Dr. John Bartok replaced the test tube carefully in its rack with fingers that shook a little. His pleasant, plump-cheeked face had the blank look of a man who has had too great a shock to assimilate, so that for the moment he feels nothing.

He got to his feet. Absently he ran stained, spatulate-tipped fingers through his thinning blond hair and groped for a cigarette. The liquid in the test tube caught a pinpoint of light from the fluorescents and thin fumes escaped upwards from its surface. Dr. Bartok shuddered slightly and turned toward the broad plate glass window that spread clear across the front of the chrome and enamel lab.

Behind him the swinging door squeaked. A hearty voice boomed greeting. "Well, John!"

Dr. Goodwin, head of the Nelson Foundation, thought it part of his duties to boom heartily. He was a plump man in morning coat and striped trousers with the inevitable pince-nez on a black ribbon. He was a greeter, a hand-pumper, a speech-maker and a born politician. He was also a surprisingly able physicist. His work on gamma and beta particles, publicized by his own skilled hand, had won him the directorship of the Nelson Foundation. He gave his men a free hand in their research, but he thought it his right to snoop into their work at all times so that few of them had any illusion of freedom.

John Bartok did not turn. "I've got it, Dr. Goodwin," he said and continued to look out the window.

The manicured lawns of the Nelson Foundation were a well ordered green in the spring sunshine. A marble wing of the building rose to his right. Through the slits of partly-drawn blinds he could see laboratory workers at their jobs. Below, a girl assistant walked briskly along one of the curving paths that wound through the flower beds, her white stockings and shoes twinkling under the crisp blue of her cape.

"Got it? You've got it?" Goodwin repeated.

Bartok turned and placed his back to the window, releasing his grasp on the sanity of sun and grass.

"The new weapon," he said flatly. "The final—the irresistible weapon. The thing we've been searching for ever since the stalemate of the atom bomb."

Excitement flamed in Goodwin's round face. His color mounted. "Where? What is it?" he demanded. "I'll phone General McComber!"

Bartok jerked his cigarette towards the test tube, which still gave off faint white fumes.

"This?" Goodwin stared at it fascinated. "It's—"

"It's the deadliest thing a man's imagination could ever hope to conceive," Bartok said flatly. He was not boasting. He was stating a simple fact. "Beside it the atom bomb is like a child's pea-shooter."

Goodwin edged back instinctively.

"Oh, it's harmless enough by itself," Bartok said. "It's a catalyst. Add it to something else, and—" He drew a deep breath. "Remember the scare the atom bomb threw into the world? And the menace of radioactive dust? And germ warfare? Remember people talking about the end of the human race as though it were just around the corner?" He nodded gloomily at the test tube. "Well, there it is, the real thing, if that stuff ever gets loose."

The uneasiness had fled Goodwin's face. His jowls were alight with excitement. He seized Bartok's hand, pumping it like a driveshaft.

"Congratulations," he beamed. "This puts us at the top of the heap again. It'll mean a lot to the Nelson Foundation, John. And to you. I've got to call the War Department!"

On his way to the door he stopped, turned back. "By the way," he asked. "What is it you add that stuff to, to make it the potent weapon?"

Bartok shifted his eyes and looked unhappy. "I'd rather not say," he muttered. "Not till I think this out a little more."

"Think what out?"

"I'm afraid of that stuff," Bartok said reluctantly. "It's—it's far more dangerous—well, sir, this time it's not just scare talk. It really could mean the end of man, the end of the world!"

"Nonsense," Goodwin said. "That's not for you to judge, John! We've got to turn it over to the War Department! It will make us the strongest nation on earth, give us control! Just the threat of having it will be enough. We won't have to use it!"

Bartok gave him a cynical glance and said nothing.

"Look." Goodwin's plump finger pointed to a newspaper lying open on a lab bench. Headlines flared.

ATOMIC WEAPONS NOT TO BE USED

REPRISALS FEARED

Top ranking military men believe atomic weapons will not be used in future wars because belligerents will fear reprisals, it was revealed today by a confidential source close to high officials in the War Department. The one way to victory and the best hope of avoiding war is to maintain the nation's war machine in a state of immediate readiness.
"You see?" Goodwin said. "Anything as terrible as you say this is could never be used. Just having it—and letting other nations know we have it—is insurance against anybody jumping on us. Keep them scared to death."

