“Something is already known. It is ascertained that the South American rivers possess some fishes peculiar to them. Were these fishes then created in these separate water-systems as they now exist, or have they been transferred thither from some other water-bed? If not born there, how did they come there? Is there, or has there ever been, any possible connection between these water-systems? Are their characteristic species repeated elsewhere? Thus we narrow the boundaries of the investigation, and bring it, by successive approaches, nearer the ultimate question. But the first inquiry is, How far are species distinct all over the world, and what are their limits? Till this is ascertained, all theories about their origin, their derivation from one another, their successive transformation, their migration from given centres, and so on, are mere beating about the bush. I allude especially to the fresh-water fishes, in connection with this investigation, on account of the precision of their boundaries. Looking at the matter theoretically, without a positive investigation, I do not expect to find a single species of the Lower Amazons above Tabatinga.[6] I base this supposition upon my own observations respecting the distribution of species in the European rivers. I have found that, while some species occur simultaneously in the many upper water-courses which combine to form the Rhine, the Rhone, and the Danube, most of them are not found in the lower course of these rivers; that, again, certain species are found in two of these water-basins and do not occur in the third, or inhabit only one and are not to be met in the two others. The brook trout, for instance (_Salmo Fario_), is common to the upper course and the higher tributaries of all the three river-systems, but does not inhabit the main bed of their lower course. So it is, also, and in a more striking degree, with the Salmling (_Salmo Salvelinus_). The Huchen (_Salmo Hucho_) is only found in the Danube. But the distribution of the perch family in these rivers is, perhaps, the most remarkable. The Zingel (_Aspro Zingel_) and the Schrætzer (_Acerina Schrætzer_) are only found in the Danube; while _Acerina cernua_ is found in the Danube as well as in the Rhine, but not in the Rhone; and _Aspro asper_ in the Danube as well as in the Rhone, but not in the Rhine. The Sander (_Lucioperca Sandra_) is found in the Danube and the other large rivers of Eastern Europe, but occurs neither in the Rhine nor in the Rhone. The common perch (_Perca fluviatilis_), on the contrary, is found both in the Rhine and Rhone, but not in the Danube, which, however, nourishes another species of true Perca, already described by Schaeffer as _Perca vulgaris_. Again, the pickerel (_Esox Lucius_) is common to all these rivers, especially in their lower course, and so is also the cusk (_Lota vulgaris_). The special distribution of the carp family would afford many other striking