Chapter XVII: The Distribution of Freshwater Fishes

Having shown above that numerous marine fishes enter fresh waters, and that some of them have permanently established themselves therein, we have to eliminate from the category of freshwater fishes all such adventitious elements. They are derived from forms, the distribution of which is regulated by other agencies, and which, therefore, would obscure the relations of the faunæ of terrestrial regions if they were included in them. They will be mentioned with greater propriety along with the fishes constituting the fauna of the brackish water.

True freshwater fishes are the following families and groups only:--

Dipnoi with 4 species.
Acipenseridæ and
Polyodontidæ „ 26 „
Amiidæ „ 1 „
Polypteridæ. „ 2 „
Lepidosteidæ. „ 3 „
Percina „ 46 „
Grystina „ 11 „
Aphredoderidæ „ 1 „
Centrarchina „ 26 „
Dules „ 10 „
Nandidæ „ 7 „
Polycentridæ „ 3 „
Labyrinthici „ 30 „
Luciocephalidæ „ 1 „
Gastrosteus „ 10 „
Ophiocephalidæ „ 31 „
Mastacembelidaæ „ 13 „
Chromides „ 105 „
Comephoridæ „ 1 „
Gadopsidæ „ 1 „
Siluridæ „ 572 „
Characinidæ „ 261 „
Haplochitonidæ „ 3 „
Salmonidæ (3 genera
excepted) „ 135 „
Percopsidæ „ 1 „
Galaxiidæ „ 15 „
Mormyridæ (and
Gymnarchidæ) „ 52 „
Esocidæ „ 8 „
Umbridæ „ 2 „
Cyprinodontidæ „ 112 „
Heteropygii „ 2 „
Cyprinidæ „ 724 „
Kneriidæ „ 2 „
Hyodontidæ „ 1 „
Osteoglossidæ „ 5 „
Notopteridæ „ 5 „
Gymnotidæ „ 20 „
Symbranchidæ „ 5 „
Petromyzontidæ „ 12 „
--------------
Total 2269 species.
==============

As in every other class of animals, these freshwater genera and families vary greatly with regard to the extent of their geographical range; some extend over the greater 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 scarcer towards the periphery as the type recedes more from its central home, some outposts being frequently pushed far beyond the outskirts of the area occupied by it. But there are not wanting those remarkable instances of closely allied forms occurring, almost isolated, at most distant points, without being connected by allied species in the intervening space; or of members of the same family, genus, or species inhabiting the opposite shores of an ocean, and separated by many degrees of abyssal depths. We mention of a multitude of such instances the following only:--

_A_. Species identical in distant continents--

1. A number of species inhabiting Europe and the temperate parts of eastern North America, as _Perca fluviatilis_, _Gastrosteus pungitius_, _Lota vulgaris_, _Salmo solar_, _Esox lucius_, _Acipenser sturio_, _Acipenser maculosus_, and several Petromyzonts.

2. _Lates calcarifer_ is common in India as well as in Queensland.

3. _Galaxias attenuatus_ inhabits Tasmania, New Zealand, the Falkland Islands, and the southernmost part of the South American continent.

4. Several Petromyzonts enter the fresh waters of Tasmania, South Australia, New Zealand, and Chili.

_B._ Genera identical in distant continents--

1. The genus _Umbra_, so peculiar a form as to be the type of a distinct family consisting of two most closely allied species only, one of which is found in the Atlantic States of North America, the other in the system of the Danube.

2. A very distinct genus of Sturgeons, _Scaphirhynchus_, consisting of two species only, one inhabiting fresh waters of Central Asia, the other the system of the Mississippi.

3. A second most peculiar genus of Sturgeons, _Polyodon_, consists likewise of two species only, one inhabiting the Mississippi, the other the Yang-tse-kiang.

4. _Amiurus_, a Siluroid, and _Catostomus_, a Cyprinoid genus, both well represented in North America, occur in a single species in temperate China.

5. _Lepidosiren_ is represented by one species in tropical America, and by the second in tropical Africa (_Protopterus_).

6. _Notopterus_ consists of three Indian and two West African species.

7. _Mastacembelus_ and _Ophiocephalus_, genera characteristic of the Indian region, emerge severally by a single species in West and Central Africa.

