The _Mumums_ brought up the rear now, apparently resigned to the fact that their puny efforts to prevent the theft of the secret had failed. Knowing that he would no longer be confronted by their unwinking gaze was a comfort to Farr, and he moved quickly across the sweltering valley. Moving rapidly as he was, he had no time to avoid crashing into the towering wall of rock that loomed suddenly in his path. Strangely he felt no pain as he clambered erect, but the very unexpectedness of the collision stunned him, confusing his befuddled mind even more. There should be no wall here, yet here it was. Farr could not deny that, though he could have sworn it had not been here when he entered the canyon. There was nothing to do but walk around it. The cliff stretched a hundred yards to either side of him. He began moving along it, a fierce anxiety to escape this infernal place beating in his brain. The inscrutable _Mumums_ followed, pattering along on bare feet. He had covered what seemed like forty yards, when he stopped and stared in puzzlement at the craggy precipice. He looked back along the wall, then ahead, peered up at its dizzy heights, then down at its smooth base. Color drained from his face and his shoulders slumped in defeat. He was beaten. Farr knew it. Knew too why that other adventurer had never left the valley, why his brain was spinning and whirling like a mad dervish. The ray in the temple--he could easily guess now what it had done to him. For stone does not move of its own power, and the cliff _had_ moved. Its terminations still reared a hundred yards in either direction from him! It was an illusion, that wall, an illusion conjured by his own ray-impregnated mind. But for him it was real. He could spend eternity walking along its face, yet never would he reach the end of the barrier. He watched dully as a _Mumum_ scampered past and melted into the cliff. Maddening to know that the wall existed only in his own mind. He tried to concentrate, tried to nullify the terrible force that had invaded his brain. If he could do that, the cliff-illusion would vanish. Veins stood out on his forehead with the effort; but the wall remained, seemingly solid as ever to his touch. The ray-force was too powerful. Farr wanted to scream, wanted to hurl himself at the barrier and pound and tear at it with all the wild energy of a trapped animal. But he couldn't. His emotions were played out. He could only stand like a burnt-out robot, his apathetic eyes following the antics of the _Mumums_ as they popped in and out of the wall-illusion, taunting him, trying to drive him mad with the realization that only he could not pass through it to freedom. The Ancients had triumphed. The life secret would remain in this valley, eternally guarded by the ingenious ray that warped men's minds and made them see illusions that to them were insuperable actualities. Not until a Master Intellect claimed the heritage for the human race, would it ever leave here.