372 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



Evidence for these conclusions has been given in detail in the foregoing 

 pages. The basis for the entire discussion is a list of the species giving 

 their geographical distribution. Such a list has been prepared and follows 

 this discussion. 



The points of strategic importance for ichthyic chorology in South 

 America are, therefore, {a) western Colombia and Panama ; {d) Guayaquil 

 and Peru to the Amazon, across the Andes ; [c) the tableland of Guiana, 

 Archiguiana ; [d) the Rio San Francisco, with the Rio Parahyba and the 

 headwaters of the Tiete and Rio Grande, in Archamazona, and [e) the 

 area between the Rio Negro and the La Plata. 



Note on Lake Titicaca and its Fauna. 



In conversation with Prof G. Steinmann the latter called attention to 

 his views concerning Lake Titicaca, which he had the kindness to write 

 out for me. 



"The region at present covered by Lake Titicaca was formerly, as late 

 as late Tertiary times, a normally drained area. Its drainage was south- 

 east toward the Amazon. It was not till the Glacial epoch that the glaciers 

 of the high Andes pushed their moraines into the drainage valley and 

 formed the lake. It is, therefore, a glacial-dam lake which did not retain 

 its Amazonian drainage, but flowed over the low watershed southeast to- 

 ward the undrained high plateau of Bolivia. There is no evidence that 

 the lake was formerly connected with the Pacific. Not only are there no 

 marine formations in the inter-Andean high plateaus of northern Bolivia 

 and southern Peru belonging to the Diluvial time, but also those of Ter- 

 tiary times are lacking. For this reason also the fauna of Titicaca cannot 

 be explained as a relict, but must have arisen from the ocean by migration 

 in a roundabout way through former rivers and lakes." 



In my account of Lake Titicaca it was assumed that the origin of the 

 lake from an arm of the sea was without question, and an attempt 

 was made to explain its fauna on that basis. It was assumed that the 

 genus Orestias gave rise to numerous species, some of which succeeded 

 in crossing the divide into neighboring rivers. The explanation of Stein- 

 mann would obviate the difficulty of originating numerous species from 

 one type in a restricted unit environment. General observation every- 

 where gives evidence that segregated individuals of a given species tend 

 to diverge from the central type, not that a species mutates into a large or 



