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.