Travels on the Amazon

by Alfred Russel Wallace

Pará itself, for variety of species, is perhaps the best locality for diurnal Lepidoptera; six hundred distinct kinds may be obtained within a day's walk of the city. At Santarem I had increased my collection to seven hundred species, at Barra to eight hundred, and I should have brought home with me nine hundred species had my collections arrived in safety. Mr. Bates, who has paid more exclusive attention to insects, states that he has now obtained twelve hundred species,—a wonderful collection to be made by one person, in a country without any variation of climate or of physical features, and no part of it elevated five hundred feet above the level of the sea.

E. Geographical Distribution of Animals.

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.