"... all physical actions on the sub-microscopic level, on the level of molecules and atoms and sub-atomic particles and quanta of energy--all these actions," the chief physicist told Jason Wall, "are reversible. If you can control the reversal, you can return matter, energy, and space to its former state. Doing that, you travel through time. Therefore--" "Never mind the details," Jason Wall snapped. "That's your department. I only want to know this: will it work. Will it take a man back through time." "Yes, but--" "Very well. I'll go." "But we haven't figured out a way to return. If you go, you won't come back. You'll have to spend the rest of your life back there." The rest of his life. Jason Wall smiled. The rest of his life could be measured in pain-wracked months, possibly only in weeks. Fifteen minutes after his discussion with the chief physicist, he sat down in the time chair. Anthropologists had been consulted for the final stages of the project. There would be no mistakes. He would go where and when he had to go.... "Ready, sir?" "Ready," said Jason Wall. Ready to destroy the human race-- His vision flashed and blurred. Time moved backward for him. * * * * * A forest trail. Animals used it, had carved it out of the wall of jungle. And the first man? Armed with a revolver, Jason Wall left the now useless time-chair and hid himself beside the trail. He waited three days, living on berries and a small marsupial creature he had caught with his bare hands. If First Man was around, he didn't want to frighten him off with gun-fire. At last, First Man came. He was, Jason Wall observed with objective detachment, a noble-looking creature. The first true man. Over six feet tall, perfectly proportioned. He looked quite the healthiest man Jason Wall had ever seen. If looks meant anything, he had never known a day of disease in his life, and never would. Jason Wall's determination to kill grew. He did not have to wait long. When First Man came by his hiding place he stood up, pointed the revolver, and fired it point-blank. He was, naturally, ready for the end. The death of First Man ought to mean the death of all men, the sudden blotting out, in all ages, of all mankind and all traces of mankind. First Man fell, mortally wounded. Blood gushed from his nostrils; he died. And Jason Wall went on existing. He didn't understand. It made no sense. The death of First Man should have brought all humanity in all future ages to an instant, painless end. A woman, he thought. There must be a woman. Already with child, perhaps, and therefore, the mother of all the human race.... Jason Wall followed the forest trail, his revolver ready. If the woman turned out to be as beautiful as the man had been handsome, Jason Wall would not relish his job. He'd always had a soft-spot, the one soft-spot in his makeup, for beautiful women. He found her in a little clearing before a cave.