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TIME AND SPACE] which they may be regarded as derived. In no instance do these types show a structure which may be termed archaic when compared with their surface allies. That these fishes are localized in their vertical distribution, between the 100-fathoms line, often taken as the arbitrary limit of the bathybial fauna, and the depth of 2750 fathoms, the lowest point whence fishes have been procured, there is little doubt. But our knowledge is still too fragmentary to allow of any general conclusions, and the same applies to the horizontal distribution. Yet the same species may occur at most distant points; as these fishes dwell beyond the influence of the sun’s rays, they are not affected by temperature, and living in the Arctic zone or under the equator makes little difference to them. A great deal of evidence has been accumulated to show the gradual transition of the surface into the bathybial forms; a large number of surface fishes have been met with in deep water (from 100 to 500 fathoms), and these animals afford no support to Alexander Agassiz’s supposition of the existence of an azoic zone between the 200-fathoms line and the bottom.

Brackish-water fishes occur also in salt and fresh water, in some localities at least, and belong to various groups of Teleosteans. Sticklebacks, gobies, grey mullets, blennies are among the best-known examples. The facility with which they accommodate themselves to changes in the medium in which they live has enabled them to spread readily over very large areas. The three-spined stickleback, for instance, occurs over nearly the whole of the cold and temperate parts of the northern hemisphere, whilst a grey mullet (Mugil capito) ranges without any appreciable difference in form from Scandinavia and the United States along all the Atlantic coasts to the Cape of Good Hope and Brazil. It would be hardly possible to base zoo-geographical divisions on the distribution of such forms.

The fresh-water fishes, however, invite to such attempts. How greatly their distribution differs from that of terrestrial animals has long ago been emphasized. The key to their mode of dispersal is, with few exceptions, to be found in the hydrography of the continents, latitude and climate, excepting of course very great altitudes, being inconsiderable factors, the fish-fauna of a country deriving its character from the headwaters of the river-system which flows through it. The lower Nile, for instance, is inhabited by fishes bearing a close resemblance to, or even specifically identical with, those of tropical Africa, thus strikingly contrasting with the land-fauna of its banks. The knowledge of the river-systems is, however, not sufficient for tracing areas of distribution, for we must bear in mind the movements which have taken place on the surface of the earth, owing to which present conditions may not have existed within comparatively recent times, geologically speaking; and this is where the systematic study of the aquatic animals affords scope for conclusions having a direct bearing on the physical geography of the near past. It is not possible here to enter into the discussion of the many problems which the distribution of fresh-water fishes involves; we limit ourselves to an indication of the principal regions into which the world may be divided from this point of view. The main divisions proposed by Günther in the 9th edition of the Encyclopædia Britannica still appear the most satisfactory. They are as follows:—

I. —Characterized by Acipenseridae. Few Siluridae. Numerous Cyprinidae, Salmonidae, Esocidae, Percidae. 1. Europaeo-Asiatic or Palaearctic Region. Characterized by absence of osseous Ganoidei; Cobitinae and Barbus numerous. 2. North American or Nearctic Region. Characterized by osseous Ganoidei and abundance of Catostominae; but no Cobitinae or Barbus.

II. —Characterized by the development of Siluridae. A. Cyprinoid Division. Characterized by presence of Cyprinidae, Mastacembelidae. Anabantidae, Ophiocephalidae. 1. Indian Region. Characterized by absence of Dipneusti, Polypteridae, Mormyridae and Characinidae. Cobitinae numerous. 2. African Region. Characterized by presence of Dipneusti, Polypterid and Mormyrid; Cichlid and Characinid numerous. B. Acyprinoid Division. Characterized by absence of Cyprinidae and the other families mentioned above. 1. Tropical American or Neotropical Region. Characterized by presence of Dipneusti; Cichlidae and Characinidae numerous; Gymnotidae and Loricariidae. 2. Tropical Pacific Region. Includes the Australian as well as the Polynesian Region. Characterized by presence of Dipneusti. Cichlidae and Characinidae absent.

III. —Characterized by absence of Cyprinidae and scarcity of Siluridae. Haplochitonidae and Galaxiidae represent the Salmonids and Esoces of the northern zone. One region only. 1. Antarctic Region. Characterized by the small number of species; the fishes of (a) The Tasmanian subregion; (b) The New Zealand subregion; and (c) The Patagonian or Fuegian subregion being almost identical.

Although, as expressed in the above synopsis, the resemblance between the Indian and African regions is far greater than exists between them and the other regions of the equatorial zone, attention must be drawn to the marked affinity which some of the fishes of tropical Africa show to those of South America (Lepidosirenidae, Characinidae, Cichlidae, Nandidae), an affinity which favours the supposition of a connexion between these two parts of the world in early Tertiary times.

The boundaries of Günther’s regions may thus be traced, beginning with the equatorial zone, this being the richest.

—Roughly speaking, the borders of this zoological zone coincide with the geographical limits of the tropics of Cancer and Capricorn; its characteristic forms, however, extend in undulating lines several degrees both northwards and southwards. Commencing from the west coast of Africa, the desert of the Sahara forms a 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, crosses through Persia and Afghanistan 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 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 little studied, but the tropical fishes of that region follow the principal watercourse, the Murray river, far towards the south and probably to its mouth. The boundary-line then stretches to the north 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 southward to embrace the system of the Rio de la Plata.

The four regions into which the equatorial zone is divided arrange themselves into two well-marked divisions, one of which is characterized by the presence of Cyprinid fishes, combined with the development of Labyrinthic Percesoces (Anabantidae and Ophiocephalidae) and Mastacembelids, whilst in the other these types are absent. The boundary between the Cyprinoid and Acyprinoid division seems to follow the now exploded Wallace’s line—a line drawn from the south of the Philippines between Borneo and Celebes, and farther south between Bali and Lombok. Borneo abounds in Cyprinids; from the Philippine Islands a few only are known, and in Bali two species have been found; but none are known from Celebes or Lombok, or from islands situated farther east.

The Indian region comprises Asia south of the Himalayas and the Yang-tse-Kiang, and includes the islands to the west of Celebes and Lombok. Towards the north-east the island of Formosa, which also by other parts of its fauna shows the