He beamed upon the silent Bartok. "I'll call General McComber to come down at once." The swinging door squealed and flapped behind his confident exit.

Dr. John Bartok took his cigarette and himself into the chair in front of the lab table and slumped down to stare at the gently fuming test tube. He was tired and he was confused. He was a little afraid of the responsibility thrust so suddenly upon him.

There was no simple decision to be made here. He could not deny his country a weapon it might possibly need desperately in some unknown future emergency. To withhold it might mean destruction. And to give it—to turn it over to some thick-necked general who thought of war principally as a road to promotions and glory? Bartok was under no illusions as to the average military mind, American or otherwise.

Yet he shrank from the thought of making so great a decision himself, of bucking the Foundation and the War Department by himself. Perhaps Goodwin was right, the weapon need never be used. Its threat alone might be enough. In any case, perhaps Congress or the people, or the professional trouble shooters, should decide, not John Bartok, laboratory man.

Miserably he rubbed his hair until it stood up in wild disarray on his pink scalp. He leaned forward to stare into the sauterne colored fluid. The fumes rose to his nostrils, they tickled and made him want to sneeze. Suddenly he was dizzy, the lab began to heave unsteadily about him.

Alarmed, Bartok pulled back, tried to climb to his feet. The movement made his head spin wildly. The whole lab floated up into a narrowing funnel above him and the darkness contracted about him like overlapping layers of a blanket.

After a long time there was light. He opened heavy lids to see a red glow of sun. Somewhere there was a guttural, familiar chanting. The smell of woodland was in his nostrils. Under him the foam rubber cushion had grown suddenly hard and uncomfortable. His vision cleared and amazement froze him.

He was squatted on a ledge in the face of a rough cliff-like slope. Before him a red sunset blazed over a wild, jungle-thick forest. Behind him was a cave mouth in which a fire flickered. From its depths came the chanting which was so strangely familiar.

Bartok looked down at himself. His limbs were all but naked, only a wolfskin draped about his hips served as both garment and belt. Arms and legs were hugely muscled, thick with furry hair. A beard spread its bushy length upon his hirsute chest.

He was neither alarmed nor frightened. He was Krug of the Cave Dwellers, and this was his home. Behind him was a bustle and Ngeel, the chief, came out on the ledge. He was a stocky, powerful man, wearing the leopard skin of authority, plus a bustling official manner, plus a belief that his tribesmen were his by right.

Ngeel squatted down beside Krug and chuckled deep in his throat. "They come, the Forest Men," he grunted. "They think to kill. We shall kill many and there will be much feasting."

Krug stared, troubled, out over the sea of waving branches. A dim thought struggled for expression in his mind.

"Always there is fighting," he said. "Always killing. We are men, not beasts. There is food for all. Must the women always weep for the dead, must children always wail for mothers and fathers slain?"

Ngeel's bushy eyebrows drew down. "In my father's day," he said, "men were truly wiser. Now we breed a race of fools. The world is very old, my father told me, and there has always been war. Men will always kill. Do you think to stop it with words? If we kill more of the Forest Men than they kill us, there will be no more war."

Krug shrugged massive shoulders. He leaned back against the rock and began again to twist and plait some strips of skin.

"What is that?" Ngeel asked.

"Something I am making. I had thought if I took—"

"They come!" Ngeel rose suddenly, began to roar a warning. "They come!"

Out of the forest shadows streamed a broken mass of men. They were armed with clubs, spears sharpened and hardened in the fire, and stones. They charged the cliff front.

Stones hurtled down upon them, and they gave ground momentarily, leaving one or two dark forms stretched out. Then Ngeel and the men of the Caves swung down the steep slope to give battle.

There was the snarling shock of combat, vicious but short. The Forest Men broke and fell back to the trees. Ngeel called back his blood-maddened warriors, who would have followed. The chief was canny, he knew the Forest Men held the advantage in the shadowy aisles that were their home.

From the fringes of the wood, the Forest Men shouted taunts, flung an occasional spear or stone which fell short. The Cave Men huddled at the base of the cliff, licking their wounds.

Up above, Krug worked busily at his plaiting. Ngeel climbed to his ledge.

"This is bad," said the chief. "They keep us from hunting, even from water, if they stay."

