sciænoids and the herring tribe give to the South American fauna a peculiarly marine flavor. But all these form but a small fraction of the entire fauna, and their elimination would make little inroad on the number of species. All are recent additions from the sea.
There are also in South America a number of undoubted relicts of former times, and if one should judge by the interest excited by the genera Synbranchus, Lepidosiren, Arapaima and Osteoglossum, it would seem that these genera formed an important element in the present fauna; but they are so few in number that they also might be eliminated without any appreciable depreciation in the variety of the South American fauna.
After eliminating these, then, we come to the reigning element in the present fauna, the element now in its prime and best suited to contribute to the elucidation of the methods and paths of divergent
and convergent evolution and the paths of dispersal. Chief of these elements is the superorder Ostaryphysiæ, composed of the Characinidæ with about 500 species; the Gymnotidæ with about 30 species, and the various families of catfishes with about 500 species. Pœciliidæ, dominant in middle America, contributed materially to the fauna—about 45 species.
The largest contribution, aside from the characins and the catfishes, is furnished by the Cichlidæ, with about 145 species, in south and middle America.
The great variety of South American fishes is due to the divergence in the types of characin, catfish, cichlid and pœecilid.
3. Paucity of Middle American Fauna.
The fresh-water fauna of middle America is poor. No river excepting the Lerma harbors as many as 50 species. The genera south of the isthmus of Tehuantepec are practically all South American with the addition of pœcilids.
