• HISTOEY OF EIVER AMAZON 361 



gradual elevation and consequent slow change of the marine 

 character of an ancient lagoon is afiorded by Lake Titicaca 

 on the borders of Peru. This lake, with a length of eighty 

 miles, lies in a mountain valley over 12,000 feet above sea- 

 level, and occupied not long ago a much larger area. To sup- 

 pose that this region should have risen from sea-level to such a 

 height, and still preserve the remnants of an ancient marine 

 fauna dating back to the period when it was a gulf of the 

 Pacific, would seem a very bold theory. Professor Suess,* 

 indeed, expresses the opinion that the presence of a marine 

 fauna in Lake Titicaca cannot be regarded as a sufficient proof 

 of the theory that the lake was at sea-level within recent 

 geological times. I quite concur with Professor Suess in so 

 far as the assumption of a recent elevation is concerned, but 

 we have reason to believe that certain ancient forms of animal 

 life, particularly among aquatic groups, have transmitted 

 their specific characters unchanged to their modern descen- 

 dants. It is conceivable, therefore, and even possible, that the 

 striking affinity of the fauna of Lake Titicaca to that of the 

 Pacific coast may have been preserved, although the actual 

 junction of the lake with the sea took place perhaps as far back 

 as early Tertiary times. The theory of the recent elevation was 

 first mooted by Mr. A. Agassiz,f owing to the discovery in the 

 lake of eight species of the marine amphipod AUorchestes, one 

 of which (A. dentatus) differs but slightly from a form still 

 inhabiting the Strait of Magellan. That fact alone might 

 be attributable to accidental dispersal, although the enormous 

 difference in height between the sea and the lake, and the 

 circumstance of there being eight different species of AUor- 

 chestes, would be difficult to explain on that theory. But 

 besides this marine crustacean other members of a marine 

 fauna have been shown to exist in Lake Titicaca, and thus 

 the case against accidental dispersal has assumed a stronger 

 position. The fish fauna consists of a catfish (Pigidium 

 rivulatum"), belonging to a genus which is very widely distri- 

 buted all over South America, and several species of Orestias. 

 The latter genus is quite confined to Lake Titicaca, and since 



* Suess, E., " Antlitz der Brde," I., p. 693. 

 t Agassiz, A., " Lake Titicaca," p. 287. 