8. _Symbranchus_ has two Indian and one South American species.

9. _Prototroctes_, the singular antarctic analogue of _Coregonus_, consists of two species, one in the south of Australia the other in New Zealand.

10. _Galaxias_ is equally represented in Southern Australia, New Zealand, and the southern parts of South America.

_C._ Families identical in distant continents--

1. The _Labyrinthici_, represented in Africa by 5, and in India by 25 species.

2. The _Chromides_, represented in Africa by 25, and in South America by 80 species.

3. The _Characinidæ_, represented in Africa by 35, and in South America by 226 species.

4. The _Haplochitonidæ_, represented in Southern Australia by one, in New Zealand by one, and in Patagonia by a third species.

This list could be much increased from the families of _Siluridæ_ and _Cyprinidæ_, but as these have a greater range than the other Freshwater fishes, they do not illustrate with equal force the object for which the list has been composed.

* * * * *

The ways in which the dispersal of Freshwater fishes has been effected were various; they are probably all still in operation, but most work so slowly and imperceptibly as to escape direct observation; perhaps, they will be more conspicuous, after science and scientific inquiry shall have reached to a somewhat greater age. From the great number of freshwater forms which we see at this present day acclimatised in, gradually acclimatising themselves in, or periodically or sporadically migrating into, the sea, 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 grandest 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 have been easily 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 of an exchange of a number of species previously 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 Siluroids (_Clarias_, _Callichthys_), Eels, etc., are enabled to wander for some distance over land, and may thus reach a water-course leading them thousands of miles from their original home. Finally, fishes or their ova may be accidentally carried by waterspouts, by aquatic birds or insects, to considerable distances.

Freshwater fishes of the present fauna were already in existence when the great changes of the distribution of land and water took place in the tertiary epoch; and having stated 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 disconnection of certain families or genera. It is not necessary to assume that there was a continuity of land 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 at so distant localities; it suffices to assume that the distances were lessened by intervening archipelagoes, or that an oscillation 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 it 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 on the continents of Africa, South America, and Australia, and their present distribution is evidently the consequence of their wide range in palæozoic and secondary epochs; the proof of their high antiquity can be found in their fossil remains only. For, though the Siluroids have a still greater range, their wide distribution is of comparatively recent date, as the few fossil remains that have been found belong to the tertiary epoch. The rapidity of dispersal of a type depends entirely on its facility to accommodate 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 respiratory functions, and readily survive a change of water.

* * * * *

To trace the geological sequence of the distribution of an ichthyic type, and to recognise 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; or, as Mr. Wallace truly observes, “any attempt to exhibit the regions of former geological ages in combination with those of our own period must lead to confusion.” 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 to decide as far as we can, in an account of the several zoo-geographical divisions, 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?

2. Which of them are to be considered to be _autochthont_ species, that is, forms which came in the tertiary epoch or later into existence within the area to which they are still limited, or from which they have since spread?

3. Which are the forms which must be considered to be _immigrants_ from some other region?

The mode of division of the earth’s surface into zoological regions or areas now generally adopted, is that proposed by Mr. Sclater, which recommends itself as most nearly agreeing with the geographical divisions. These regions are as follows:--

I. PALÆOGÆA.

1. The _Palæarctic_ region; including Europe, temperate Asia,
and North Africa.

2. The _Ethiopian_ region; including Africa, south of the
Sahara, Madagascar, and the Mascarene Islands; also
Southern Arabia.

