I. Equatorial Zone.

Roughly speaking, the borders of this zoological zone coincide with the geographical limits of the tropical zone, the tropics of the Cancer and Capricorn; its characteristic forms, however, extend in undulating lines several degrees north and southwards. Commencing from the west coast of Africa the desert of the Sahara forms a well-marked boundary between the equatorial and northern zones; as the boundary approaches the Nile it makes a sudden sweep towards the north as far as Northern Syria (Mastacembelus, near Aleppo, and in the Tigris; Clarias and Chromides, in the lake of Galilee); crosses through Persia and Afghanistan (Ophiocephalus), to the southern ranges of the Himalayas, and follows the course of the Yang-tse-Kiang, which receives its contingent of equatorial fishes through its southern tributaries. Its continuation through the North Pacific may be considered to be indicated by the tropic which strikes the coast of Mexico at the southern end of the Gulf of California. Equatorial types of South America are known to extend so far northwards; and by following the same line the West India Islands are naturally included in this zone.

Towards the south the equatorial zone embraces the whole of Africa and Madagascar, and seems to extend still farther south in Australia, its boundary probably following the southern coast of that continent; the detailed distribution of the freshwater fishes of South-Western Australia has been but little studied, but the few facts which we know show that the tropical fishes of Queensland follow the principal water-course of that country, the Murray River, far towards the south and probably to its mouth. The boundary-line then stretches northwards of Tasmania and New Zealand, coinciding with the tropic until it strikes the western slope of the Andes, on the South American Continent, where it again bends southwards to embrace the system of the Rio de la Plata.

The equatorial zone is divided into four regions:—

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.

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.

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, etc.

The following is the list of the forms of Freshwater fishes inhabiting this region:—

Dipnoi [Australia, Neotrop.]—
Lepidosiren annectens1species.
Polypteridæ2„
Percina (Cosmopol.)—
Lates [India, Australia]1„
Labyrinthici [India]5„
Ophiocephalidæ [India]1„
Mastacembelidæ [India]3„
Chromides [South America]—
Chromis23„
Hemichromis5„
Paretroplus1„
Siluridæ—
Clariina [India]14„
Silurina [India, Palæarct.]11„
Bagrina [India]10„
Pimelodina [South America]2„
Ariina[23] [India, Australia, S. Amer., Patagonia]4„
Doradina [South America]—
Synodontis15„
Rhinoglanina [India]2„
Malapterurina3„
Characinidæ [South America]—
Citharinina2„
Nannocharacina2„
Tetragonopterina—
Alestes14„
Crenuchina—
Xenocharax1„
Hydrocyonina—
Hydrocyon4„
Distichodontina10„
Ichthyborina2„
Mormyridæ (Gymnarchidæ)51„
Cyprinodontidæ—
Carnivoræ [Palæarct., India, S. America—
Haplochilus [India, South America]7„
Fundulus [Palæarct., Nearct.]1„
Cyprinidæ [Palæarct., India, North America]—
Cyprinina [Palæarct., India, N. America—
Labeo [India]6„
Barynotus [India]2„
Abrostomus2„
Discognathus lamta[24] [India]1„
Barbus [Palæarct., India]35„
Rasborina [India]1„
Danionina [India]—
Barilius [India]3„
Abramidina [Palæarct., India, N. America]—
Pelotrophus2„
Kneriidæ2„
Osteoglossidæ [India, Australia, South America]—
Heterotis1„
Pantodontidæ1„
Notopteridæ [India]2„
255species.

