Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/691

673 FRESHWATER FISHES.] ICHTHYOLOGY have found their way into Africa. On the other hand, the number of species is much less, viz., 255, being only two-fifths of the known Indian species. The small de gree of specialization and localization 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 uninter rupted continuity of the fish fauna from the 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. Unlike India, Africa does not possess either alpine ranges or outlying archipelagoes, the fresh waters of which would swell the number of its indigenous species ; but, when its fauna becomes better known than at present, the great difference in the number of species between this and the Indian regions may possibly be somewhat lessened. The most numerously represented families are the Silu- roids, with 61 species ; the Cyprinoids, with 52 ; the MormyridcBt with 51 ; the Charadnidce, with 35 ; and the Chromides, with 29. There is not, therefore, that great preponderance of the first two families over the rest which we noticed in the Indian region ; in Africa there is a comparatively greater variety of distinct freshwater types, making the study of its fauna an unflagging plea sure such as is scarcely reached in the study of the other region. With the forms peculiar to it there are combined those of India as well as of South America. In tropical Africa there are still remnants of Ganoids, Protopterus (Lepidosiren) annectens and Polypterus bickir, with the singularly modified Calamoichthys. The first two range from east to west, and are accompanied by an Osteoglossoid (ffeterotis), which has hitherto been found in the Nile and on the west coast only. Autochthonous and limited to this region are the Mormyridce, Pan- todontidae, and Kneriidce, a singular type somewhat akin to the loaches. Of Siluroid genera the most charac teristic are Synodontis, Rhinoglanis, and the electric Malapterurus ; of Characinoids, Citharinus, Alestes, Xeno- charax, Hydrocyon, Distichudon, Ichthyborus. _ The regions to which Africa (like India) has least similarity are again the North American and Antarctic. Its affinity with the Europe-Asiatic region consists only in Its having received, like the latter, a branch of the Cypri noids, the African carps and barbels, which on the whole resemble Inrlian more than Europo-Asiatic forms. Its similarity to Australia is limited to the two regions possess ing Dipnoous and Osteoglossoid types. But its relations to the other two regions of the equatorial zone are near and of great interest. 1. Africa has in common with India the Siluroid groups of the Uarnna, Silurina, and Bagrina, and more especially the small it very natural family of Notoptcridce, represented by three species India, and by two on the west coast of Africa. It would be hazardous to state at present in which of the two re-ions these fishes rst made their appearance, but the discovery of remains of Noto- pteruice and Silunna in Tertiary deposits of Sumatra points to the Indian region as their original home. We are in less doubt about the other fishes common to the two regions ; they are clearly im migrants into Africa from the east, and it is a remarkable fact that e immigrants have penetrated to tin 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 Ophioctphalidce and Mnstncembclulai, a few species of which have penetrated to the west coast, while, singularly enough, they are absent from the eastern rivers ; the Ariina, represented by several species, of which one or two are identical with Indian, having ex tended their range along the intervening coasts to the east coast of Africa. 1 he 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 in 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&quot; families of fishes, the Lhromides and Charadnidce, 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 char- acteristic of tropical America, have three representatives in Africa viz., Pimelodus platychir, I. balai/i,and Auchcnoglanis biscutatuf the Doradina are another Siluroid group restricted to these two continents. 1 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 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 con siderable period, and are still sufficiently alike to preserve the iden tity of a number of peculiar freshwater forms on hoth sides of the Atlantic. Africa and South America are, moreover, the only con tinents which have produced in freshwater fishes, though in very different families, one of the most extraordinary modifications of an organthe conversion, that is, of muscle into an apparatus creating electric force. The boundaries of the Neotropical or Tropical American 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 descrip tion of the relations between the South and North Ameri can faunas. Out of the 40 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 (672) exceeds that of every other region, even of the Indian, with which, in regard to the comparative development uf families, the neotropical region shows a close analogy, as will be seen from the following table : Indian. Siluridae, 200 species; Cyprinidsp, 330; Labyrinthici, 25 ; Ophiocephalidce, 30 ; Mastacembelida?, 10. Neotropical Siluridre, 276 species ; Characinida?, 226 ; Chro mides, 80 ; Cyprinodontidffi, 60 ; Gymnotida?, 20. In both regions the great number of species is due to the development of numerous local forms of two families, the Characinidai in the New World taking the place of the Cypnnidoe of the Old. To these are added a few smaller families with a moderately large number of species, which, however, is only a fraction of that of the leading fami lies, the remainder being represented by a few species only. The number of genera within each of the two re gions belonging to the two principal families is also singularly alike ; the Indian region having produced about 45 Siluroid and as many Cyprinoid genera, whilst the Neo tropical region is tenanted by 54 Siluroid and 40 Chara- 1 We have left out of consideration here the Ariina and Cyprmo- donts, which can pass with impunity through salt water, and are&quot;sprca&amp;lt;J over much larger areas.