3. The _Indian_ region; including India south of the
Himalayas, to Southern China, Borneo, and Java.

4. The _Australian_ region; including Australia, the
Pacific Islands, Celebes, and Lombock.

II. NEOGÆA.

5. The _Nearctic_ region; including North America to
Northern Mexico.

6. The _Neotropical_ region; including South America, the West
Indies, and Southern Mexico.

Comparatively few classes and orders of animals have been carefully studied with regard to their geographical distribution, but the majority of those which have been examined show that the difference of latitude is accompanied by a greater dissimilarity of indigenous species than that of longitude, and that a main division into an old world and new world fauna is untenable. More especially the Freshwater fishes, with which we are here solely concerned, have been spread in _circumpolar_ zones, and in a but limited degree from north to south. No family, much less a genus, ranges from the north to the south, whilst a number of families and genera make the entire circuit, and some species more than half of the circuit round the globe within the zone to which they belong. Not even the Cyprinoids and Siluroids, which are most characteristic of the freshwater fauna of our period, are an exception to this. Temperature and climate, indeed, are the principal factors by which the character of the freshwater fauna is determined; they form the barriers which interfere with the unlimited dispersal of an ichthyic type, much more than mountain ranges, deserts, or oceans. Hence the tropical zone is an impassable barrier to the northern Freshwater fish in its progress towards the south; where a similarly temperate climate obtains in the southern hemisphere, fish-forms appear _analogous_ to those of the north, but _genetically_ and _structurally distinct_.

The similarity which obtains in fishes at somewhat distant points of the same degree of longitude, rarely extends far, and is due to the natural tendency of every animal to spread as far as physical conditions will permit. Between two regions situated north and south of each other there is always a debateable border ground, in parts of which sometimes the fishes of the one, sometimes those of the other, predominate, and which is, in fact, a _band_ of demarcation. Within this band the regions overlap each other; therefore, their border _lines_ are rarely identical, and should be determined by the northern and southernmost extent of the most characteristic types of each region. Thus, for instance, in China, a broad band intervenes between temperate and tropical Asia, in which these two faunæ mix, and the actual northern border line of the tropical fauna is north of the southern border line of temperate Asia.

It is the aim of every philosophical classification to indicate the degree of affinity which obtains between the various divisions; but the mode of division into six equivalent regions, as given above, does not fulfil this aim with regard to Freshwater fishes, the distribution of which allows of further generalisation and subdivision. The two families, _Cyprinidæ_ and _Siluridæ_, of which the former yields a contingent of one-third, and the latter of one-fourth of all the freshwater species known of our period, afford most valuable guidance for the valuation of the degrees of affinity between the various divisions. The Cyprinoids may be assumed to have taken their origin in the Alpine region, dividing the temperate and tropical parts of Asia; endowed with a greater capability of acclimatising themselves in a temperate as well as tropical climate than any other family of freshwater fishes, they spread north and south as well as east and west; in the preglacial 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, and well adapted for surviving changes of the water in which they live, for living in mud or sea-water, flourish most in the tropical climate, in which this type evidently had its origin. They came into existence after the Cyprinoids, fossil remains being known only from tertiary deposits in India, none from Europe. They rapidly spread over the areas of land within the tropical zone, reaching northern Australia from India, and one species even immigrated into the Sandwich Islands, probably from South America. The Coral Islands of the Pacific still remain untenanted by them. Their progress into temperate regions was evidently slow, only very few species penetrating into the temperate parts of Asia and Europe; and the North American species, although more numerous, showing no great variety of structure, all belonging to the same group (_Amiurina_). Towards the south their progress was still slower, Tasmania, New Zealand, and Patagonia being without representatives, whilst the streams of the Andes of Chili are inhabited by a few dwarfed forms identical with such as are characteristic of similar localities in the more northern and warmer parts of the South American continent.

After these preliminary remarks we propose the following division of the fauna of Freshwater fishes:--

I. THE NORTHERN ZONE.--Characterised by Acipenseridæ. Few
Siluridæ. Numerous Cyprinidæ. Salmonidæ, Esocidæ.

1. _Europo-Asiatic or Palæarctic Region_.--Characterised by
absence of osseous Ganoidei; Cobitidæ and Barbus numerous.

2. _North American Region_.--Characterised by osseous Ganoidei,
Amiurina, and Catostomina; but no Cobitidæ or Barbus.

II. THE EQUATORIAL ZONE.--Characterised by the development of
Siluridæ.

A. _Cyprinoid Division_.--Characterised by presence of Cyprinidæ
and Labyrinthici.

1. _Indian Region_.--Characterised by [absence of Dipnoi[18]]
Ophiocephalidæ, Mastacembelidæ. Cobitidæ numerous.

2. _African Region_.--Characterised by presence of Dipnoi and
Polypteridæ. Chromides and Characinidæ numerous. Mormyridæ.
Cobitidæ absent.

B. _Acyprinoid Division_.--Characterised by absence of Cyprinidæ
and Labyrinthici.

1. _Tropical American Region_.--Characterised by presence of
Dipnoi. Chromides and Characinidæ numerous. Gymnotidæ.

