THE AMAZON DISTRICT. 327
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, Brachiums satanas, zxidjacchus 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
