362 PATAGONIAN EXPEDITIONS : ZOOLOGY. 



Of the genera peculiar to the western slope Protisiiis and Gastropteyns 

 are of marine origin not related to Atlantic slope forms. They are found 

 at high altitudes and are remnants of early colonists from the sea. Lebi- 

 asina is a genus of CharacidcB very closely related to Piabitcina of wide 

 distribution on the Atlantic slope. Saccodon is related to Paradon, dis- 

 tributed from the La Plata to Colombia and to Heiuiodusixom. the Orinoco 

 to the San Francisco, the Peruvian Amazon and the upper courses of the 

 Paraguay. From one or the other it is probably an offshoot, sufficiently 

 remote in time to be generically different. All of the genera are very 

 probably among the oldest on the continent. 



It is seen that the present distribution of the fresh-water fishes does not 

 require an interoceanic waterway for its explanation. Does the present 

 distribution of fresh-water fishes offer any objection to such an inter- 

 oceanic connection in recent times ? Two or three things are opposed to 

 this supposition. One is the paucity of the Pacific fauna in general, and 

 the second the absence of many Atlantic slope families, especially the 

 Lepidosirenidce, Aspredinidce, HypopJithahnida^, CalUchtliyida, Electro- 

 p/iondce, Osfeoglossidce, Arapamida; and many dominant Atlantic slope 

 genera, all of them lowland forms, and the third the absence of all 

 tropical American forms, except Pygidium from Lake Titicaca. 



If the reigning fauna had been present on the eastern slopes of the Andes 

 even in outline at a time of the interoceanic canal, a very much larger per 

 cent, of the Atlantic slope forms ought to be found on the Pacific slope, 

 unless the Pacific slope streams were uninhabitable, or, unless the fauna be- 

 came extinct, after the separation of the oceans and the streams were 

 repopulated by new emigrants from the east. 



The meagreness of the Pacific slope fauna is probably in great part due 

 to the small rivers and precarious water supply. It can scarcely be sup- 

 posed that a fauna became extinct to be replaced by a similar one. It is 

 quite certain that its present fauna has been derived from forms whose 

 wide distribution shows them to be especially fit to cross existing barriers 

 and which were probably the very first to reach the Atlantic slope of the 

 Andes. All of them are old genera, among the oldest on the continent. 



If the present fauna had been in existence along the Andes at the time 

 Lake Titicaca was much lower, it ought to have received a fair sprinkling 

 at least of the fresh-water fauna, but it contains little but marine types. 



All of these considerations lead me to conclude that the present fauna 



