62 FISHES OF WESTERN SOUTH AMERICA 
mann, 1922b, d; 1923a). Although most of the stocks passed through periods in 
which they inhabited the eastern highlands, it was not until the Amazon developed 
its great freshwater basin that it became the greatest hatchery of species known. 
From there Haseman (1912), Pearson (1937b) have traced the highways of dis- 
persal in various directions, while Eigenmann, |. ¢., found that the fishes of the 
Pacific slopes had to traverse a circuitous route around the low northern Andes of 
Colombia to reach their present habitats. 
Pearson finds two major sources of the population of the Paraguay, the Gua- 
poré, and of the Bolivian headwaters of the Madeira, the latter more nearly resem- 
bling the upper Amazonian fauna. Our fishes from the upper Peruvian Amazon 
and its tributaries should therefore exhibit close kinship with those of Bolivia, and 
they do. This is true, not only for the lowland species, but we find a number of 
fishes such as the Hemibrycons inhabiting regions of intermediate elevation in the 
tropical rain forests of both Peru and Bolivia. In Peru as in Bolivia and La Plata, 
the changes in a fauna as we proceed from the Amazon southward are similar, 
that is, changes mostly by subtraction. None of these facts aid in explaining the 
origins of the respective families, nor do we find any facts which would support 
Haseman’s theory as to a North American origin of Characin fishes. 
The present study differs from those previously made: in the age of the forma- 
tions which determine the types of habitat; in the large-scale topographic relief; 
and in the length of the period of occupancy. Like most of the continent, it is 
part of a unit uncontaminated from north, west, or south, and having its only 
family ties with the East. Numerous families are of tropical South American 
origin, and are confined to that continent, such as: Loricaridae, Astroblepidae, 
Callichthyidae, Aspredinidae, Hypophthalmidae, Pygidiidae, Gymnotidae; also 
numerous characin subfamilies. 
The fauna has developed with the continent itself, and despite the youth- 
fulness of our region, sufficient time has elapsed to differentiate the Pacific slope 
fishes completely from those of the Atlantic side, at least to species, and sometimes 
to distinct genera. 
The upthrust of the Andes as described above has created barriers more rapidly 
than weathering processes could smooth them down. This has resulted in the 
creation of: (a) the land-locked Titicaca-Poop6 basin, and of numerous areas, large 
and small, throughout our area, having mature topography and high elevation, 
plains, lakes, marshes; (b) a second, subtropical, zone to which some of the inhabi- 
tants of the first zone have escaped; (c) a zone of still lower elevation to which 
certain genera of Amazonian fishes have been able to ascend; and (d) the tropical 
lowlands, richly and abundantly populated from the various centers of dispersal 
of the Amazon system. 
The first and most elevated zone is occupied almost exclusively by fishes de- 
scended from those which were elevated with the mountains themselves. The 
second zone consists of limited localities with suitable terrain to encourage the 
further evolution of highland fishes, such as the Astroblepus of southern Ecuador. 
The third zone whose extent is to be described below is marked off from those 
above by greater or less extent of rapids, and represents the farthest reach of a 
