NEOTROPICAL KEGION. 233 



blance, 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 inter- 

 vening 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 conti- 

 nents which have produced in Freshwater fishes, though in 

 very different families, one of the most extraordinary modi- 

 fications of an organ — the conversion, that is, of muscle into 

 an apparatus creating electric force. 



C. The boundaries of the Tropical American {Neotropical) 

 Eegion 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 ISTorth American faunae. 

 The following Freshwater fishes inhabit this region : — ■ 



Dipnoi [Australia, Africa] — 



Lepidosiren paradoxa . ■ .1 species. 



PolycentridcB . . . • • 3 „ 

 Chromides [Africa] — 



Heros, Acara, Cichla, etc. . . 80 „ 



{Lmcifiiga . . . ■ . 2 „) 



