It was neither night nor day, but a sort of nether world of twilight. The huge fern-like plants, flashing phosphorescence under the green corona light, seemed to close in after Cappy Upjohn and Terry Hall like prison bars in the windows of a dungeon.

Cappy, who was leading the way, paused and waited for Terry. As the huge, leonine man turned, his lips curled into a taunting smile.

Terry's eyes moved from side to side, watching the weird shadows, dodging the sweep of the giant ferns as they moved in the wind.

"Scared!" Cappy ridiculed.

"I can't help it!" the younger man said. "This place gives me the creeps."

Cappy's great laugh echoed above the howl of the winds. "This is Mercury. Half day, half night everlastingly. Right here is the battleground of roasting heat and perpetual cold. A twenty-mile strip of habitable land between two kinds of hell. What the devil did you expect, Tenderfoot?"

Cappy grunted in disgust, turned and picked his way through the ferns. Terry, his jaw set grimly, followed. Cappy had been through all this before. Twice he'd landed on Venus, and he'd been with the only previous expedition to Mercury. But Terry knew that fear was a human emotion, and that there were things even Cappy was afraid of.

The wind died a moment. Between an opening in the ferns Terry caught a glimpse of a ghostly face, more simian than an ape's, less human than a man's. At the same time he felt something that was like a breeze through his brain. A painless stab of thought,

"Cappy—look!" Terry pointed at the face peering through the opening in the ferns, and his hand clawed at the rifle he had slung over his shoulder. In all respects it was like an old-fashioned gun, but it fired a deadly bullet that was capable of complete annihilation of whatever it hit. A single bullet from a flourobeam gun was powerful enough to wipe out a steel ball ten feet in diameter.

But Terry's hands slipped away from his gun. He recognized his action as more than fear. It was panic.

Cappy had been too surprised at the sight of the grinning face to notice Terry's action. Now the youth had control of himself.

"Great guns! This is something new, lad! A living creature on Mercury!"

"I tried to tell you, Cappy!" Terry said. "I've seen 'em. I've felt 'em for the past hour!"

"Felt 'em? Did they touch you?"

"It's something I can't describe. It's like poking a finger into our brains. It doesn't hurt, but it feels uncomfortable. It's like being watched by someone you can't see."

Cappy's boldness seemed to tarnish a little. A suggestion of a shudder seemed to pass over the man. He straightened and shook it off.

"Pooh! Imagination, Terry!"

Cappy took a step toward the creature. The eyes seemed to flash. Perhaps it was a reflection from the corona streamers stretching above the horizon to the west. Cappy halted as the creature seemed to shrink away.

"Hold on!" Cappy called. "Don't go away! We've got to see that creature, Terry. Get it back! Can you stop it!"

There was no mistaking that the animal was intelligent. Perhaps it felt the same emotions that raced through Terry's body.

"Let me try," Terry said. His heart was pounding, but science had to know if intelligent life existed in Mercury's twilight zone.

Terry stripped the gun from his shoulders and laid it on the ground. He pushed passed Cappy and walked toward the creature. Terry's hands were extended, palms outward.

The creature hesitated. Its eyes flashed again and once more Terry felt something cold in his skull.

The ferns parted and the creature stepped out into view.

It had two stubby legs, two long arms. Its head was pear-shaped and hairless, and its body was an ovoid ball, bloated and ugly. But its eyes were almost human except that they seemed to flash fire.

Terry halted, facing the creature.

"Terryhall of the earth!" The creature's voice echoed through the ferns.

"You—you speak English!" The words that fell from Terry's lips mirrored his surprise. In his mind flashed a name for this creature. Something like Zombie, the living dead.

"Not Zombie. Name is Chomby. I am not dead, not even living dead."

"You read my mind! That's what I've felt for the past hour. You were studying my thoughts, and you learned my language by reading my mind!"

Chomby's rubbery lips tried to imitate a grin, but it achieved only a hideous travesty.

Chomby's hand rubbed over the leathery rags he wore for clothing and reached toward the earthman. The gesture was unmistakable. Chomby wanted to shake hands. The Mercurian had been thorough in his probe of Terry's brain. His actions were more human than Terry's under the circumstances, for Terry was afraid.

"Nothing to fear from Chomby," spoke the Mercurian.