2. _Tropical Pacific Region_.--Characterised by presence of
Dipnoi. Chromides and Characinidæ absent.

III. THE SOUTHERN ZONE.--Characterised by absence of Cyprinidæ, and
scarcity of Siluridæ. Haplochitonidæ and Galaxiidæ represent
the Salmonoids and Esoces of the Northern zone. One region
only.

1. _Antarctic Region_.--Characterised by the small number of
species; the fishes of--

_a_. The Tasmanian sub-region;
_b_. The New Zealand sub-region;
_c_. The Patagonian sub-region;

being almost identical.[19]

In the following detailed account we begin with a description of the equatorial zone, this being the one from which the two principal families of freshwater fishes seem to have spread.

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:--

A. The Indian region.
B. The African region.
C. The Tropical American region.
D. The Tropical Pacific region.

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] 1 species.
_Nandina_ 7 „
Labyrinthici [Africa] 25 „
_Luciocephalidæ_ 1 „
_Ophiocephalidæ_ [1 species in Africa] 30 „
_Mastacembelidæ_ [3 species in Africa] 10 „
Chromides [Africa, South America]
_Etroplus_ 2 „
Siluridæ--
Clariina [Africa] 12 „
_Chacina_ 3 „
Silurina [Africa, Palæarct.] 72 „
Bagrina [Africa] 50 „
Ariina [Africa, Australia, South America] 40 „
_Bagariina_ 20 „
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 „
_Semiplotina_ 4 „
Danionina [Africa] 30 „
Abramidina [Palæarct., N. Amer., Africa] 30 „
_Homalopterina_ 10 „
Cobitidina [Palæarct.] 50 „
Osteoglossidæ [Africa, Australia, S. America] 1 „
Notopteridæ [Africa] 3 „
Symbranchidæ--
_Amphipnous_ 1 „
_Monopterus_ 1 „
Symbranchus [1 species in S. America] 2 „
------------
625 species.
============

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 annectens_ 1 species.

_Polypteridæ_ 2 „

Percina (Cosmopol.)--
Lates [India, Australia] 1 „
Labyrinthici [India] 5 „
Ophiocephalidæ [India] 1 „
Mastacembelidæ [India] 3 „

Chromides [South America]--
_Chromis_ 23 „
_Hemichromis_ 5 „
_Paretroplus_ 1 „

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]--
_Synodontis_ 15 „
Rhinoglanina [India] 2 „
_Malapterurina_ 3 „

Characinidæ [South America]--
_Citharinina_ 2 „
_Nannocharacina_ 2 „
Tetragonopterina--
_Alestes_ 14 „
Crenuchina--
_Xenocharax_ 1 „
Hydrocyonina--
_Hydrocyon_ 4 „
_Distichodontina_ 10 „
_Ichthyborina_ 2 „

_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 „
_Abrostomus_ 2 „
Discognathus lamta[24] [India] 1 „
Barbus [Palæarct., India] 35 „
Rasborina [India] 1 „
Danionina [India]--
Barilius [India] 3 „
Abramidina [Palæarct., India, N. America]--
_Pelotrophus_ 2 „

_Kneriidæ_ 2 „

Osteoglossidæ [India, Australia, South America]--
_Heterotis_ 1 „

_Pantodontidæ_ 1 „

Notopteridæ [India] 2 „
------------
255 species.
============

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 paradoxa_ 1 species.

_Polycentridæ_ 3 „

Chromides [Africa]--
_Heros_, _Acara_, _Cichla_, etc. 80 „

(_Lucifuga_ 2 „)

Siluridæ--
_Hypophthalmina_ 5 „
Pimelodina [Africa, 2 species] 70 „
Ariina [Africa, India, Australia, Fuegian] 35 „
Doradina [Africa] 60 „
Hypostomatina [India] 90 „
_Aspredinina_ 9 „
Nematogenyina [Fuegian]-- 2 „
Trichomycterina [Fuegian] 2 „
_Stegophilina_ 3 „
Characinidae [Africa]--
_Erythrinina_ 15 „
_Curimatina_ 40 „
_Anastomatina_ 25 „
Tetragonopterina 80 „
Hydrocyonina 30 „
Crenuchina 1 „
Serrasalmonina 35 „

