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