Out of the 39 families or groups of freshwater fishes 15 are represented in the African region, or three more than in the Indian region; however of two of them, viz., the Ophiocephalidæ and Mastacembelidæ, a few species only have found their way into Africa. On the other hand, the number of species is much less, viz. 255, which is only two-fifths of that of the known Indian species. The small degree of specialisation and localisation is principally due to the greater uniformity of the physical conditions of this continent, and to the almost perfect continuity of the great river systems, which take their origin from the lakes in its centre. This is best shown by a comparison of the fauna of the Upper Nile with that of the West African rivers. The number of species known from the Upper Nile amounts to 56, and of these not less than 25 are absolutely identical with West African species. There is an uninterrupted continuity of the fish-fauna from west to the north-east, and the species known to be common to both extremities may be reasonably assumed to inhabit also the great reservoirs of water in the centre of the continent. A greater dissimilarity is noticeable between the west and north-east fauna on the one hand, and that of the Zambezi on the other; the affinity between them is merely generic; and all the fishes hitherto collected in Lake Nyassa have proved to be distinct from those of the Nile, and even from those of other parts of the system of the Zambezi.

Africa, unlike India, does not possess either alpine ranges or outlying archipelagoes, the fresh waters of which would swell the number of its indigenous species; but at a future time, when its fauna is better known than at present, it is possible that the great difference in the number of species between this and the Indian regions may be somewhat lessened.

The most numerously-represented families are the Siluroids, with 61 species; the Cyprinoids, with 52; the Mormyridæ, with 51; the Characinidæ, with 35; and the Chromides, with 29. There is not, therefore, that great preponderance of the two first families over the remaining, which we noticed in the Indian region; in Africa there is a comparatively greater variety of distinct Freshwater types, imparting to the study of its fauna an unflagging pleasure such as is scarcely gained by the study of the other region. With the forms peculiar to it there are combined those of India as well as South America.

In Tropical Africa there are still remnants of Ganoids: Protopterus (Lepidosiren) annectens and Polypterus bichir, with the singularly modified Calamoichthys. The two former range from east to west, and are accompanied by an Osteoglossoid (Heterotis) which has hitherto been found in the Nile and on the West Coast only.

Autochthont and limited to this region are the Mormyridæ, Pantodontidæ, and Kneriidæ, a singular type, somewhat akin to the Loaches. Of Siluroid genera the most characteristic are Synodontis, Rhinoglanis, and the electric Malapterurus; of Characinoids, Citharinus, Alestes, Xenocharax, Hydrocyon, Distichodon, Ichthyborus.

The regions with which Africa (like India) has least similarity are, again, the North American and Antarctic. Its affinity with the Europe-Asiatic region consists only in having received, like this latter, a branch of the Cyprinoids, the African Carps and Barbels resembling, on the whole, more Indian than Europo-Asiatic forms. Its similarity to Australia is limited to the two regions possessing Dipnoous and Osteoglossoid types. But its relations to the two other regions of the equatorial zone are near and of great interest.

1. Africa has, in common with India, the Siluroid group of Clariina, the Silurina, and Bagrina; and more especially the small but very natural family of Notopteridæ, represented by three species in India, and by two on the west coast of Africa. It would be hazardous to state at present in which of the two regions these fishes first made their appearance, but the discovery of remains of Notopteridæ and Silurina in tertiary deposits of Sumatra points to the Indian region as their original home. We can have less doubt about the other fishes common to both regions; they are clearly immigrants into Africa from the East, and it is a remarkable fact that these immigrants have penetrated to the most distant limits of Africa in the west as well as in the south,—viz. the Labyrinthici, represented by two genera closely allied to the Indian Anabas; the Ophiocephalidæ and Mastacembelidæ, a few species of which have penetrated to the west coast, singularly enough being absent from the eastern rivers; the Ariina, represented by several species, of which one or two are identical with Indian, having extended their range along the intervening coasts to the east coast of Africa. The Cyprinoids also afford an instance of an Indian species ranging into Africa, viz. Discognathus lamta, which seems to have crossed at the southern extremity of the Red Sea, as it is found in the reservoirs at Aden and the hill-streams of the opposite coast-region of Abyssinia.

