African Chromides have acclimatised 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. A closer affinity between the Indian and Tropical American regions than is indicated by the character of the equatorial zone generally, does not exist. No genus of Freshwater fishes occurs in India and South America without being found in the intermediate African region, with two exceptions. Four small Indian Siluroids (Sisor, Erethistes, Pseudecheneis, and Exostoma) have been referred to the South American Hypostomatina; but 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 Symbranchidæ (Symbranchus marmoratus), which is not only congeneric with, but also most closely allied to, the Indian Symbranchus bengalensis, offers one of those extraordinary anomalies in the distribution of animals of which no satisfactory explanation can be given at present.
4. The relation of the Indian region to the Tropical Pacific region consists 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 fresh waters of tropical Australia and the South Sea Islands without having in any way changed their specific characters, as Lates calcarifer, species of Dules, Plotosus anguillaris; others (species of Arius) are but little different from 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.
Before concluding these remarks on the Indian region, we must mention 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, Euglyptosternum, Pseudecheneis, 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 the alpine rivers pass, such as Oreinus, Schizothorax, Ptychobarbus, Schizopygopsis, 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 Europo-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 are inhabited by Salmonoids. Griffith found an Oreinus and a Loach, the former in abundance, in the Helmund at Gridun Dewar, altitude 10,500 feet; and another Loach at Kaloo at 11,000 feet.
B. 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 of 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 presenting 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 macracanthus, a common fish of the Upper Nile. This infusion of African forms cannot be accounted for by any one of those accidental means of dispersal, as Hemichromis is not represented in the north-eastern parts of Africa proper, but chiefly on the west coast and in the Central African lakes.