Terry took the hand. It was leathery and cold.

"I am glad to meet you," Terry said. He forced the words from his lips. "We came to Mercury on a peaceful mission."

"You're afraid, Terryhall," Chomby replied. "I do not intend to kill you. Nor will I hurt Cappyupjohn."

Cappy brushed forward and shook hands with Chomby. He concealed a wince as he touched the creature's corpse-like flesh.

Then he unstrapped the caseknife he wore at his side and handed it to Chomby. "A gift of friendship," he said.

Chomby reached awkwardly toward the knife. As he took it, the knife fell to the ground. Chomby leaned over and fumbled. He could hardly grasp the knife. Terry noted now what had been so repulsive about Chomby's handshake. The Mercurian had no thumb.

Instead of the five-fingered hand of a human being, there were only three flabby fingers on the end of a round, gristly hand.

Terry stooped and picked up the knife. He handed it to the Mercurian who seized it in both hands and clutched it to his body.

"You want food and drink?" Chomby asked. "Terryhall and Cappyupjohn follow me to my village. Sorry I have no gift of friendship otherwise. My people are unskilled in handicraft. We can hardly make our own clothes."

Chomby led the earthmen eastward. The shadows deepened and the sky grew dark. The stars appeared; among them the brilliant blue planet that was Earth.

Again Chomby read Terry's thoughts.

"You come from that one?" he asked, pointing. "You are from the sky?"

Terry answered, but Chomby seemed to read the thoughts that Terry flashed through his mind.

"Spaceship. Machines. Rockets." The Mercurian rolled the words in his mouth. "You come from a great race, Terryhall and Cappyupjohn. My people want to learn skill with our hands, to be like you."

"It would be nice if we could read your minds," Cappy said, tossing a glance toward Terry.

"You are suspicious men," Chomby said. "I read your mind and I know you do not trust me. You fear a trap. That is why you carry your guns. But there is nothing to fear from Chomby's people. You will think them very primitive."

The cold winds lashed at the earthmen and nipped through the heavy clothing they wore. Terry and Cappy lowered a plastic windshield from their caps to protect their faces from frostbite.

Vegetation grew more scarce and at last they walked across a rocky plain toward a row of towering basalt cliffs. The feeble light that came from the outer fringe of the corona revealed a row of caves at the base of the cliffs and from these emerged a hundred or so ill-fashioned beings resembling Chomby.

The Mercurian gave no cry, nor warning of his approach. He apparently had notified his people by telepathy, for they rushed silently to meet him. Chomby lapsed into silence, turning from one of his kinsfolk to another, answering questions without speaking.

Cappy stared at the Mercurians. His leonine figure marched through the ill-shapen creatures confidently and unafraid. Doubts crept into Terry's mind. There were too many Mercurians. Despite the powerful weapons strapped to the earthmen's backs, they could not hope to escape imprisonment, if these savages sought to hold them.

Chomby led the men to a cave in the center of the village. He turned to Terry.

"My people think you are queer, Terryhall," Chomby said. "But it is evident to them that you think the same about us. But we are all one people. Terryhall and Cappyupjohn are not the same. Terryhall is afraid, while Cappyupjohn is a man of rock. Are you not of the same race?"

"More or less," Cappy said, deridingly. "Terry doesn't understand you and he fears what he doesn't understand. I've learned to control fear, so I am not afraid."

"You trust us?" Chomby seemed surprise.

"No," Cappy replied. "I don't trust anything, but I'm not afraid of you."

"What if we should intend to keep you here?" Chomby asked. "Terry sees himself as a prisoner, a slave. He fears he will never return to the earth."

"I still have my gun," Cappy said.

"There are many of us," Chomby said, his lips gaping again in that imitation grin. "We could overpower you and kill you."

"I'd take some of you with me," Cappy replied, defiantly. "I'd go out in a blaze of glory."

Chomby studied the earthman. "I believe you would. In your world such a trait would be considered admirable. You are hard, Cappyupjohn, and brave. But it is not your kind that makes your race so great. Your world is a coward's world. It was built by men who are afraid."

Cappy choked. "You lie!"

