"We do not know. We have never seen them. They were devised after the first years, when our fathers' fathers fled. But the legends are that the Tubes run everywhere beneath the crust of the earth, as the trails lie across the mountains, and that somewhere among these Tubes there is a Sun that glows, giving heat and light and power--and something else that we do not understand, a strange protection, not against us, but against a great evil--" "The Curtain!" "You know what this evil is?" "Perhaps. And I'll learn more. But tell me one thing now--why did your people flee this sickness?" The Elder's dignity was impressive. "I've told you that--what happens to their get. It is better to fight and to struggle for your life. Man was not meant to exist, as a worm does, pallid and content beneath a log." And that was as close to an expression of religious belief as O'Hara was to hear among these mountain clansmen. Security was evil in itself. Considering this, O'Hara decided that in those first years these clansmen who had fled the fat and effortless life of the atomic regions had discovered that security had an inevitable price, and it had been a price that they would not pay. They had turned their backs upon it, fleeing from it, escaping the greatest of all scientific wonders because of the greatest of all scientific mistakes--an unforeseeable error that they grasped only by seeing its result, without understanding of the cause. Yet they must have known it once. O'Hara knew it. Atomic contamination, never quite deadly, low enough to be tolerated, nonetheless had wrought its havoc in the genes of the race. It had reversed the process of evolution. The people of the lowlands, the atomic peoples, were reverting, returning toward the ape, and at a pace incredibly speeded up by some mutation within the reproductive genes that forced maturity at ten years of age, a generation every decade, ten to a century--twenty-seven since the establishment of the Atomic Curtain! But the rate of retrogression was immeasurably swifter than had been the slow climb upward since the dawn of time. For with the reversal of the process of evolution, an atomic disaster within the genes, had also come the reversal of the law that only the fittest could survive--the perfect atomic state, with security for all, was preserving and multiplying the predominant strain, those who were unfit!