“You will find that they thought they came across the tracks of some naked human foot at a great altitude. That looks at first sight like an hallucination. What more natural than that a brain excited and exhilarated by the extreme height should have interpreted certain marks in the snow as the footprints of a human being? Every bodily organ at these altitudes is exerting itself to the utmost to do its work, and the brain seizes on those marks in the snow and says ‘Yes, I’m all right, I’m doing my job, and I perceive marks in the snow which I affirm are human footprints.’ You know, even at this altitude, how restless and eager the brain is, how vividly, as you told me, you dream at night. Multiply that stimulus and that consequent eagerness and restlessness by three, and how natural that the brain should harbour illusions! What after all is the delirium which often accompanies high fever but the effort of the brain to do its work under the pressure of feverish conditions? It is so eager to continue perceiving that it perceives things which have no existence!” “And yet you don’t think that these naked human footprints were illusions,” said I. “You told me you would have thought so, if you had not known better.” He shifted in his chair and looked out of the window a moment. The air was thick now with the density of the big snow-flakes that were driven along by the squealing north-west gale. “Quite so,” he said. “In all probability the human footprints were real human footprints. I expect that they were the footprints, anyhow, of a being more nearly a man than anything else. My reason for saying so is that I know such beings exist. I have even seen quite near at hand--and I assure you I did not wish to be nearer in spite of my intense curiosity--the creature, shall we say, which would make such footprints. And if the snow was not so dense, I could show you the place where I saw him.” He pointed straight out of the window, where across the valley lies the huge tower of the Ungeheuerhorn with the carved pinnacle of rock at the top like some gigantic rhinoceros-horn. On one side only, as I knew, was the mountain practicable, and that for none but the finest climbers; on the other three a succession of ledges and precipices rendered it unscalable. Two thousand feet of sheer rock form the tower; below are five hundred feet of fallen boulders, up to the edge of which grow dense woods of larch and pine. “Upon the Ungeheuerhorn?” I asked.