HISTORY OF FIVER AMAZON 
361 
gradual elevation and consequent slow change of the marine 
character of an ancient lagoon is afforded 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 tlie presence of a marine 
fauna in Lake Titicaca cannot he 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 Allorchestes, 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 Allor¬ 
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 Erde,” I., p. 693. 
t Agassiz, A., “ Lake Titicaca,” p. 2S7. 
