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 t;ach 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 tliat what is 
