HASEMAN, GEOGRAPHICAL DISTRIB VTION IN SO UTH AMERICA 73 



either the Amazon or the La Plata, regardless of any hypothetical con- 

 nection with the eastern hemisphere, for the following reasons : 



1. Smaller volume of water, relative higher altitude, excepting parts 

 of Patagonia and strong currents. 



2. The original stock, part of which was marine in origin, did not 

 develop into so many species and genera in the restricted environments 

 as it did in the more extensive environments, because the factors active 

 in the evolution and preservation of life were neither as favorable nor as 

 numerous in the restricted environments. 



3. Many species are adapted to live in only certain environments. 

 One would not expect, therefore, to find Lepidosiren in the Alto Eio 

 Parana, the coastwise streams of eastern Brazil and Patagonia. Lepido- 

 siren lives in the vast swamps (chacos and pantanals) of Eio Paraguay 

 and Eio Amazonas. 



4. Species of more or less recent marine origin have encountered far 

 greater difficulty in entering small, rapid, rocky, shallow streams with a 

 limited supply of food than large rivers, because marine fishes are used 

 to swimming in the sea and not in rapid rivers and because the change 

 from sea to fresh water is less sudden. 



5. Some of these rivers have higher altitudes, and the number of fish 

 always bear a relation to the altitude. 



The last of the environmental complexes which needs a further con- 

 sideration is that of Patagonia. It lies between 40° and 55° south 

 latitude and is characterized by a general paucity of plants and animals, 

 especially tropical forms. Its plains are arid and forestless. Its rivers 

 are not large, because they are for the most part fed by the melting 

 snoAv on the lofty Andes. A cold Antarctic current flows along the coast, 

 which is devoid of swamps. Besides, vast tracts of Patagonia have been 

 under the sea during part of the Tertiary period. At this time, it is 

 quite possible that nearly all of its fresh-water life was exterminated, 

 and as the land rose again from the sea with the Tertiaiy elevation of 

 the Andes, the northern portion of it became semi-desert. In part of 

 this region, the rivers flowing down from the Andes dry up on the barren 

 plains. Hence, a southern migration of fishes would have been almost 

 blocked, excepting in the case of the P} r gidae, which are found everywhere 

 in South America. 



Notwithstanding all this and the fact that the Patagonian rivers have 

 few marine immigrants, its twenty-six known species of fishes contrast 

 favorably at least in number, either with the secca (dried) regions of 

 Ceara and Pernambuco, Brazil, or with similar latitudes in some parts 

 of the northern hemisphere. It is not at all strange that such tropical 



