Curt recoiled at the sudden venom in Sark's voice. "The _future_?" That was what they had in common with Dell--psychosis, systematic delusions. He had suspected danger before; now it was imminent and terrifying. "Perhaps you are one of those who regard your accomplishments with pride," Sark went on savagely, ignoring or unaware of Curt's fear and horror. "That the hydrogen bombs smashed the cities, and the aerosols destroyed the remnants of humanity seems insignificant to you beside the high technical achievement these things represent." Curt's throat was dry with panic. Irrelevantly, he recalled the pain-fired eyes of Dell and the dying scientist's words: "The responsibility for the coming destruction of civilization lies at the doors of the scientist mercenaries--" "Some of us _did_ manage to survive," said Sark, glaring at the scene of gaunt rubble. Curt could see the veins pounding beneath the thin flesh of his forehead. "We lived for twenty years with the dream of rebuilding a world, the same dream that has followed all wars. But at last we knew that the dream was truly vain this time. We survivors lived in hermetically sealed caverns, trying to exist and recover our lost science and technology. "We could not emerge into the Earth's atmosphere. Its pollution with virulent aerosols would persist for another hundred years. We could not bear a new race out of these famished and rickety bodies of ours. Unless Man was to vanish completely from the face of the Earth, we had only a single hope. That hope was to prevent the destruction from ever occurring!" Sark's eyes were burning now. "Do you understand what that means? We had to go _back_, not forward. We had to arm to fight a new war, a war to prevent the final war that destroyed Mankind." "Back? How could you go back?" Curt hesitated, grasping now the full insanity of the scene about him. "How have you _come_ back?" He waited tautly for the answer. It would be gibberish, of course, like all the mad conversation before it. * * * * * "The undisturbed flow of time from the beginning to the end--neither of which we can experience--we call the Prime Continuum," Sark replied. "Mathematically speaking, it is composed of billions of separate bands of probability running side by side. For analogy, you may liken it to a great river, whose many insignificant tributaries merge into a roaring, turbulent whole. That is the flow of time, the Prime Continuum. "You may change one of these tributaries, dam it up, turn it aside, let it reach the main stream at a different point. No matter how insignificant the tributary, the stream will not be the same after the change. That is what we are doing. We are controlling critical tributaries of the Prime Continuum, altering the hell that you scientists have so generously handed down to us.