328 ZOOLOGY OF 



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 Curicurian, 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. 

 Mrsuta, 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 

 more satisfactory, because monkeys are animals so well known 

 to the native hunters, they are so much sought after for food, 

 and all their haunts are so thoroughly searched, and the 

 localities for the separate kinds are so often the subject of 

 communication from one hunter to another, that it is quite 

 impossible that any well-known species can exist in a particular 

 district, unknown to men whose lives are occupied in forming 

 an acquaintance with the various tenants of the forests. 



On the south side of the Lower Amazon, in the neighbour- 

 hood of Para, are found two monkeys, Mycetes beelzebub and 

 Jacchus tamarin, which do not pass the river to the north. I 

 have never heard of monkeys swimming over any river, so that 

 this kind of boundary might be expected to be more definite 

 in their case than in that of other quadrupeds, most of which 

 readily take to the water. 



Towards their sources, rivers do not form a boundary 

 between distinct species ; but those found there, though rang- 

 ing on both sides of the stream, do not often extend down to 

 the mouth. 



Thus on the Upper Rio Negro and its branches are found 

 the Callithrix torquatus, Nyctipithecus trivirgatus, and Jacchus 

 (No. 21), none of which inhabit the Lower Rio Negro or 

 Amazon ; they are probably confined to the granitic districts 

 which extend from Guiana across the sources of the Rio 

 Negro towards the Andes. 



Among birds it cannot be expected that we should find 

 many proofs of rivers limiting their range ; but there is one 

 very remarkable instance of a genus, the three known species 



