422 Evolution and Distribution of Fishes 



richest In genera; while three are African. But the Hydro- 

 cyonina or River wolves, and the Citharinina, deserve 

 special attention, for these include genera the greater 

 number of which are found in S. America, but some have 

 reached Africa. So putting all of the above facts together, 

 one strongly inclines to regard S. America as the centre of 

 evolution. 



If we accept such, the wide expansion of the species over 

 S. America seems to point to the conclusion that that 

 continent has witnessed no sudden fundamental geologic 

 changes, except those of erosion and related phenomena, 

 over its larger northeastern area in recent geologic time. 

 But the entire absence of Characinidae from the southern 

 and southeastern coastal parts, might be due to an average- 

 ly overcold condition, that would prove injurious to an es- 

 sentially tropical or sub-tropical group of fishes. Or it may 

 be that during steady elevation and even folding of strata 

 in the west during the elevation of the Andes, the land on 

 the south-eastern side was under the sea, a view that has 

 been demonstrated as correct by the Princeton Patagonian 

 Expedition, and that has been fully accepted by subsequent 

 workers. 



A highly interesting feature however is the presence 

 of ten or more species, along the coastal and western 

 Andean regions, which belong to genera that extend over 

 the entire eastern half of the continent. Thus Tetragon- 

 opterus brevirostris and T. microphthalmiis are encountered 

 in the "Western Andes of Ecuador" and from the "Pacific 

 coast of Guatemala." A possible explanation is that mem- 

 bers of the family had become distributed over the entire 

 width of the continent, before any marked elevation of the 

 Andes with attendant faulting and probable volcanic action, 

 had taken place. When such happened, outlier-species may 

 have been separated on the western Andean side, that have 

 evolved into the species now found there. But as an alter- 

 nate possibility, It must also be kept In mind that about the 

 time now under consideration free communication existed 

 between the Pacific and the Gulf of Mexico. So It may 

 be that accustomment to a marine environment was effected 

 further north. 



