Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/692

674 674 ICHTHYOLOGY [DISTRIBUTION cinoid geaera. 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). Autochthonous and limited to this region are the Pvlycentridce ; all the non-African genera of Chromides and Characinidce ; of Siluroids, the Hypophtkalmina, Aspredi- nina, and Stegophilina, and the majority of Pimelodina, llypostomatina, and Doradina ; the herbivorous Cyprino- dotits or Limnophagce, and numerous insectivorous Cypri- nodonts or Carnivorw ; and the Gymnotidce (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. AVe have again to draw attention to the unexplained presence in South America of a representative of a truly Indian type (not found iu 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 speci fically 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 faunas mix with each other. We need not enter into the details of the physical features of Central America and Mexico ; the broken ground, the diversity of climate (produced by different altitudes) within limited districts, the hot and moist alluvial plains surrounding the Mexican Gulf, offer a variety of conditions most favourable to the intermixture of the types from the north and the south. Still the interchange of peculiar forms appears to be only beginning ; none have yet pene trated beyond the debateable ground, and it is evident that the land connexion 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 which is the only island in the West Indies that 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 Chromic! (an Acara), and SymbrancJms marmoratus. All these fishes are found in Central America, and, as they belong to forms known to eater brackish water more or less freely, it is evident that they have crossed either from that region or from the main land of South America. But with them there came a re markable North American type, Lepidosteas. 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 brackish-water 1 ikes along the coast ; it probably crossed into Cuba from Florida. A perfectly isolated type of fishes inhabits the subterranean waters in the caves of Cuba (two species of Lucifaga). The eyes are absent, or quite rudimentary, as in most other cave animals. Singularly, it belongs to a family (Ophidiidce) 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 tem perate 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, how ever, belong to the Siluroids and Cyprinodonts. The former are small, dwarfed forms (Arges, Stygoyenes, Brontes, Astroblepus, Trichomyderus, Ereinojihilus), 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), ex ceed the usual small size of the other members of this family, and are covered with thick scales, but have lost their ventral fins. Some of these alpine forms, like Trichomyc- terus, follow the range of the Andes far into the Southern Temperate region. The majority are found at a height of 1 5,000 feet above the level of the sea, and a few still higher. The Tropical Pacific Region includes all the islands east of Wallace s line, New Guinea, Australia (with the excep tion 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 the poorest, not only in point of the number of its species generally (36), but also in the possession of peculiar forms. The paucity of freshwater fishes is due, in the first place, to the arid climate and the deficiency of water in the Aus tralian continent, as well as to the insignificant size of the freshwater courses in the smaller islands. Still this can not 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, so far as has been ascertained, tenanted by seven freshwater fishes only, viz., 2 Arius, 2 Plof.osus, 1 Anabas, 1 Opkiocephahis, 1 Monopterus, all of which are the com monest species of the Indian region. New Guinea has not yet been explored, but, from the faunaB 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 favour able to the development of freshwater fishes have not pro duced 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 exist ence 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 the Liassic and Triassic formations of North America, England, Germany, and India ; 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. Osteoglosmm, of which no fossil remains have yet 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 Tdeostd. All of them migrated through the intervening parts of the ocean from India. Most of the Plotosina, some of the Arii, Dides, and Atherinichthys, also Nanno- perca (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 undistinguishable from Indian specimens.