Page:Popular Science Monthly Volume 68.djvu/532

528 The origin and distribution of the fresh-water fishes of tropical South America have come about as follows: In the earliest tertiary tropical America consisted of two land areas, Archiguiana and Archamazona, separated by the lower valley of the Amazon, which was still submerged. There was a land mass, Helenis, between Africa and South America, possibly in contact with Guiana in South America and some point in tropical Africa.

This land mass was inhabited, among other things, by Lepidosirenidæ, Pceciliidæ, Characinidæ, Cichlidæ and Siluridæ. This land-mass sank beneath the surface of the ocean, forcing the fauna in two directions, towards Africa and towards South America, exterminating all types not moved to the east or the west. From these two rudiments have developed the present diverse faunas of Africa and South America, each reinforced by intrusives from the ocean and neighboring land areas and by autochthonous development within its own border. The one fauna can not be said to have been derived directly from the other.

The connection between Africa and South America existed before the origin of present genera and even before the origin of some of the present subfamilies and families, some time before the earlier tertiary. There has never been any exchange between Africa and South America since that time. There must have been an intimate connection between these two continents, for there is no evidence such as identical species or genera on the two coasts to indicate an occasional or accidental exchange of types across the Atlantic since the formation of existing genera, therefore such an interchange across the ocean probably never took place. The east Brazilian land mass south of the Amazon (Archamazona) must have become stocked from the western end of Helenis, or Archiguiana, very early, for it contains many genera peculiar to the region, indicating a long separation, and tertiary freshwater deposits in this area contain existing genera of fresh-water fishes.

When, later, the Cordilleras arose out of the ocean at a distance from Archiguiana and Archamazona too great to be traversed by colonists from them, their developing streams and arms of the sea, connected with brackish, and, later, fresh-water lakes, all became populated with marine types. In the north where they later came into competition with immigrants from Archiguiana most of them were exterminated with the continued elevation of the land. On the south, which was not, or not so easily, reached by immigrants, Orestias, Gastropterus and Protistius remain in the high Cordilleras of southern Peru as relicts of these marine species. Later, as the distance between the