Numbers of small ravines emerge from the barren slopes flanking the Rio Grande, and streams of inconsiderable size pour their water into the larger artery. All these openings are filled with brush and low trees; we followed up one of them and, within a few hours, reached a habitation called Bella Vista. The shambling structure stood on the edge of a clearing planted in sugar-cane. Dense jungles of wild cane and brush bounded the plantation. As I was already convinced that the Rio Grande is the avenue up which the chaco bird-life was penetrating into the higher regions, we determined to remain at Bella Vista sufficiently long to substantiate my views; it required only one day for this purpose. The species that had been found in limited numbers farther up, and that seemed to belong to a strange fauna, exist in abundance at Bella Vista; among them are brown-shouldered orioles, white anis (_Guira_), fork-tailed goatsuckers, white-throated toucans (_Ramphastos_) and many others. Pigeons (_Leptoptila_) were so numerous that they suffered for lack of food. I am unable to say whether there had been an abnormal increase in the number of the species, or if the food-supply was unusually low; but one thing is certain--they were in a very emaciated condition and some of them had become so weakened that flight was impossible, and they fell an easy prey to the natives or predatory animals. I also noticed that all the pigeons were infested with parasites, but the weaker individuals were covered with them, including many winged, fly-like bird-ticks (_Hippoboscidæ_) that skipped among the feathers at bewildering speed, and finally flew away with a loud buzz; sometimes the repulsive insects settled on our hands or faces, when it was almost impossible to displace them, owing to their agile movements and to their clinging ability caused by the hooks on their feet. This furnished a very good illustration of the survival of the fittest, and one that I believe is typical of what happens in many instances. Owing, perhaps, to unusual or long-continued favorable conditions, the species had become exceedingly numerous. So long as there was no shortage in the food-supply, the birds were able to hold their own and keep increasing; but, as the season of famine approached, as I believe it must occasionally do, though not necessarily at regular intervals, the weaker individuals were the first to feel the pinch of a reduced subsistence which automatically rendered them still less suited to obtain a livelihood. Their rapidly failing vigor also prevented them from coping with their natural enemies--whether parasitic or predaceous, so that they were soon eliminated and only those that entered the struggle in the strongest, healthiest condition stood a reasonable show of surviving.