10 A JOURNEY IN BRAZIL. 



to one, our record may not be lost. We must try not to 

 mix the fishes of different rivers, even though they flow 

 into each other, but to keep our collections perfectly dis- 

 tinct. You will easily see the vast importance of thus 

 ascertaining the limitation of species, and the bearing of 

 the result on the great question of origin. 



" Something is already known. It is ascertained that 

 the South American rivers possess some fishes peculiar to 

 them. Were these fishes then created in these separate 

 water-systems as they now exist, or have they been trans- 

 ferred thither from some other water-bed ? If not born 

 there, how did they come there ? Is there, or has there 

 ever been, any possible connection between these water-sys- 

 tems ? Are their characteristic species repeated elsewhere ? 

 Thus we narrow the boundaries of the investigation, and 

 bring it, by successive approaches, nearer the ultimate 

 question. But the first inquiry is, How far are species 

 distinct all over the world, and what are their limits ? Till 

 this is ascertained, all theories about their origin, their 

 derivation from one another, their successive transforma- 

 tion, their migration from given centres, and so on, are 

 mere beating about the bush. I allude especially to the 

 fresh-water fishes, in connection with this investigation, 

 on account of the precision of their boundaries. Looking 

 at the matter theoretically, without a positive investigation, 

 I do not expect to find a single species of the Lower Amazons 

 above Tabatinga.* I base this supposition upon my own ob- 



* This anticipation was more than confirmed by the result of the journey. It 

 is true that Mr. Agassiz did not go beyond the Peruvian frontier, and therefore 

 could not verify his prophecy in that region. But he found the localization of 

 species in the Amazons circumscribed within much narrower limits than he ex- 

 pected, the whole length of the great stream, as well as its tributaries, being 

 broken up into numerous distinct faunae. There can be no doubt that what is 