2. No such direct influx of species and genera has occurred from South America into Africa. Yet the affinity of their Freshwater fishes is striking. Two of the most natural families of fishes, the Chromides and Characinidæ, are peculiar, and (with the exception of Etroplus) restricted to them. The African and South American Dipnoi are closely allied to each other. The Pimelodina, so characteristic of Tropical America, have three representatives in Africa, viz., Pimelodus platychir, P. balayi, and Auchenoglanis biscutatus; the Doradina are another Siluroid group restricted to these two continents.[25] Yet, with all these points of close resemblance, the African and South American series are, with the exception of the two species of Pimelodus, generically distinct; which shows that the separation of the continents must have been of an old date. On the other hand, the existence of so many similar forms on both sides of the Atlantic affords much support to the supposition that at a former period the distance between the present Atlantic continents was much less, and that the fishes which have diverged towards the East and West are descendants of a common stock which had its home in a region now submerged under some intervening part of that ocean. Be this as it may, it is evident that the physical conditions of Africa and South America have remained unchanged for a considerable period, and are still sufficiently alike to preserve the identity of a number of peculiar freshwater forms on both sides of the Atlantic. Africa and South America are, moreover, the only continents which have produced in Freshwater fishes, though in very different families, one of the most extraordinary modifications of an organ—the conversion, that is, of muscle into an apparatus creating electric force.

C. The boundaries of the Tropical American (Neotropical) Region have been sufficiently indicated in the definition of the Equatorial zone. A broad and most irregular band of country, in which the South and North American forms are mixed, exists in the north; offering some peculiarities which deserve fuller attention in the subsequent description of the relations between the South and North American faunæ. The following Freshwater fishes inhabit this region:—

Dipnoi [Australia, Africa]—
Lepidosiren paradoxa1species.
Polycentridæ3„
Chromides [Africa]—
Heros, Acara, Cichla, etc.80„
(Lucifuga2„)
Siluridæ—
Hypophthalmina5„
Pimelodina [Africa, 2 species]70„
Ariina [Africa, India, Australia, Fuegian]35„
Doradina [Africa]60„
Hypostomatina [India]90„
Aspredinina9„
Nematogenyina [Fuegian]—2„
Trichomycterina [Fuegian]2„
Stegophilina3„
Characinidae [Africa]—
Erythrinina15„
Curimatina40„
Anastomatina25„
Tetragonopterina80„
Hydrocyonina30„
Crenuchina1„
Serrasalmonina35„
Cyprinodontidæ—
Carnivoræ [Europe, Asia, N. America, India, Africa]30„
Limnophagæ31„
Osteoglossidæ [Africa, India, Australia]2„
Gymnotidæ20„
Symbranchidæ [India]1„
672species.

Out of the 39 families or groups of Freshwater fishes, 9 only are represented in the Tropical American region. This may be accounted for by the fact that South America is too much isolated from the other regions of the Equatorial zone to have received recent additions to its fauna. On the other hand, the number of species exceeds that of every other region, even of the Indian, with which, in regard to the comparative development of families, the Neotropical region shows great analogy, as will be seen from the following Table:—

INDIAN.   NEOTROPICAL.
Siluridæ 200 sp. Siluridæ 276 sp.
Cyprinidæ 330 sp. Characinidæ 226 sp.
Labyrinthici 25 sp. Chromides 80 sp.
Ophiocephalidæ 30 sp. Cyprinodontidæ 60 sp.
Mastacembelidæ 10 sp. Gymnotidæ 20 sp.

In both regions the great number of species is due to the development of numerous local forms of two families, the Characinidæ taking in the New World the place of the Cyprinidæ of the Old World. Thereto are added a few smaller families with a moderately large number of species, which, however, is only a fraction of that of the leading families, the remainder of the families being represented by a few species only. The number of genera within each of the two regions of the two principal families is also singularly alike; the Indian region having produced about 45 Siluroid and as many Cyprinoid genera, whilst the Neotropical region is tenanted by 54 Siluroid and 40 Characinoid genera. These points of similarity between the two regions cannot be accidental; they indicate that agreement in their physical and hydrographical features which in reality exists.

Of Ganoids, we find in Tropical America one species only, Lepidosiren paradoxa, accompanied by two Osteoglossoids (Osteoglossum bicirrhosum and Arapaima gigas).