"We have read your minds, earthmen," Chomby said. "We understand the civilization from the mental pictures you carry in your minds. You have great brains, but skilled as they are they are nothing in comparison with ours. It was not your brain that made your world, but your love of security. You feared wild beasts, so you killed them. You feared hunger, so you stored food. You built weapons to defend yourself against enemies. You sought out new worlds in fleeing from dangers of the old. If your race had not known fear, it would have never done these things."

"Men died to win that security," Cappy replied. "That wasn't cowardice, was it?"

"They died fighting, which meant that they hoped to win. Their chance of victory was more attractive than the insecurity that would come with defeat," Chomby replied. "My people wish to keep you here. We want to learn more about the emotion called fear that has made your civilization so great."

"You'll never keep us!" growled Cappy. His hand automatically touched the gun on his shoulder.

"That is the fear I speak of," Chomby said. "You would rather die than be a prisoner. In a different way Terryhall has the same emotion. But he does not risk so recklessly. He would rather avoid the situation that makes fighting necessary. He is a brave man, too, Cappyupjohn, but in a different way."

"He's yellow!" Cappy said vehemently. "He's worse than no help at all."

The earthmen rested, but they were not left alone. Chomby seemed always to be near. Although Chomby insisted the earthmen were not prisoners, he made it plain they would not be allowed to leave for a time.

"We must study you," he said. "In turn, you may study us."

"I'm warning you, Chomby," Cappy roared. "We'll stand just so much of this thing. We'll stay until we've studied you enough, but when we want to go, we're going—or we'll die trying."

"That time is not here," Chomby said, wrinkling his lips again.

The icy mental probing of the Mercurians grew familiar to Terry and Cappy. There was nothing the earthmen could think of that these eerie, repulsive-looking creatures did not understand. That the Mercurians read Terry's thoughts so easily was often embarrassing, for Terry knew that they were aware of his repugnance toward them, as well as Terry's distrust and fear.

But Chomby and his people seemed to accept Terry's opinion of them understandingly. Not one of them made a move to remove either Terry's or Cappy's guns.

After a first period of mental probing Chomby urged the earthmen to instruct the Mercurians in some simple crafts. Now the earthmen enjoyed the sense of superiority that previously had been a monopoly of the Mercurians. The simplest pieces of handiwork were almost beyond, the Mercurians. The hands of these creatures, without thumbs and with stiff fingers, were clumsy. Weaving was an arduous task. Construction of a simple, primitive thatched dwelling was attempted and abandoned, when Cappy found that it would require months to complete. It was not because the Mercurians did not understand what had to be done—they knew this the instant the terrestrials pictured the idea in their minds. But the tasks were nearly impossible for the Mercurians.

Terry taught a few of the creatures to write, but the rest could not master the process, although every one of them had learned to read by watching Terry's mind at work.

Cappy grew more contemptuous of the Mercurians as he watched their bungling efforts at the simplest human arts.

"We haven't anything to be afraid of from these creatures, Terry," he said.

Terry shook his head.

"They've got brains, Cappy. They know everything we know and a lot of things we don't. They read us like a book. They know our thoughts before we know them ourselves."

"It isn't thought that wins battles. It's power. Oh, brains help, but only when used in the application of the proper weapons. Why with our guns and fists we probably could lick this whole village. I'm sure they haven't coordination enough to fight hand-to-hand with us singly."

"But they're too smart to let us out-maneuver them," Terry said. "There's a principle that seems to work in society that makes me afraid of these Mercurians. In the history of our own planet, it has always been the brains which exploited the brawn among men. People who thought of things usually took ascendancy over those who tried to progress by the sweat of their brow. For instance, everyone knows Columbus discovered America, but who knows the name of the man who built the ships he sailed in? LeCompton designed the first successful spacecraft, but the name of the man who tooled the intricate parts of its mechanism and made it successful has been completely forgotten."

"You mean we're likely to become slaves of these—these Zombies?"

