DISTRIBUTION AND MIGRATIONS 273 



which are known to have secondarily returned to a marine 

 habitat, and certainly reached Australia by sea, and have there 

 again formed fresh- water species. 



In most respects Madagascar bears a marked resemblance to 

 the Australian region, being characterised by the complete 

 absence of Ostariophysi, with the sole exception of a species of 

 a marine family of Cat-fishes (Ariidae). The characteristic 

 African families of Carps {Cyprinidae) , Characins (Characidae, 

 Citharinidae) , Cat-fishes {Clariidae, Mochocidae, Amphiliidae, etc.), 

 and so on, are all absent, and only the Perch-like Cichhds 

 {Cichlidae) are common to the two regions. But the Cichlids 

 are not entirely confined to fresh water, certain species being 

 known to thrive in brackish or even salt water, and all those 

 found in Madagascar are of the estuarine type. It may be 

 concluded, therefore, that the severance of Madagascar from 

 Africa took place very long ago, probably at about the same 

 time that Australia became detached from Asia, and that any 

 fresh-water fishes found there to-day have reached the island 

 subsequently from the sea. 



In the Neotropical region the characteristic fishes might be 

 expected to be somewhat similar to those of North America, but, 

 in point of fact, the faunas of the two regions of the New World 

 are of a totally different nature. South America may be said 

 to be inhabited by two distinct fresh-water fish-faunas, diflferent 

 in composition and in origin. The Patagonian fauna, occupy- 

 ing the region south of a line drawn from Valparaiso to Bahia 

 Blanca, is very poor in species, and consists mainly of immi- 

 grants from the sea which are more or less permanently 

 estabhshed in the rivers, plus a few stragglers (Characins and 

 Cat-fishes) from the north. The region from the La Plata 

 River northwards to Central America is inhabited by a fauna 

 extremely rich in genera and species, which bears a marked 

 similarity to that of Africa. The tribe of Characins [Characi- 

 formes) is found in the African and Neotropical regions and 

 nowhere else, and of the six families one [Characidae] is found 

 in both continents (Fig. loi), four are exclusively South 

 American, and one occurs only in Africa. The family common 

 to the Old and New World presents a much greater diversity 

 and number of genera and species in South America, and 

 although there are no genera common to both, Alestes in Africa 

 is very closely related to Brycon of South America. The peculiar 

 Eel-like Gymnotids [Gymnotiformes) are confined to the Neo- 

 tropical region, but there are no Carps, Loaches, Suckers, or 