Autochthont and limited to this region are the Polycentridæ, all the non-African genera of Chromides and Characinidæ; of Siluroids, the Hypophthalmina, Aspredinina, and Stegophilina, and the majority of Pimelodina, Hypostomatina, and Doradina; the herbivorous Cyprinodonts or Limnophagæ, and numerous insectivorous Cyprinodonts or Carnivoræ; and the Gymnotidæ (Electric eel).

The relations to the other regions are as follows:—

1. The resemblances to the Indian and Tropical Pacific regions partly date from remote geological epochs, or are partly due to that similarity of physical conditions to which we have already referred. We have again to draw attention to the unexplained presence in South America of a representative of a truly Indian type (not found in Africa), viz. Symbranchus marmoratus. On the other hand, a direct genetic affinity exists between the Neotropical and African regions, as has been noticed in the description of the latter, a great part of their freshwater fauna consisting of descendants from a common stock.

2. A comparison of the specifically Neotropical with the specifically North American types shows that no two regions can be more dissimilar. It is only in the intervening borderland, and in the large West Indian Islands, that the two faunæ mix with each other. We need not enter into the details of the physical features of Central America and Mexico—the broken ground, the variety of climate (produced by different altitudes) within limited districts, the hot and moist alluvial plains surrounding the Mexican Gulf, offer a diversity of conditions most favourable to the intermixture of the types from the north and the south. But yet the exchange of peculiar forms appears to be only beginning; none have yet penetrated beyond the debatable ground, and it is evident that the land-connection between the two continents is of comparatively recent date: a view which is confirmed by the identity of the marine fishes on both sides of Central America.

Cuba—and this is the only island in the West Indies which has a number of freshwater fishes sufficient for the determination of its zoo-geographical relations—is inhabited by several kinds of a perch (Centropomus), freshwater mullets, Cyprinodonts, one species of Chromid (an Acara), and Symbranchus marmoratus. All these fishes are found in Central America, and as they belong to forms known to enter brackish water more or less freely, it is evident that they have crossed from the mainland of South America or from Central America. But with them there came a remarkable North American type, Lepidosteus. Lepidosteus viridis, which is found in the United States, has penetrated on the mainland to the Pacific coast of Guatemala, where it is common at the mouth of the rivers and in brack-water lakes along the coast; it probably crossed into Cuba from Florida. A perfectly isolated type of fishes inhabits the subterranean waters of the caves of Cuba (two species of Lucifuga). The eyes are absent or quite rudimentary, as in most other cave animals. Singularly, it belongs to a family (Ophidiidæ), the members of which are strictly marine; and its nearest ally is a genus, Brotula, the species of which are distributed over the Indo-Pacific Ocean, one only occurring in the Caribbean Sea. This type must have witnessed all the geological changes which have taken place since Cuba rose above the surface of the sea.

A similar mixture of forms of the Tropical and Temperate types of Freshwater fishes takes place in the south of South America; its details have not yet been so well studied as in the north; but this much is evident that, whilst in the East Tropical forms follow the Plate river far into the Temperate region, in the West the Temperate Fauna finds still a congenial climate in ranges of the Andes, situated close to, or even north of, the Tropic.

Like the Indian region, the Tropical American has a peculiar Alpine Fauna, the Freshwater fishes of which, however, belong to the Siluroids and Cyprinodonts. The former are small, dwarfed forms (Arges, Stygogenes, Brontes, Astroblepus, Trichomycterus, Eremophilus), and have a perfectly naked body, whilst the representatives in the lowlands of, at least, the first four genera are mailed. The Alpine Cyprinodonts, on the other hand, (Orestias) exceed the usual small size of the other members of this family, are covered with thick scales, but have lost their ventral fins. Some of these Alpine forms, like Trichomycterus, follow the range of the Andes far into the southern temperate region. The majority reach to a height of 15,000 feet above the level of the sea, and a few are found still higher.