"Since we came to this village they've changed," Terry said. "These Zombies—as you call them—are learning fear. They see their world, lashed by cold and hot winds, freezing and roasting, as an insecure place. A violent storm might burn up their food supply, or freeze their crops. For the first time they've seen fear as a safeguard to their future. Now they want to build. They want places to store food; homes to protect them from cold. They want means of transportation, to escape uninhabitable spots. They know the value of fear, but they cannot conquer it because they are physiologically incapable of conquering their environment. When they realize this—if they don't realize it already—they'll force us to conquer their environment for them. Every earthman unfortunate enough to land on Mercury will become a slave—"

"Great Scott, Terry!" Cappy exclaimed. "I believe you are right. Your fear has been warning us all along to get out of this place. Get your gun ready, we're going back to the space ship—"

The light from the entrance of the cave was blotted out. Chomby stood before them, followed by a horde of his fellow beings.

"You recognized fear too late, Cappyupjohn," Chomby said.

The two earthmen tried to level their guns, but the Mercurians attacked too quickly. The leathery fists struck home and the guns slipped from the earthmen's grasps.

A transformation occurred in front of the row of caves in the basalt cliff. Windbreaks appeared in the openings in the wall. Rude machines were set up to build houses of stone and covered passages from cave to cave. From the fibrous ferns Terry constructed rude looms for weaving cloth. Stone mills for grinding the pulpy fruit of the Mercurian trees into flour were designed.

How long the two earthmen had been prisoners on Mercury they had no way of telling, for there was neither night nor day, nor seasons in the twilight zone. But the Earth had disappeared over the south horizon and reappeared in the north and Cappy estimated that two-thirds of the Mercurian year of 88 days had passed.

"We won't be here another year," Terry said.

Cappy snorted. "I wouldn't bet."

"They're getting careless," Terry pointed out. "They used to have a dozen men guarding us day and night. If we even got a little too far away from the village, we'd be shoved back. Now only one Zombie is guarding us. We're allowed to go almost anywhere, except near the spaceship."

"They read our minds, so they're always two jumps ahead of us, Terry. No. The principle of brains over brawn can't be beaten. We're licked."

"I'm not," Terry announced. "Once you called me a coward—you said I was yellow. But a coward isn't the man who is afraid, it's the man that lets fear get the upperhand. You're being a coward now, Cappy. You're admitting that Chomby and his pals have the Injun sign on us. I'm not admitting it. It isn't brains that makes men the rulers of nine planets, and it isn't fear. Man has something else that gives him a physiological edge. I'm going to find out what that is. When we find it, we'll be free men again."

"When you find it, the Mercurians will know. They'll be ready to keep us from using the weapon—whatever it is—before we know we've got it."

"Some day we'll have a chance. Some day we'll have a chance to slug our guard and get back to the spaceship—"

Terry paused. He looked at the dozing Mercurian at the mouth of the cave. Even as Terry looked the Mercurian roused out of his sleep. The thought thread had roused the guard from a deep slumber.

"Now!" Terry yelled. "Now's our chance!"

Terry sprang. The guard tried to seize the stone club at his side, but his clumsy hand was not made for swift action. Terry was on him before the guard could send out a mental alarm and the young earthman's fist crashed against the base of the guard's skull.

Terrestrial muscles, built for a heavier force of gravity, delivered a sledgehammer blow. The guard toppled forward. Terry leaped over the figure and darted into the open.

Another figure rose before Terry, but again Terry's fist smashed.

Cappy was beside Terry now and together they raced toward the path that led in the direction of the spaceship.

"They'll follow us!" Cappy said. "They can trail us by our thoughts. If we miss the ship and have to double back, we'll run into them."

"We won't miss!" Terry said.

The sudden formulation of the escape plan had been too quick for even the Mercurian brains to block. Terry and Cappy were racing into the forest of ferns far ahead of their pursuers. The coordinated muscles of the earthmen were far more capable of traveling swiftly than those of the Mercurians behind and sounds of pursuit grew fainter in the distance.

But Terry knew that these creatures were dogged. They would follow until the spaceship's rockets blasted loose from the planet.

"Remember," Terry whispered, "don't act on any set plan. Don't plan what you'll do when you meet 'em. If you do they'll be prepared for it. Act on impulse, before they know what you intend to do."

"I'll remember," Cappy panted.

Was it impulse that made terrestrials the masters of the universe? Terry wondered. It hardly seemed logical, yet impulse had given them their first chance of freedom. But impulse might lead them astray. First thoughts are not always the best thoughts. True enough, man had made some strides by accident, but far more of his greatest discoveries and most useful inventions had been the result of years of labor and careful planning. No, it wasn't impulse. Some other weapon had to be used to defeat the Mercurians decisively. Terry and Cappy might escape through luck, but some day there would be a final, decisive battle that would employ the one thing that gave man an advantage over the semi-human monsters of the first planet.

