CHARACTERISTICS AND DISTRIBUTION OF LAKES 639 
from sea to sea, by way of the San Juan River and I^ake Nicaragua, 
was commenced in 1889 by a United States company, but the work 
was suspended when the United States Government took up the 
Panama Canal scheme. Lake Managua is 40 miles in length by 
^5 miles in maximum breadth, with an area of approximately 500 
square miles and a maximum depth of about 90 feet. The tempera- 
ture of the water taken in March 1906 at several points at a depth 
of 12 feet was 83° F. (28° "3 C). The same temperature was observed 
at the northern end of Lake Nicaragua in 18 feet of water. 
The fish . fauna of these lakes is rather poor considering their size, 
and comprises about 30 species of true fresh-water fishes, the majority 
of which are endemic.^ The presence of marine fishes in the lakes is 
interesting ; with one or two possible exceptions, these are all shore 
fishes of the Atlantic coast, such as run up all suitable rivers. 
Mr Tate Regan informs me in a letter that the marine element is a 
modern one, and does not in any way indicate that the lakes were 
formerly marine, but only that they were recently accessible from 
the sea. One of the peculiar ichthyic features of Lake Nicaragua is 
the red, or partially red, cichlids or moj arras. 
During the glacial epoch a fairly warm temperature must have South 
prevailed in inter-tropical South America, so that the running waters 
suffered no serious arrest, and the rivers, except in the sub- Antarctic 
lands of the extreme south, where indications of former glaciation on 
a vast scale are still in evidence, have excavated their beds down to 
their natural levels, drained most of the old lacustrine basins, and 
effaced the greater number of the falls and rapids which formerly 
abounded in many districts. 
The continent of South America was in former geological periods 
probably occupied by an inland sea surrounded by elevated masses of 
land. The eastern portion (the Brazilian highlands) was very much 
higher than the western (the Andean chain), and as the latter gradually 
rose, and the former was lowered by subsidence and denudation, the 
sea was broken into two or three secondary basins, the northern portions 
becoming transformed into the fluvial valleys which now constitute 
the Orinoco and Amazon systems, and the southern into the valley 
now traversed by the Parana-Paraguay River and its tributaries. This 
appears to be the explanation of the interconnnunications between the 
river systems ; the Orinoco is linked to the Amazon by the Cassequiare, 
and the Amazon to the Parana by an intricate network of channels. 
The slopes of the divide in the latter case are so gently inclined that 
the slightest cause suffices to divert the currents from one basin to 
the other. " Due to their horizontality, all the plains, from the 
^ Biologia Gentrali Americana: Pisces, by C. Tate Regan, London, 1906-8. 
