A yet more remarkable passage in the 'Journal of the "Beagle,"' though entered under the account of events observed in the year 1834, must almost certainly have been written somewhat later, and subsequently to Darwin's first reading of Malthus's momentous work, 'The Principle of Population,' which (as we know from his own pen) formed a cardinal point in the great biologist's mental development. It runs as follows in the published journal:[1]--'We do not steadily bear in mind how profoundly ignorant we are of the conditions of existence of every animal; nor do we always remember that some check is constantly preventing the too rapid increase of every organised being left in a state of nature. The supply of food, on an average, remains constant; yet the tendency in every animal to increase by propagation is geometrical, and its surprising effects have nowhere been more astonishingly shown than in the case of the European animals run wild during the last few centuries in America. Every animal in a state of nature regularly breeds; yet in a species long established any great increase in numbers is obviously impossible, and must be checked by some means.' Aut Malthus aut Diabolus. And surely here, if anywhere at all, we tremble on the very verge of natural selection. It would be impossible to follow young Darwin in detail through his journey to Buenos Ayres, and up the Parana to Santa Fé, which occupied the autumn of 1833. In the succeeding year he visited Patagonia and the Falkland Islands, having previously made his first acquaintance with savage life among the naked Fuegians of the extreme southern point of the continent. Some of these interesting natives, taken to England by Captain Fitzroy on a former visit, had accompanied the 'Beagle' through all her wanderings, and from them Darwin obtained that close insight into the workings of savage human nature which he afterwards utilised with such conspicuous ability in the 'Descent of Man.' Through Magellan's Straits the party made their way up the coasts of Chili, and Darwin had there an opportunity of investigating the geology and biology of the Cordillera. The year 1835 was chiefly spent in that temperate country and in tropical Peru; and as the autumn went on, the 'Beagle' made her way across a belt of the Pacific to the Galapagos archipelago.