Midday came; Seer's hot little sun hung directly overhead. The Morganizing equipment hummed and grumbled to itself. The Tournier wires flashed blue sparks.
Nothing happened.
The hours dragged by, Arnold read every available manual on rodent control. Gregor dug out a pack of tattered cards and morosely played solitaire. The equipment murmured and buzzed, exactly as its manufacturers guaranteed. Enough power was consumed to light a medium-sized village.
Not a single rodent corpse was produced.
By evening, it was apparent that slegs were not susceptible to Morganizing or Tournierizing. It was time for dinner and a conference.
"What could make them so elusive?" Gregor puzzled, sitting worriedly on a kitchen chair with a can of self-heating hash.
"A mutation," Arnold stated.
"Yeah, that could do it. Superior intelligence, adaptability…" Mechanically, Gregor ate his hash. All around the kitchen, he could hear the patter of countless little sleg feet, slipping in and out of holes, staying just out of sight.
Arnold opened an apple pie. "They must be a mutation, and a damned clever one. We'd better catch one quick and find out what we're up against."
But catching one was no easier than killing a thousand. The slegs stayed out of sight, ignoring traps, lures, snares and doped bait.
At midnight, Arnold said, "This is ridiculous."
Gregor nodded abstractedly. He was putting the finishing touches on a new trap. It was a large sheet metal box with two sides left invitingly open. If a sleg were foolish enough to enter, a photo-electric cell closed the sides with the speed of a lightning bolt.
"Now we'll see," Gregor said. They left the box in the kitchen and went into the living room.
At two-thirty in the morning, the sides slammed shut.
They hurried in. Within the metal box, they could hear a frantic scurrying and squealing. Gregor turned on the lights and up-ended the box. Although he knew that no rat born could climb the polished sides of the trap, he withdrew the cover with great care, an inch at a time.
The squealing increased.
They eagerly peered into the trap, half prepared to see a rat in full soldier's uniform, waving a white flag.
They saw nothing. The box was empty.
"He couldn't have gotten out!" Arnold exclaimed.
"And he didn't gnaw through. Listen!"
Inside the box, the squealing continued, accompanied by frantic scratching sounds, as though a rat were trying to scramble up the sides of the trap.
Gregor put his hand in and felt cautiously around, "Ouch!" He jerked his hand back. There were two small toothmarks on his forefinger.
The noise within the empty box increased.
"We seem to have captured an invisible rat," Gregor said blankly.
The Seerian was vacationing at the Majestic Hotel, in the Catakinny Cluster. It took almost two hours to reach him by interstellar telephone.
Gregor started the conversation by shouting, "You never said anything about invisible slegs!"
"Didn't I?" the Seerian asked. "Careless of me. What about it?"
"It's a breach of contract, that's what!" Gregor yelled.
"Not at all. My lawyer, who happens to be vacationing with me, says that invisibility in animals comes under the classification of Natural Protective Coloration, and therefore need not be mentioned as a hazardous or unique condition. For legal purposes, the courts don't even admit a state of invisibility exists, as long as some means of detection is possible. They call it Relative Dimness and it is not allowed as permissible distress in an extermination contract."
Gregor was momentarily stunned.
"We poor farmers must protect ourselves, you know," the Seerian continued. "But I have perfect faith in your ability to cope. Good day."
"He's protected, all right," Arnold admitted, putting down the extension telephone. "If we clean out these invisible rats, he's got a bargain. If we don't, he collects forfeitures."
"Invisible or not," Gregor said, "Morganizing ought to work on them."
"But it doesn't," Arnold pointed out.
"I know. But why doesn't it work? Why don't traps work? Why doesn't the Tournierizing work?"
"Because the rats are invisible."
"That shouldn't matter. They still sniff like rats, don't they? They still hear like rats. They still think like—or do they?"
"Well," Arnold said, "if this invisibility is a true mutational change, it's possible that their sensory apparatus has changed, too."
Gregor frowned. "And a change in their sensory equipment would call for a change in our applied stimulus. Now all we need to know is how these slegs differ from the norm."
"Aside from their invisibility, you mean," Arnold said.
But how do you test the sensory apparatus of an invisible rat? Gregor began by constructing a maze out of the Seerian's choicer furniture. Its walls were designed to light up when an invisible sleg brushed by. In that way, the rodents' movements could be traced.
Arnold experimented with stains and dyes, searching for something that would return the slegs to visibility. One high-potency dye took momentary hold. A sleg appeared as though by magic, blinking slowly, his nose quivering. He looked at Arnold with maddening calm, then fearlessly turned his back. His rapid metabolic rate converted the dye almost immediately and he faded from view.
Gregor captured ten slegs and tried to run them through his maze. They were unbelievably uncooperative. Most of them refused to move at all. They sniffed disdainfully at the food he gave them, toyed with it a few moments, then ignored it. Even light electric shocks budged them only a few inches.
But the tests did give the answer to the failure of Morganizing and Tournierizing,
Like all large-scale extermination systems, they were based upon the concept of "normal" rodents. These normals could be tricked or scared into certain behavior patterns by stimulation of their hunger or fear drives. It was the norm among rodents that the systems destroyed.
Everything was fine as long as the norm represented a high percentage of the rodent population. But as the slegs had changed, their norm had changed, too. These slegs had adapted to invisibility.
They could no longer be panicked, for they had discovered that nothing chased them. And since they had no reason to flee, they could eat anywhere, at any time. Therefore, they were invariably well fed and in no mood to explore enticing smells, shapes or sounds.
Both Morganizing and Tournierizing could be adapted and would destroy slegs. But only a few. Only those rodents who had not adapted to invisibility—the unaverage ones. And this only served to reinforce the change in the others.
But what had happened to the natural enemies of the sleg, the forces acting to maintain an ecological balance? In order to find out, Gregor and Arnold made a frantic survey of the fauna of Bitter Lug.
Bit by bit, they reconstructed what must have happened.
The slegs had enemies on Seer—flying hangs, drigs, tree skurls and omenesters. These unimaginative creatures had been unable to cope with the sudden change. For one thing, they were visual hunters, using smell only as an auxiliary. Although sleg scent was powerful in their nostrils, seeing was believing, not smelling. So they ate each other and left the slegs alone.
And the slegs increased and increased…
And AAA Ace could find nothing to check them.
"We're tackling this at the wrong end," Gregor said, after a fruitless week. "We should find out why they became invisible. Then we'd know how to deal with them."
"Mutation," Arnold insisted dogmatically.
"I don't believe it. No animal has ever mutated into invisibility. Why should the slegs be the first?"