At Maldonado within the distance of a morning's walk no fewer than eighty species of birds were collected, most of them exceedingly beautiful. Darwin's observations on the molothri (representatives of our cuckoos), the tyrant fly-catchers, and the carrion-feeding hawks are most attractive reading. Rio Negro, much further south, was next visited, and the fauna of a salt lake examined. The adaptation of creatures to live in and near brine struck him as wonderful. "Well may we affirm," says he, "that every part of the world is habitable! Whether lakes of brine, or those subterranean ones, hidden beneath volcanic mountains--warm mineral springs--the wide expanse and depths of the ocean--the upper regions of the atmosphere, and even the surface of perpetual snow--all support organic beings." Here he found reason to believe that all the great plains which he was surveying had been raised above the sea level in a modern geological period. Our naturalist started by land for Bahia Blanca and Buenos Ayres on August 11, 1833, and we have the record: "This was the first night which I had ever passed under the open sky, with the gear of the recado for my bed. There is high enjoyment in the independence of the Gaucho's life, to be able at any moment to pull up your horse, and say, 'Here we will pass the night.' The deathlike stillness of the plain, the dogs keeping watch, the gipsy group of Gauchos making their beds round the fire, have left in my mind a strongly-marked picture of this first night, which will not soon be forgotten." After an interesting _rencontre_ with General Rosas, Bahia Blanca was reached, and at Punta Alta were found many of the fossil bones which Owen subsequently described, this point being a perfect catacomb, as Darwin terms it, for monsters of extinct races. The remains of nine great kinds of quadrupeds chiefly allied to the sloths were found embedded on the beach within a space of about two hundred yards square; and these were associated with shells of molluscs of still existing species. Here was indeed a remarkable fact to germinate in the great naturalist's mind. It bore full fruit at a later date. An important theory then current, that large animals require a luxuriant vegetation, was overthrown at the same time, for there was every reason to believe that the sterility of the surrounding country was no new thing. The South American ostrich and many other animals here afforded material for important observations.