A narrative of travels on the Amazon and Rio Negro,

by Alfred Russel Wallace

There is no part of natural history more interesting or instructive, than the study of the geographical distribution of animals.

It is well known that countries possessing a climate and soil very similar, may differ almost entirely in their productions. Thus Europe and North America have scarcely an animal in common in the temperate zone; and South America contrasts equally with the opposite coast of Africa; while Australia differs almost entirely in its productions from districts under the same parallel of latitude in South Africa and South America. In all these cases there is a wide extent of sea separating the countries, which few animals can pass over; so that, supposing the animal productions to have been originally distinct, they could not well have become intermixed.

In each of these countries we find well-marked smaller districts, appearing to depend upon climate. The tropical and temperate parts of America and Africa have, generally speaking, distinct animals in each of them.

On a more minute acquaintance with the animals of any country, we shall find that they are broken up into yet smaller local groups, and that almost every district has peculiar animals found nowhere else. Great mountain-chains are found to separate countries possessing very distinct sets of animals. Those of the east and west of the Andes differ very remarkably. The Rocky Mountains also separate two distinct zoological districts; California and Oregon on the one side, possessing plants, birds, and insects, not found in any part of North America east of that range.

But there must be many other kinds of boundaries besides these, which, independently of climate, limit the range of animals. Places not more than fifty or a hundred miles apart, often have species of insects and birds at the one, which are not found at the other. There must be some boundary which determines the range of each species; some external peculiarity to mark the line which each one does not pass.

These boundaries do not always form a barrier to the progress of the animal, for many birds have a limited range, in a country where there is nothing to prevent 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 believed, 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 Jacchus bicolor. These are all found close up to the margins of the Rio Negro and Amazon, but never on the opposite 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 Curicuriarí, 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 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 neighbourhood of Pará, 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.