236 FISHES. 



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 repre- 

 sentative of a truly Indian type (not found in Africa), viz. 

 Symhranchus 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 Keotropical with the 

 specifically North American types shows that no two regions 

 can be more dissimilar. It is only in the intervening border- 

 land, and in the large West Indian Islands, that the two 

 faunse mix with each other. We need not enter into the 

 details of the physical features of Central America and Mexico 

 — the broken ground, the variety of climate (produced by dif- 

 ferent altitudes) within limited districts, the hot and moist 

 alluvial plains surrounding the Mexican Gulf, offer a diver- 

 sity of conditions most favourable to the intermixture of the 

 types from the north and the south. But yet the exchange 

 of peculiar forms appears to be only beginning ; none have yet 

 penetrated beyond the debateable ground, and it is evident that 

 the land-connection between the two continents is of com- 

 paratively recent date : a view which is confirmed by the 

 identity of the marine fishes on both sides of Central America. 



Cuba — and this is the only island in the West Indies 

 which has a number of freshwater fishes sufficient for the 

 determination of its zoogeographical relations — is inhabited 

 by several kinds of a perch (Centropomus), freshwater muUets, 

 Cyprinodonts, one species of Chromid (an Acara), and Sym- 

 hranchus marmoratvs. All these fishes are found in Central 

 America, and as they belong to forms known to enter brack- 

 ish water more or less freely, it is evident that they have 

 crossed from the mainland of South America or from Central 



