600 LOCALIZATION OF FISH. 



of the river. Thus, the fishes found at the bottom of a lake 

 covering, perhaps, a square mile in extent when the waters 

 are lowest, will appear near the shores of the same lake 

 when, at the season of high- water, it extends over a much 

 wider area. In the same way, fishes which gather near the 

 mouth of a rivulet at the time of low- water, will be found as 

 high as its origin at the period of high-water ; and those 

 which inhabit the larger igarapes on the sides of the Amazon, 

 when they are swollen by the rise of the river, may be found 

 in the Amazon itself when the stream is low. There is not a 

 single fish known to ascend from the sea to the higher courses 

 of the Amazon at certain seasons, and to return regularly to 

 the ocean. 



The striking limitation of species within different areas 

 does not, however, exclude the presence of certain kinds of 

 fish simultaneously throughout the whole Amazonian basin. 

 The piraracu, for instance, is found everywhere from Peru to 

 Para ; and so are a few other species more or less extensively 

 distributed over what may be considered distinct ichthyolo- 

 gical fauna. But these wide-spread species are not migratory. 

 They have normally and permanently a wide range -just as 

 some terrestrial animals have an almost cosmopolite character 

 while others are circumscribed within comparatively narrow 

 limits. 



Surprising indeed is the variety of species of fishes contained 

 in the Amazonian basin. Professor Agassiz, during his expe- 

 dition, collected nearly two thousand, " for the most part," as he 

 observes and which is still more surprising " circumscribed 

 within different limits, from Tabalinga to Para, where the 

 waters differ neither in temperature, nor in the nature of their 

 bed, nor in the vegetation along their borders. There are met 



