These four regions diverge into two well-marked divisions, one of which is characterised by the presence of Cyprinoid fishes, combined with the development of Labyrinthici; whilst in the other both these types are absent. The boundary between the Cyprinoid and Acyprinoid division seems to follow Wallace’s line, a line drawn from the south of the Philippines between Borneo and Celebes, and farther south between Bali and Lombock. Borneo abounds in Cyprinoids; from the Philippine Islands a few only are known at present, and in Bali two species have been found; but none are known from Celebes or Lombock, or from islands situated farther east of them.[20]

Taking into consideration the manner in which Cyprinoids and Siluroids have been dispersed, we are obliged to place the Indian region as the first in the order of our treatment; and indeed the number of its freshwater fishes, which appear to have spread from it into the neighbouring regions, far exceeds that of the species which it has received from them.

A. The Indian Region comprises the whole continent of Asia south of the Himalayas and the Yang-tse-kiang; it includes the islands to the west of Wallace’s line. Towards the north-east the island of Formosa, which also by other parts of its fauna leans more towards the equatorial zone, has received some characteristic Japanese Freshwater fishes, for instance, the singular Salmonoid Plecoglossus. Within the geographical boundaries of China the Freshwater fishes of the tropics pass gradually into those of the northern zone, both being separated by a broad debateable ground. The affluents of the great river traversing this district are more numerous from the south than from the north, and carry the southern fishes far into the temperate zone. The boundary of this region towards the north-west is scarcely better defined. Before Persia passed through the geological changes by which its waters were converted into brine and finally dried up, it seems to have been inhabited by many characteristic Indian forms, of which a few still survive in the tract intervening between Afghanistan and Syria; Ophiocephalus and Discognathus have each at least one representative, Macrones has survived in the Tigris, and Mastacembelus has penetrated as far as Aleppo. Thus, Freshwater fishes belonging to India, Africa, and Europe, are intermingled in a district which forms the connecting link between the three continents. Of the freshwater fishes of Arabia we are perfectly ignorant; so much only being known that the Indian Discognathus lamta occurs in the reservoirs of Aden, having, moreover, found its way to the opposite African coast; and that the ubiquitous Cyprinodonts flourish in brackish pools of Northern Arabia.

The following is the list of the forms of freshwater fishes inhabiting this region:[21]—

Percina—
Lates[22] [Africa, Australia]1species.
Nandina7„
Labyrinthici [Africa]25„
Luciocephalidæ1„
Ophiocephalidæ [1 species in Africa]30„
Mastacembelidæ [3 species in Africa]10„
Chromides [Africa, South America]
Etroplus2„
Siluridæ—
Clariina [Africa]12„
Chacina3„
Silurina [Africa, Palæarct.]72„
Bagrina [Africa]50„
Ariina [Africa, Australia, South America]40„
Bagariina20„
Rhinoglanina [Africa]1„
Hypostomatina [South America]5„
Cyprinodontidæ—
Carnivoræ [Palæarct., North America, Africa, South America]
Haplochilus [Africa, South America, North America, Japan]4„
Cyprinidæ [Palæarct., N. America, Africa]—
Cyprinina [Palæarct., N. America, Africa]190„
Rasborina [Africa, 1 species]20„
Semiplotina4„
Danionina [Africa]30„
Abramidina [Palæarct., N. Amer., Africa]30„
Homalopterina10„
Cobitidina [Palæarct.]50„
Osteoglossidæ [Africa, Australia, S. America]1„
Notopteridæ [Africa]3„
Symbranchidæ—
Amphipnous1„
Monopterus1„
Symbranchus [1 species in S. America]2„
625species.

In analysing this list we find that out of 39 families or groups of freshwater fishes 12 are represented in this region, and that 625 species are known to occur in it; a number equal to 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, and their undue preponderance over the other freshwater types, is therefore the principal characteristic of the Indian region. The second important character of its fauna is the apparently total absence of Ganoid and Cyclostomous fishes. Every other region has representatives of either Ganoids or Cyclostomes, some of both. However, attention has been directed to the remarkable coincidence of the geographical distribution of the Sirenidæ and Osteoglossidæ, and as the latter family is represented in Sumatra and Borneo, it may be reasonably expected that a Dipnoous form will be found to accompany it. The distribution of the Sirenidæ and Osteoglossidæ is as follows:—

Tropical America.
Lepidosiren paradoxa. Osteoglossum bicirrhosum.
Arapaima gigas.
Tropical Australia.
Ceratodus forsteri. Osteoglossum leichardti.
Ceratodus miolepis.
East Indian Archipelago.
? Osteoglossum formosum.
Tropical Africa.
Protopterus annectens. Heterotis niloticus.

Not only are the corresponding species found within the same region, but also in the same river systems; and although such a connection may and must be partly due to a similarity of habit, yet the identity of this singular distribution is so striking that it can only be accounted for by assuming that the Osteoglossidæ are one of the earliest Teleosteous types which have been contemporaries of and have accompanied the present Dipnoi since or even before the beginning of the tertiary epoch.