Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/689

671 FRESHWATER FISHES.] ICHTHYOLOGY 671 type evidently had its origin. They came into existence after the Cypriuoids, their fossil remains being found only in Tertiary deposits in India, none in Europe. They rapidly spread over the areas of land within the tropical zone, reaching Northern Australia from India, and one j species migrating even into the Sandwich Islands, pro- ; bably from South America. The coral islands of the Paciric still remain untenanted by them. Their progress into temperate regions was evidently slow, only very few species having penetrated 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 Zea land, and Patagonia being without any representative, 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. These remarks may serve to introduce the following division of the fauna of freshwater fishes : I. THE NORTHERN ZONE. Characterized by Acipenseridoe. Few Siluridre. Numerous Cyprinidie. Salmonidae, Esocidse. 1. Europo- Asiatic or Palacarctic Region. Characterized by absence of osseous Ganoidei ; Cobitidte aud Barbus numerous. 2. Nortli American Region. Characterized by osseous Ganoi- di-i, Amiurina, and Catostomina ; but no Cobitidte or Barbus. II. THE EQUATORIAL ZONE. Characterized by the development of Silurida;. A. Cyprinoid Division. Characterized by presence of Cypri- nidaj and Labyrinthici. 1. Indian Region. Characterized by [absence of Dipnoi 1 ] Ophiocephalidee, Mastacembelidse. Co bitidte numerous. 2. African Region. Characterized by presence ot Dipnoi and PolyptericUe. Chromides and Chara- cinidai numerous. Mormyridse. Cobitidte absent. A. Acyprinoid Division. Characterized by absence of Cypri- j idaj and Labyrinthici. 1. Tropical American or Neotropical Region. Char acterized by presence of Dipnoi. Chromides and Characinidaj numerous. Gymnotidte. 2. Tropical Pacific Region. Characterized by pre sence of Dipnoi. Chromides and Characinidae absent. III. THE SOUTHERN ZONE. Characterized by absence of Cypri- nidse, and scarcity of Siluridse. Haplochitonidoj and Ga- laxiidie represent the Salmonoids and Esoces of the northern zoni3. One region only. 1. Antarctic Region. Characterized by the small number of species ; the fishes of a. The Tasmanian sub-region, b. The New Zealand sub-region, and c. The Patagonian sub-region, being almost identical. 2 In the following 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. EQUATORIAL ZONE. 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 degreas both northwards 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 (Mastacembelm, near Aleppo and in the Tigris; Clarias and Chromides, in the Lake of Galilee), crosses through 1 These will probably be found. 2 We distinguish these sub-regions, because their distinction is jus tified by other classes of animals ; as regards freshwater lishes they are even less distinct than Europe aud northern Asia. Persia and Afghanistan (Ophiocephalus) to the southern ranges of the Himalayas, and follows the course of the Yang-tse-Keang, which receives its contingent of equatorial fishes through its southern tributaries. Its continuation through the North Pacific may be considered as 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 distribu tion 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 that region follow the principal water-course, 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 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 Lombok. 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 Lombok, or from islands situated farther east. Taking into consideration the manner in which Cypri noids 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 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. The Indian Region comprises the whole continent of Asia south of the Himalayas and the Yang-tse-Keang, aud 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 shows the characters of 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. Scarcely better defined is the boundary of this region towards the north-west. 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 Mastacembdus 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 almost entirely ignorant ; we only