Terry hoped he could discover that weapon now.

They had been over the route to the spaceship three times. Once in company with Chomby on their first trip to the Mercurian village. They had returned in company with a guard and come back again later.

Terry had noted a few landmarks and now he began to spot them again. A curious-shaped rock; a spring of moulten metal; a deep fissure in the soil.

But as they traveled toward the sun's corona the air grew warmer. Vegetation became profuse and the trail was more difficult to find. But Cappy had kept his eyes open, too. Between them they made their way, slower and slower.

Suddenly Terry stopped. An icy finger had passed over his brain. A wordless thought flashed into his consciousness.

"This is Chomby, Terryhall. Go back, or you will be unmercifully killed."

Cappy looked at Terry. The same thought had flashed through his brain.

"I am ahead of you, earthmen. I am waiting in front of the spaceship. In my hands I hold a flourobeam gun. You cannot pass me, earthmen!"

Cappy groaned.

"They're still two jumps ahead, Terry! They've outfigured us again. They knew we might get away by accident. So they put Chomby out here to intercept us and he's armed with our weapons!"

Terry was afraid. He knew the power of a flourobeam. He'd seen it blast rocks into powder and he knew that a man could never survive its charge. One blast from the gun could wipe out all trace of Terry and Cappy.

Terry's face paled. Then, suddenly, he moved forward.

"Come on, Cappy! You said I was yellow once. Let's see who's yellow now!"

"But that gun! This isn't being brave! It's foolhardy. The better part of valor is knowing when you're licked."

"Come on," Terry said, moving ahead.

"You're a fool, Terry," Cappy said. "But you've got guts! Now listen to reason!"

But Terry would not listen. He moved forward. Cappy, sweating, came following.

"It's suicide!" cried the older man.

In the center of a clearing stood the spaceship. The streaks of the corona revealed a figure, huge, hideous and ape-like, standing before the locks.

It was Chomby, holding the flourobeam gun, aiming at the two terrestrials who emerged from the forest.

"If you come closer, I'll shoot!" It was a thought, not a voice, that flashed toward the men, but the words were as clear as if Chomby had spoken.

"We're licked, Terry!" Cappy said. "I'll surrender."

Slowly Cappy's hands went over his head.

"And what about you, Terryhall?"

Terry stood in the clearing, looking at the Mercurian. The young man's face was pale and green beneath the corona glow.

"This is a war of the worlds, Chomby," he said. "It's a fight between your race and mine. It's a Waterloo for one of us."

"You haven't a chance, Terryhall. You know the power of this gun."

Terry's lips tightened into a thin, straight line. His body crouched for an instant and then he sprang. The terrestrial muscles sent him shooting toward Chomby. His feet touched the ground again, and like steel springs he shot forward a second time.

Chomby's fingers tightened on the trigger of the weapon. They seemed to twitch.

Cappy closed his eyes.

An inhuman scream rent the Mercurian air. But the gun did not go off.

Cappy opened his eyes to see Terry and the monster rolling on the ground. Terry's fingers were closed about the Mercurian's throat. Chomby struggled feebly, and then lay still.

Terry rose, picked up the gun and motioned to Cappy.

"Put down your hands and start moving your legs. Get into those locks before the rest of them get here!"

Cappy's jaw worked up and down, but words did not come from his throat. Somehow he moved. He ran into the locks and a moment later he was at the controls.

"Let 'er go!" Terry said.

Cappy, still speechless, pressed the charger. The rockets roared. The machine lurched skyward and Mercury was left behind.

"What happened?" Cappy asked.

"I just remembered that when we were last carrying the guns we had the safety catches on. The catches are released with the thumb. Chomby couldn't do it! He couldn't have shot me, even though he wanted to. The safety catches are such a small piece of the flourobeam mechanism that we never think of them. Chomby didn't have a chance to read our minds about the safety catch until it was too late."

"Terry," Cappy said. "I'll take back everything I said about you being yellow. You've got guts! More than I have, Terry."

"Guts? Hell, Cappy, I was scared to death every second."