Page:The naturalist on the River Amazons 1863 v1.djvu/290

similar to those where it exists under a constant normal form. In these districts selection has not operated, or it is suitable to the conditions of life there prevailing, that the species should exist under an instable form. But in the adjoining moister forests, as the result shows, the local conditions were originally more favourable to one of these varieties than to the others. The selected one, therefore, increased more rapidly than its relatives; and the fact of the entire absence of these latter from an area whence they are now separated only by a few miles, points to the conclusion that they could not there maintain their ground. Those individuals of successive broods which were still better suited to the new conditions would for the same reasons be preferred over their relatives; and this process going forward for a few generations, the extreme form of H. Thelxiope would be reached. At this point the race became well adapted to the new area, which we may suppose to have been at that epoch in process of formation as the river plains became dry land, at the last geological changes in the level of the country. In the higher and drier areas of Guiana and the neighbouring countries, H. Melpomene has been the selected form; in the lower and more humid regions of the Amazons, H. Thelxiope has been preferred. An existing proof of this perfect adaptation is shown by the swarming abundance of the species; the derivation of H. Thelxiope from H. Melpomene is made extremely probable by the existence of a complete series of connecting links; and lastly, its permanent establishment is made evident by its refusal to intercross with its parent form, or revert