D. The Tropical Pacific Region includes all the islands east of Wallace’s Line, New Guinea, Australia—with the exception of its south-eastern portion,—and all the islands of the Tropical Pacific to the Sandwich group. Comparing the area of this region with that of the others, we find it to be not only the poorest in point of the number of its species generally, but also in that of the possession of peculiar forms, as will appear from the following list:—

Dipnoi [Neotrop., Africa.]
Ceratodus2species.
Percidæ [Cosmopol.]—
Lates (calcarifer) [India]1„
Nannoperca1„
Oligorus [New Zealand]1„
Dules [India]8„
(Macquaria)1„
Labyrinthici—
Anabas (scandens) [India]1„
Ophiocephalidæ—
Ophiocephalus (striatus) [India]1„
Atherinidæ [Brack-water]—
Atherinichthys2„
Osteoglossidæ [India, Africa, Neotrop.]1„
Siluridæ—
Plotosina [India]9„
Ariina [India, Africa, Neotrop.]7„
Symbranchidæ—
Monopterus (javanicus) [India]1„
Total36species.

The paucity of freshwater fishes is due, in the first place, to the arid climate and the deficiency of water in the Australian continent, as well as to the insignificant size of the freshwater courses in the smaller islands. Still this cannot be the only cause: the large island of Celebes, which, by its mountainous portions, as well as by its extensive plains and lowlands, would seem to offer a favourable variety of conditions for the development of a freshwater Fauna is, as far as has been ascertained, tenanted by seven Freshwater fishes only, viz. 2 Arius, 2 Plotosus, 1 Andbas, 1 Ophiocephalus, 1 Monopterus, all of which are the commonest species of the Indian region. New Guinea has not yet been explored, but, from the faunæ nearest to this island, we expect its freshwater fishes will prove to be equally few in number, and identical with those of Celebes and North Australia; a supposition confirmed by the few small collections which have reached Europe. Finding, then, that even those parts of this region, which are favourable to the development of Freshwater fishes, have not produced any distinct forms, and that the few species which inhabit them, are unchanged, or but slightly modified Indian species, we must conclude that the whole of this area has remained geologically isolated from the other regions of this zone since the commencement of the existence of Teleostei; and that, with the exception of Ceratodus and Osteoglossum, the immigration of the other species is of very recent date.

Fossil remains of Ceratodus have been found in Liassic and Triassic formations of North America, England, Germany, and India; and it is, therefore, a type which was widely spread in the Mesozoic epoch. Although it would be rash to conclude that its occupation of Australia dates equally far back, for it may have reached that continent long afterwards; yet it is evident that, as it is one of the most ancient of the existing types, so it is certainly the first of the Freshwater fishes which appeared in Australia. Osteoglossum, of which no fossil remains yet have been found, is proved by its distribution to be one of the oldest Teleosteous types. There must have been a long gap of time before these ancient types were joined by the other Teleostei. All of them migrated through the intervening parts of the ocean from India. Most of the Plotosina, some of the Arii, Dules, and the Atherinichthys, also Nannoperca (allied to Apogon), were among the earliest arrivals, being sufficiently differentiated to be specifically or even generically (Cnidoglanis, Nannoperca) distinguished; but some others, like Anabas scandens, Lates calcarifer, Dules marginatus, must have reached the Australian continent quite recently, for they are indistinguishable from Indian specimens.

In South-western Australia a mingling of the scanty fauna with that of the southern temperate parts takes place. Oligorus macquariensis (The Murray Cod), which has a congener on the coast of New Zealand, ascends high up the Murray river, so that we cannot decide whether this Percoid should be located in the Tropical or Temperate part of Australia. Several Galaxias also extend to the confines of Queensland, and will probably some day be found members of this region.

In the smaller Pacific islands the Freshwater fishes exhibit a remarkable sameness: two or three species of Dules, several Eels, an Atherine, or some Gobies, Mullets, and other fishes which with equal readiness exchange fresh for salt water, and which would at once reach and occupy any streams or freshwater lakes that may be formed on an island.

The Sandwich Islands are the only group among the smaller islands which are tenanted by a Siluroid, a species of Arius, which is closely allied to Central American species, and, therefore, probably immigrated from Tropical America.