THE AMAZON DISTRICT. 
471 
them flying in every direction,— as in the case of the 
nightingale, which is quite unknown in some of our 
western counties. Rivers generally do not determine 
the distribution of species, because, when small, there 
are few animals which cannot pass them; but in very 
large rivers the case is different, and they will, it is be- 
lieved, be found to be the limits, determining the range 
of many animals of all orders. 
With regard to the Amazon and its larger tributaries, 
I have ascertained this to be the case, and shall here 
mention the facts which tend to prove it. 
On the north side of the Amazon, and the east of the 
Rio Negro, are found the following three species of 
monkeys, Ateles paniscus, Brachiurus satanas, and Jac- 
elms bicolor. These are all found close up to the margins 
of the Rio Negro and Amazon, but never on the oppo- 
site banks of either river ; nor am I able to ascertain 
that either of them have ever been found in any other 
part of South America than Cayenne or Guiana, and the 
eastern part of Venezuela, a district which is bounded 
on the south and west by the Amazon and Rio Negro. 
The species of Pithecia, No. 14 of my list, is found 
on the west side of the Rio Negro for several hundred 
miles, from its mouth up to the river Curicuriari, but 
never on the east side, neither is it known on the south 
side of the Upper Amazon, where it is replaced by an 
allied species, the P. irrorata (P. hirsuta, Spix), which, 
though abundant there, is never found on the north 
bank. These facts are, I think, sufficient to prove that 
these rivers do accurately limit the range of some species, 
and in the cases just mentioned, the evidence is the 
