12 Members of the Gymnotidæ. a, Sternarchorhynchus curvirostris (Boulenger); b, Sleatogenys elegans (Steindachner); c, Sternarchus brasiliensis (Reinhardt); d, Eigenmannia virescens (Valenciennes); e, Giton fasciatus (Pallas); f, Rhamphichthys maramoratus (Castelnau). arisen in South America from the Characinidæ and Siluridæ since the separation of the two continents.
Similarly, other families found in Africa and not in South America have either arisen in Africa since that time or have immigrated from the east.
A land connection, whether a land bridge, intermediate continent or land wave, between the two continents is imperative. This land connection must have existed before the origin of existing genera and before many of the existing families.
B. Conclusions and Bird's-eye View of the Problems.
America, south of the Tropic of Cancer, contains four (or five) distinct faunas. These faunas are the Transition, the Mexican, the South American and the Patagonian.
The northernmost, or Transition fauna, is characterized largely by instrusive elements from both the north and the south. It is found on the Atlantic slope from the tropic to the Isthmus of Tehuantepec and on the Pacific slope of this Isthmus.
The Mexican fauna is unique, and occupies a narrow strip including
