A narrative of travels on the Amazon and Rio Negro,

by Alfred Russel Wallace

Towards their sources, rivers do not form a boundary between distinct species; but those found there, though ranging 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 Río 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 of which are separated by rivers, namely, the three species of the genus Psophia, P. crepitans (Linn.), P. viridis (Spix), and P. leucoptera (Spix). The P. crepitans is the common trumpeter of Guiana; it extends into the interior all over the country, beyond the sources of the Rio Negro and Orinooko, towards the Andes, and down to the Amazon, both east and west of the Rio Negro, but is never found on the south side of the Amazon.

The P. viridis is found in the forests of Pará, at Villa Nova, on the south bank of the Amazon, and up to the Madeira, where it is found at Borba, on the east bank.

The P. leucoptera, a most beautiful white-backed species, is found also on the south bank of the Amazon, at São Paulo, at Ega, at Coarí, and opposite the mouth of the Rio Negro, but not east of the Madeira, where the green-backed species commences. These birds are all great favourites in the houses of the Brazilians, and all three may sometimes be seen domesticated at Barra, where they are brought by the traders from the different districts in which they are found. They are inhabitants of the dense forests, and scarcely ever fly; so that we see the reason why the rivers should so sharply divide the species, which, spreading towards each other from different directions, might otherwise become intermingled. It is not improbable that, if the two Brazilian species extend as far as the sources of the Madeira, they may be found inhabiting the same district.

Of the smaller perching-birds and insects, which doubtless would have afforded many interesting facts corroborative of those already mentioned, I have nothing to say, as my extensive collection of specimens from the Rio Negro and Upper Amazon, all ticketed for my own use, have been lost; and of course in such a question as this, the exact determination of species is everything.

The two beautiful butterflies, Callithea sapphira and C. Leprieuri, which were originally found, the former in Brazil, and the latter in Guiana, have been taken by myself on the opposite banks of the Amazon, within a few miles of each other, but neither of them on both sides of that river.

Mr. Bates has since discovered another species, named after himself, on the south side of the Amazon; and a fourth, distinct from either of them, was found by me high up in one of the north-western tributaries of the Rio Negro, so that it seems probable that distinct species of this genus inhabit the opposite shores of the Amazon.

The cock of the rock, Rupicola cayana, is, on the other hand, an example of a bird having its range defined by a geological formation, and by the physical character of the country. Its range extends in a curving line along the centre of the mountainous district of Guiana, across the sources of the Rio Negro and Orinooko, towards the Andes; it is thus entirely comprised in the granite formation, and in that part of it where there are numerous peaks and rocks, in which the birds make their nests.

Whether it actually reaches the Andes, or occurs in the same district with the allied R. Peruviana, is not known, but personal information obtained in the districts it inhabits, shows that it is confined to the narrow tract I have mentioned, between 1° south and 6° north latitude, and from the mountains of Cayenne to the Andes, south of Bogotá.