Cyprinodontidæ--
Carnivoræ [Europe, Asia, N. America,
India, Africa] 30 „
_Limnophagæ_ 31 „
Osteoglossidæ [Africa, India, Australia] 2 „
_Gymnotidæ_ 20 „
Symbranchidæ [India] 1 „
------------
672 species.
============

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.]
_Ceratodus_ 2 species.
Percidæ [Cosmopol.]--
Lates (calcarifer) [India] 1 „
_Nannoperca_ 1 „
Oligorus [New Zealand] 1 „
Dules [India] 8 „
(_Macquaria_) 1 „
Labyrinthici--
Anabas (scandens) [India] 1 „
Ophiocephalidæ--
Ophiocephalus (striatus) [India] 1 „
Atherinidæ [Brack-water]--
Atherinichthys 2 „
Osteoglossidæ [India, Africa, Neotrop.] 1 „
Siluridæ--
Plotosina [India] 9 „
Ariina [India, Africa, Neotrop.] 7 „
Symbranchidæ--
Monopterus (javanicus) [India] 1 „
-----------
Total 36 species.

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.

II. NORTHERN ZONE.

The boundaries of the Northern Zone coincide in the main with the northern limit of the Equatorial Zone; but at three different points they overlap the latter, as has been already indicated. This happens in, and east of, Syria, where the mixed faunæ of the Jordan and the rivers of Mesopotamia demand the inclusion of this territory into the Northern Zone as well as the Equatorial; in the island of Formosa, where a Salmonoid and several Japanese Cyprinoids flourish; and in Central America, where a _Lepidosteus_, a Cyprinoid (_Sclerognathus meridionalis_), and an Amiurus (_A. meridionalis_) represent the North American fauna in the midst of a host of tropical forms.

A separate _Arctic_ Zone does not exist for Freshwater fishes; ichthyic life becomes extinct towards the pole as soon as the fresh water remains frozen throughout the year, or thaws for a few weeks only; and the few fishes which extend into high latitudes, in which lakes are open for two or three months in the year, belong to types in no wise differing from those of the more temperate south. The highest latitude at which fishes have been obtained is 82° lat. N., whence the late Arctic Expedition brought back specimens of Charr (_Salmo arcturus_ and _Salmo naresii_).

The ichthyological features of this zone are well marked: the Chondrosteous Ganoids or Sturgeons, and the families of _Salmonidæ_ and _Esocidæ_ are limited to, and characteristic of, it; Cyprinoids flourish with the Salmonoids, both families preponderating in numbers over the others, whilst the Siluroids are few in number and in variety.

The two regions in which this zone is divided are very closely related to one another, and their affinity is not unlike that which obtains between the sub-regions of the Southern Zone. The subjoined list will show their close agreement with regard to families as well as species. Several of the latter are common to both, viz.--_Acipenser sturio_, _A. maculatu_s, _Perca fluviatilis_, _Gastrosteus pungitius_, _Salmo salar_, _Esox lucius_, _Lota vulgaris_, _Petromyzon marinus_, _P. fluviatilis_, and _P. branchialis_; and all recent investigations have resulted in giving additional evidence of the affinity, and not of the diversity of the two regions.

In Europe and temperate Asia, as well as in North America, mountain ranges elevated beyond the line of perpetual snow would seem to offer physical conditions favourable for the development of a distinct alpine fauna. But this is not the case, because the difference of climate between the mountain districts and the lowlands is much less in this zone than in the Equatorial. Consequently the alpine freshwater fishes do not essentially differ from those of the plains; they are principally Salmonoids; and in Asia, besides, mountain-barbels and Loaches. _Salmo orientalis_ was found by Griffith to abound in the tributaries of the Bamean river at an altitude of about 11,000 feet.

