_Separate and Overlapping Areas._--The species of a genus are distributed in two ways. Either they occupy distinct areas which do not touch each other and are sometimes widely separated, or they touch and occasionally overlap {18} each other, each species occupying an area of its own which rarely coincides exactly with that of any other species of the same genus. In some cases, when a river, a mountain-chain, or a change of conditions as from pasture to desert or forest, determines the range of species, the areas of two species of the same genus may just meet, one beginning where the other ends; but this is comparatively rare. It occurs, however, in the Amazon valley, where several species of monkeys, birds, and insects come up to the south bank of the river but do not pass it, while allied species come to the north bank, which in like manner forms their boundary. As examples we may mention that one of the Saki monkeys (_Pithecia monachus?_) comes up to the south bank of the Upper Amazon, while immediately we cross over to the north bank we find another species (_Pithecia rufibarbata?_). Among birds we have the green jacamar (_Galbula viridis_), abundant on the north bank of the Lower Amazon, while on the south bank we have two allied species (_Galbula rufoviridis_ and _G. cyaneicollis_); and among insects we have at Santarem on the south bank of the Amazon, the beautiful blue butterfly, _Callithea sapphira_, while almost opposite to it, at Monte-alegre, an allied species, _Callithea Leprieuri_ is alone found. Perhaps the most interesting and best known case of a series of allied species, whose ranges are separate but conterminous, is that of the beautiful South American wading birds, called trumpeters, and forming the genus Psophia. There are five species, all found in the Amazon valley, but each limited to a well-marked district bounded by great rivers. On the north bank of the Amazon there are two species, one in its lower valley extending up to the Rio Negro; and the other in the central part of the valley beyond that river; while to the south of the Amazon there are three, one above the Madeira, one below it, and a third near Para, probably separated from the last by the Tocantins river.