Page:Popular Science Monthly Volume 68.djvu/529

Rh These are all marine families, some of which have also developed fresh-water forms in Europe and North America as well as in South America. The fresh-water forms of South America and Africa are local adaptations of marine families that require no change in the present condition to account for their origin.

The second group comprises the Lepidosirenidæ, Osteoglossidæ, Siluridæ, Characinidæ, Pœciliidæ and Cichlidas. Of these the Lepidosirenida are relicts of a former widely distributed group, and it requires no land connection to satisfactorily account for their presence in Africa and South America. The Pœciliidæ live indifferently in marine, brackish water or fresh water. They reach their maximum development in the fresh waters of Mexico and Central America. The marine species are found along the shores, not at sea, and there is, therefore, at present, no known means of getting them from the American to the African shore. Nevertheless, Fundulus is found on both sides of the Atlantic, and there must have been an intermigration much more recent than the youngest possible land connection between Africa and South America, or else there has been a very long persistence of this type. A land connection, while not absolutely required for this family, would be very convenient.

The Siluridæ are in part marine. All of the South American forms of Siluridæ can be derived from the marine Tachisurinæ, and the same is probably true of the American members of the family. Furthermore, the catfishes are found in North America, Europe and Asia and have been recorded in North America from the Tertiary. A land connection between Africa and South America is, therefore, not absolutely required to account for their presence in both continents, though, as in the case of the Pœciliidæ, such a connection would be very convenient.

The Cichlidæ and Characinidæ are abundant in tropical America and in Africa, a few species of Cichlidæ being also found in India. There is no known means by which these two forms could have crossed the existing gap between Africa and South America. There has been no exchange of species in recent times, for there is no species or genus common to the two continents. The South American and African elements of these two families must have been derived from some intermediate land mass or must have gone from one continent to the other over a land bridge. That this connection, whatever it was, must have been obliterated before the tertiary, is evidenced by the facts that the tertiary deposits of Taubaté and Parana show existing genera and that there are many South American types, as the Gymnotidæ, Electrophoridæ, Bunocephalidæ, Loricariidæ, Argiidæ, Pygidiidæ, Callichthyidæ, Hypophthalmidæ and others not found in Africa that have all