Europo-Asiatic. N. American.
_Acipenseridæ_--
Acipenser 9 species. 12 species.
Scaphirhynchus 2 „ 1 „
Polyodon 1 „ 1 „
_Lepidosteidæ_ 0 „ 3 „
_Amiidæ_ 0 „ 1 „
Percina [Cosmopol.] 10 „ 30 „
Grystina [Australia, New Zealand] 0 „ 2 „
_Centrarchina_ 0 „ 26 „
_Aphredoderidæ_ 0 „ 1 „
Cottidæ [partly marine]--
_Cottus_ 3 „ 8 „
_Ptyonotus_ 0 „ 1 „
_Gastrosteidæ_ 5 „ 5 „
_Comephoridæ_ 1 „ 0 „
Gadidæ [marine]--
_Lota_ 1 „ 1 „
Siluridæ--
Silurina [India, Africa] 5 „ 0 „
Bagrina 2 „ 0 „
_Amiurina_ 1 „ 17 „
_Salmonidæ_ 90 „ 45 „
_Percopsidæ_ 0 „ 1 „
_Esocidæ_ 1 „ 7 „
_Umbridæ_ 1 „ 1 „
Cyprinodontidæ Carnivoræ [India,
Africa, Neotrop.] 9 „ 30 „
_Heteropygii_ 0 „ 2 „
Cyprinidæ--
_Catostomina_ 1 „ 25 „
Cyprinina [India, Africa] 80 „ 30 ”
_Leuciscina_ 60 „ 70 „
_Rhodeina_ 10 „ 0 „
Abramidina [India, Africa] 44 „ 10 ”
Cobitidina [India] 20 „ 0 ”
_Hyodontidæ_ 0 „ 1 „
Petromyzontidæ [Southern Zone] 4 „ 8 ”
-----------------------------
360 species. 339 species.
=============================

A. THE EUROPO-ASIATIC (PALÆARCTIC) REGION.--Its western and southern boundaries coincide with those of the Northern Zone, so that only those which divide it from North America have to be indicated. Behring’s Strait and the Kamtschatka Sea have been conventionally taken as the boundary, but this is shown to be artificial by the fact that the animals of both coasts, as far as they are known at present, are not sufficiently distinct to be referred to two distinct regions. As to the freshwater fishes those of North-western America and of Kamtschatka are but imperfectly known, but there can be little doubt that the same agreement exists between them as is the case with other classes of animals. The Japanese islands exhibit a decided Palæarctic fish-fauna, which includes Barbus and Cobitioids, forms strange to the North American fauna. A slight influx of tropical forms is perceived in the south of Japan, where two Bagrina (_Pseudobagrus aurantiacus_ and _Liocassis longirostris_) have established themselves for a considerable period, for both are peculiar to the island, and have not been found elsewhere.

In the east, as well as in the west, the distinction between the Europo-Asiatic and North American regions disappears almost entirely the farther we advance towards the north. Of four species of the genus Salmo known from Iceland, one (_S. salar_) is common to both regions, two are European (_S. fario_ and _S. alpinus_), and one is a peculiarly Icelandic race (_S. nivalis_). As far as we know the Salmonoids of Greenland and Baffin’s Land they are all most closely allied to European species, though they may be distinguished as local races.

Finally, as we have seen above, the Europo-Asiatic fauna mingles with African and Indian forms in Syria, Persia, and Afghanistan. _Capoëta_, a Cyprinoid genus, is characteristic of this district, and well represented in the Jordan and rivers of Mesopotamia.

Assuming that the distribution of Cyprinoids has taken its origin from the alpine tract of country dividing the Indian and Palæarctic regions, we find that this type has found in the temperate region as equally favourable conditions for its development as in the tropical. Out of the 360 species known to exist in the Palæarctic regions, no less than 215 are Cyprinoids. In the countries and on the plateaus immediately joining the Himalayan ranges those mountain forms which we mentioned as peculiar to the Indian Alps abound and extend for a considerable distance towards the west and east, mixed with other _Cyprinina_ and _Cobitidina_. The representatives of these two groups are more numerous in Central and Eastern Asia than in Europe and the northern parts of Asia, where the _Leuciscina_ predominate. _Abramidina_ or Breams are more numerous in the south and east of Asia, but they spread to the extreme north-western and northern limits, to which the Cyprinoid type reaches. The _Rhodeina_ are a small family especially characteristic of the East, but with one or two off-shoots in Central Europe. Very significant is the appearance in China of a species of the _Catostomina_, a group otherwise limited to North America.

