20 FRESH-WATER FISHES OF SOUTH AMERICA EIGENMANN. 



The validity of tliis proviuce, as of all the others considered here, 

 will become much more appareut if the number of peculiar species of 

 other than peculiar genera (see List iv, foot-notes) are taken into account. 

 Omitting alpine forms, such as Pygidium, the species, with few excep- 

 tions, are peculiar — the few excepted species being inhabitants of Cen- 

 tral America, from where they have very probably emigrated. As 

 several forms are now found on both the eastern and western sloi)es of 

 Panama, the isthmus does not seem to be a barrier to the migration of 

 fresh-water fishes at present, and many of the lowland species of east- 

 ern genera now inhabiting this proviuce may, within comparatively 

 recent time, have been derived from the east by way of Panama. That 

 the mountains of Panama are a greater barrier than the ocean is clearly 

 seen by noting the species found in the Rio Magdalena which are also 

 found in some of the other eastern slope streams, but are not found in 

 the Pacific proviuce. 



List VI would indicate that the Amazons and the region to the north- 

 ward constitute a well marked subregion or a province. The validity 

 of this i^rovince seems doubtful in the face of this seeoiiug preponder- 

 ance of evidence. The greater portion of the Parana and Paraguay 

 are unexplored, and it is tolerably safe to predict that many of the 

 genera enumerated as i)eculiar to this province will be found in some 

 portion of the La Plata sj^stem. The explorations of N'atterer in the 

 Cujaba full}' warrant such a forecast. He found several Amazonian 

 genera in this river which had not before been recorded from the La 

 Plata system, many of which have not again been taken in its lower 

 courses. Dr. D. S. Jordan* has lately called attention to the fact men- 

 tioned many years ago by Robert SchomburgA, that there is at times a 

 connection between the Amazon and La Plata systems. Dr. Jordan 

 says: "Prof. John C. Branner calls my attention to a marshy upland 

 which separates the valley of the La Plata from that of the Amazon, 

 and which permits the free movement of fishes from the Paraguay 

 River to the Tapajos. It is well known that through the Cassiquiare 

 River the Rio Negro, another branch of the Amazon, is joined to the 

 Orinoco River. It is thus evident that almost all the waters of eastern 

 South America form a single basin, so far as fishes are concerned."! 



The large number of genera found in the Amazons and La Plata which 

 do not occur in the rivers of southeastern Brazil (see List iii) would lead 

 one to conclude that the Amazonian genera reach the La Plata system 

 directly, even if such connections as are known to exist were not known. 



* Science Sketches, 120, foot-uote 1. 



tTlie American Naturalist of April, 1888, contains the following: "M. ChafFaujon, 

 the well known explorer of the Orinoco, has carefully studied the communication 

 between that river and tlie Amazon, by means of the Cassifjuiare, and comes to tlio 

 conclusion that it is of recent origin. Tiie rapid current of the Orinoco, as it passes 

 througli a gorge only 90 yards wide in the clay deposits, undermines the banks, and 

 this action, combined with actual overllow in the rainy season, has produced a perma- 

 nent channel. The clay deposits on the left bank have a slope towards the Amazon. 