"They will not stay," Krug said.

He stood up. In his hand he held the cup-like piece between them. Krug searched about until he found a round stone about the size of his fist. He placed it in the leather cup.

As Ngeel watched, hairy mouth agape, Krug began to swing the loaded contraption round his head. Faster and faster it went, until it whistled through the air and dissolved into a blur. Then he let one of the thongs slip from his fingers.

Sheer luck played its part that day as it so often does. Had the stone fallen harmlessly, the effect would have been lost. But the astonished cavemen saw the missile scream through the air with a force that no hand-thrown rock could match. Out, out, it arched, far beyond any man's throwing distance.

A Forest Man, staring, slack-jawed, was slow to duck. The stone took him full in the face with sodden crunch. There was a horrid, strangled scream, he tossed wide his huge arms and fell dead amidst his terror stricken fellows.

With howls of dismay, the Forest Men fled into the shadows of the wood.

Krug was a little astonished himself at the appalling success of his first attempt. But the effect upon Ngeel was even more striking. Always the politician, he recovered swiftly.

"This is magic," he said. "It makes us the most powerful tribe in the land. The Forest Men shall fear us and the men of the marshes and the river tremble at our name!" Shrewdly he added, "There will be no more wars, now, Krug. Your magic is too strong. None shall dare stand against us!"

Musing, he stroked his beard. "Man is old, my father said," he muttered, "and always there has been war. But now is an end to war, for with magic that kills at such a distance, war is too terrible. There will be no more wars!"

And looking down at him, Krug wrestled with the astonishing idea that by inventing a terrible weapon of war, he had made war impossible. The weapon was too terrible to use.

So there was peace. The tribe hunted in the fringes of the forest and along the river bank and others fled before their approach. There was peace for the space of a moon.

Then a hunter came racing with the word that the Forest Men were coming.

"Coming?" Ngeel's bushy brows drew down in stupefied disbelief. "They have forgotten so soon? They need another lesson!"

Confident, Krug stood upon the rocky shelf and waited. And alongside him stood a dozen warriors of the tribe, all armed with slings, all trained by him. Each carried smooth round stones in a pouch at his belt. They were the most terrible fighting force the world had yet seen.

Shadows moved at the edge of the wood, broke free and slid out into the sunlight.

"They come!" Ngeel grunted.

At Krug's word, the slingmen reached for stones. The slings began to whirl overhead, rise in pitch to a whistle. Then the missiles arched out into the air. The Cave Men and their women shouted as the stones flew. They heard the heavy thump as the projectiles landed. But there were no screams from the Forest Men, there were no crushed and bleeding forms stretched upon the grass. The invaders came on.

And now, as they came closer, the Cave Men saw why. Each of the Forest Men carried before him a round leather shield stretched on a tough green sapling bent into a hoop. The strong rawhide took the full shock of a flung stone and let it rebound without harm. Safe behind their moving forts, the Forest Men came swiftly on, to reach and climb the wall. And now again came the snarling shock of combat, as men met and strove hand to hand with club and knife and teeth.

Krug flung down his useless sling and snatched up his club. A Forest Man rushed upon him with mighty bludgeon upraised. It wavered in the air and then melted away. Layers of darkness unwrapped about him. Shining white walls swam and flowed into place to settle solidly. Dr. John Bartok opened his eyes upon the familiar sights of his laboratory.

He raised his head. The test tube rested as before in its rack. The thin white fumes curled lazily upward. He drew back in belated alarm. He could still hear the snarling roar of that savage battle, still smell the blood and heat of war and death. Good heavens, was it the fumes? Had they anaesthetized him? Had he dreamed it all?

He found a cigarette and sat back, away from the test tube, brooding. Snatches of words floated through his mind.

"Too terrible to use—"

"This will end wars—"

"All will fear us. Just having it will be enough. We will never have to use it."

The cigarette burned down. He got up. His face was pale and set.

The swinging door squeaked and Dr. Goodwin bustled in, face glowing with excitement.

"John, here's General McComber. He came right down by helicopter when I told him—John, where is it? John, what are you doing?" His usually controlled voice rose to a shriek.

Dr. John Bartok turned from the sink. He shook the last few drops of water from the glass.

"I'm just rinsing out this test tube," he said.