The Cyprinoids, in their dispersal from the south northwards, are met from the opposite direction by the Salmonoids. These fishes are, without doubt, one of the youngest families of _Teleostei_, for they did not appear before the Pliocene era; they flourished at any rate during the glacial period, and, as is testified by the remnants which we find in isolated elevated positions, like the Trout of the Atlas, of the mountains of Asia-Minor, and of the Hindu Kush, they spread to the extreme south of this region. At the present day they are most numerously represented in its northern temperate parts; towards the south they become scarcer, but increase again in numbers and species, wherever a great elevation offers them the snow-fed waters which they affect. In the rivers of the Mediterranean Salmonoids are by no means scarce, but they prefer the upper courses of those rivers, and do not migrate to the sea.

The Pike, Umbra, several species of Perch and Stickleback, are also clearly autochthont species of this region. Others belong to marine types, and seem to have been retained in fresh water at various epochs: thus the freshwater Cottus (Miller’s Thumb); _Cottus quadricornis_, which inhabits lakes of Scandinavia, whilst other individuals of the same species are strictly marine; the Burbot (_Lota vulgaris_); and the singular _Comephorus_, a dwarfed and much-changed Gadoid which inhabits the greatest depths of Lake Baikal.

Remnants of the Palæichthyic fauna are the _Sturgeons_ and _Lampreys_. The former inhabit in abundance the great rivers of Eastern Europe and Asia, periodically ascending them from the sea; their southernmost limits are the Yangtse-kiang in the east, and rivers flowing into the Adriatic, Black and Caspian Seas, and Lake Aral, towards the centre of this region. None are known to have gone beyond the boundaries of the Northern Zone. If the Lampreys are justly reckoned among Freshwater fishes, their distribution is unique and exceptional. In the Palæarctic region some of the species descend periodically to the sea, whilst others remain stationary in the rivers; the same has been observed in the Lampreys of North America. They are entirely absent in the Equatorial Zone, but reappear in the Temperate Zone of the Southern Hemisphere. Many points of the organisation of the Cyclostomes indicate that they are a type of great antiquity.

The remaining Palæarctic fishes are clearly immigrants from neighbouring regions: thus _Silurus_, _Macrones_, and _Pseudobagrus_ from the Indian region; _Amiurus_ (and, as mentioned above, _Catostomus_) from North America. The Cyprinodonts are restricted to the southern and warmer parts; all belong to the carnivorous division. The facility with which these fishes accommodate themselves to a sojourn in fresh, brackish, or salt water, and even in thermal springs, renders their general distribution easily comprehensible, but it is impossible to decide to which region they originally belonged; their remains in tertiary deposits round the Mediterranean are not rare.

* * * * *

_B_. The boundaries of the NORTH AMERICAN or NEARCTIC REGION have been sufficiently indicated. The main features and the distribution of this fauna are identical with those of the preceding region. The proportion of Cyprinoid species to the total number of North-American fishes (135:339) appears to be considerably less than in the Palæarctic region, but we cannot admit that these figures approach the truth, as the Cyprinoids of North America have been much less studied than those of Europe; of many scarcely more than the name is known. This also applies in a great measure to the Salmonoids, of which only half as many as are found in the Palæarctic region have been sufficiently described to be worthy of consideration. North America will, without doubt, in the end show as many distinct races as Europe and Asia.

Cyprinoids, belonging to genera living as well as extinct, existed in North America in the tertiary period. At present, _Cyprinina_, _Leuciscina_, and _Abramidina_ are well represented, but there is no representative of the Old World genus _Barbus_, or of the _Cobitidina_[26]; _Rhodeina_ are also absent. On the other hand, a well-marked Cyprinoid type is developed--the _Catostomina_, of which one species has, as it were, returned into Asia. Very characteristic is the group of _Centrarchina_, allied to the Perch, of which there are some thirty species; two _Grystina_. Of the Sticklebacks there are as many species as in Europe, and of Pike not less than seven species have been distinguished. _Umbra_ appears to be as local as in Europe. Some very remarkable forms, types of distinct families, though represented by one or two species only, complete the number of North American autochthont fishes--viz., _Aphredoderus_, _Percopsis_, _Hyodon_, and the _Heteropygii_ (_Amblyopsis_ and _Chologaster_). The last are allied to the Cyprinodonts, differing from them in some points of the structure of their intestines. The two genera are extremely similar, but _Chologaster_, which is found in ditches of the rice-fields of South Carolina, is provided with eyes, and lacks the ventral fins. _Amblyopsis_ is the celebrated Blind Fish of the Mammoth Cave of Kentucky: colourless, eyeless, with rudimentary ventral fins, which may be occasionally entirely absent.

A peculiar feature of the North American Fish Fauna is that it has retained, besides the Sturgeons and Lampreys, representatives of two Ganoid families, _Lepidosteus_ and _Amia_. Both these genera existed in tertiary times: the former occurs in tertiary deposits of Europe as well as North America, whilst fossil remains of _Amia_ have been found in the Western Hemisphere only.

It is difficult to account for the presence of the _Amiurina_ in North America. They form a well-marked division of the _Bagrina_, which are well represented in Africa and the East Indies, but absent in South America; it is evident, therefore, they should not be regarded as immigrants from the south, as is the case with the Palæarctic Siluroids. Nor, again, has the connection between South and North America been established sufficiently long to admit of the supposition that these Siluroids could have spread in the interval from the south to the northern parts of the continent, for some of the species are found as far north as Pine Islands Lake (54° lat. N.)[27]

III. SOUTHERN ZONE.

The boundaries of this zone have been indicated in the description of the Equatorial Zone; they overlap the southern boundaries of the latter in South Australia and South America, but we have not at present the means of exactly defining the limits to which southern types extend northwards. This zone includes Tasmania with at least a portion of South-eastern Australia (_Tasmanian sub-region_), New Zealand and the Auckland Islands (_New Zealand sub-region_), and Chili, Patagonia, Terra del Fuego, and the Falkland Islands (_Fuegian sub-region_). No freshwater fishes are known from Kerguelen’s Land, or from islands beyond 55° lat. S. The southern extremity of Africa has to be excluded from this zone so far as Freshwater fishes are concerned.

This zone is, with regard to its extent as well as to the number of species, the smallest of the three; yet its ichthyological features are well marked; they consist in the presence of two peculiar families, each of which is analogous to a northern type, viz. the _Haplochitonidæ_, which represent the _Salmonidæ_, _Haplochiton_ being the analogue of _Salmo_, and _Prototroctes_ that of _Coregonus_; and the _Galaxiidæ_, which are the Pikes of the Southern Hemisphere.

Although geographically widely separate from each other, the Freshwater fishes of the three divisions are nevertheless so closely allied that conclusions drawn from this group of animals alone would hardly justify us in regarding these divisions as sub-regions. One species of _Galaxias_ (_G. attenuatus_) and the three Lampreys are found in all three, or at least two, sub-regions.

_Freshwater Fishes of the Southern Zone._

Tasmanian. N. Zealand. Fuegian.

_Percichthys_ ... ... 3

Siluridæ--
_Diplomystax_ ... ... 1
_Nematogenys_ ... ... 1
Trichomycterina [Neotrop.] ... ... 5

_Gadopsidæ_ 1 ... ...

(_Retropinna_ ... 1 ...)

_Haplochitonidæ_ 1 1 1

_Galaxiidæ_ 6 5 4

Petromyzontidæ 3 1 3
-- -- --
11 8 18

But little remains to be added in explanation of this list; _Percichthys_ is in Chili the autochthont form of the cosmopolitan group of _Percina_. _Diplomystax_, an Arioid fish of Chili, and _Nematogenys_ seem to have crossed the Andes from Tropical America at a comparatively early period, as these genera are not represented on the eastern side of South America; the _Trichomycterina_ occur on both sides of the Andes, which they ascend to a considerable height. _Retropinna_ is a true Salmonoid, allied to, and representing in the Southern Hemisphere the Northern Smelt, _Osmerus_. In both these genera a part of the specimens live in the sea, and ascend rivers periodically to spawn; another part remain in rivers and lakes, where they propagate, never descending to the sea, this freshwater race being constantly smaller than their marine brethren. That this small Teleostean of the Northern Hemisphere should reappear, though in a generically modified form, in New Zealand, without having spread over other parts of the Southern Zone, is one of the most remarkable, and at present inexplicable facts of the geographical distribution of freshwater fishes.