Arnold shrugged his shoulders. "Consider the chameleon. There are insects that look like twigs. Other resemble leaves. Some fish can counterfeit the ocean bottom so perfectly—"
"Yes, yes," Gregor said impatiently, "that's camouflage. But invisibility—"
"Some kinds of jellyfish are transparent enough to be considered invisible," Arnold continued, "The hummingbird achieves it by dazzling speed. The shrew hides so well that few humans have ever seen one. All are moving toward invisibility."
"That's ridiculous. Nature equips each creature as best it can. But it never goes all the way by endowing one species with invulnerability from all others."
"You're being teleological," Arnold objected. "You're assuming that nature has some aim in mind, like the overseer of a garden. I maintain that it's a blind averaging process. Sure, the mean usually obtains, but there are bound to be extremes. Nature had to come up with invisibility eventually."
"Now you're being teleological. You're trying to tell me that the aim of camouflage is invisibility."
"It must be! Consider—"
"To hell with it," Gregor said wearily. "I'm not even sure what teleology is. We've been here ten days and we've captured some fifty rats, out of a population of several millions. Nothing works. Where do we go from here?"
They sat in silence. Outside, they could hear the scream of a flying hang as it dipped low over the fields.
"If only the slegs' natural enemies had some guts," Arnold said sadly.
"They're visual hunters. If they were—"
He stopped abruptly and stared at Arnold, Arnold looked puzzled for a moment. Then a slow light of comprehension dawned on his face.
"Of course!" he said.
Gregor lunged for the telephone and called Galactic Rapid Express. "Hello! Listen, this is a rush order…"
Galactic Rapid Express outdid themselves. Within two days, they deposited ten small boxes on the pocked lawn at Barney Spirit.
Gregor and Arnold brought the boxes inside and opened one. Out stepped a large, sleek, proud, yellow-eyed cat. She was of Earth stock, but her hunting capabilities had been improved with a Lyraxian strain.
She stared somberly at the two men and sniffed the air.
"Don't get your hopes too high," Gregor told Arnold as the cat stalked across the room. "This is outside all normal cat experience."
"Shh," Arnold said. "Don't distract her."
The cat stood, her head cocked delicately to one side, listening to several hundred invisible slegs amble disdainfully past her.
She wrinkled her nose and blinked several times.
"She doesn't like the setup," Gregor whispered.
"Who does?" Arnold whispered back.
The cat took a cautious step forward. She raised a forepaw, then lowered it again.
"She isn't catching on," Gregor said regretfully. "Maybe if we tried terriers—"
The cat suddenly lunged. There was a wild squealing and she was gripping something invisible between her forepaws. She mewed angrily and bit. The squealing stopped.
But other squeals took its place and ratlike shrieks and rodent cries of terror. Gregor released four more cats, keeping the remaining five as his second team. Within minutes, the room sounded like a miniature abattoir. He and Arnold had to leave. The noise was nerve-shattering.
"Time for a celebration," Arnold said, opening one of the brandy bottles he had packed.
"Well," said Gregor, "it's a little early—"
"Not at all. The cats are at work, all's well with the world. By the way, remind me to order a few hundred more cats."
"Sure. But what if the slegs turn cautious again?"
"That's the beauty of it," Arnold said, pouring two stiff shots. "As long as the slegs are this way, they're meat for the cats. But if they revert to their old habits—if they become truly ratlike—we can use the Morganizer."
Gregor could find no argument. The slegs were caught between the cats and the Morganizer. Either way, the place should be back to normal in another week, in plenty of time for a sizable bonus.
"A toast to the Earth cat," Arnold proposed.
"I'll drink to that," Gregor said. "To the staunch, down-to-Earth, common-sense Earth cat."
"Invisible rats can't faze her."
"She eats 'em if they're there or not," Gregor said, listening to the sweet music of carnage going on throughout the farmhouse.
They drank quite a number of toasts to the various attributes of the Earth cat. Then they drank a solemn toast to Earth. After that, it seemed only proper to toast all the Earth-type suns, starting with Abaco.
Their brandy gave out when they reached Glostrea. Fortunately, the Seerian had a cellar well stocked with local wines.
Arnold passed out while proposing a toast to Wanlix. Gregor managed to last through Xechia. Then he laid his head on his arms and went to sleep.
They awoke late the next day with matching headaches, upset stomachs and flashing pains in the joints. And just to make matters worse, not one of their staunch, down-to-Earth, common-sense Earth cats was to be found.
They searched the farmhouse. They looked in the barns, through the meadows, across the fields. They dug up sleg holes and peered into an abandoned well.
There was no sign of a cat—not even a wisp of fur.
On all sides, the slegs scampered merrily by, secure in their cloak of invisibility.
"Just when the cats were doing so well," Arnold mourned. "Do you suppose the slegs ganged up on them?"
"Not a chance," Gregor said. "It would be contrary to all sleg behavior. It's more reasonable to assume that the cats just wandered off."
"With all this food here?" Arnold asked. "Not a chance. It would be contrary to all cat behavior."
"Here, kitty, kitty!" Gregor called, for the last time. There was no answering meow, only the complacent squeals of a million careless slegs.
"We must find out what happened," Arnold said, walking to the boxes that housed their remaining five cats. "We'll try again. But this time we'll introduce a control element."
He removed a cat and fastened a belled collar around her neck. Gregor closed the outer doors of the farmhouse and they turned her loose.
She went to work with a vengeance and soon the chewed corpses of slegs began to appear, life—and invisibility—drained from them.
"This doesn't tell us anything," Arnold said.
"Keep on watching," Gregor told him.
After a while, the cat took a short nap, a sip of water and began again, Arnold started to doze off. Gregor watched, thinking dire thoughts.
Half of their month was now over, Gregor realized, and the sleg population was untouched. Cats could do the job; but if they gave up after a few hours, they would be too expensive to utilize. Would terriers do any better? Or would this happen to any—
He gaped suddenly and nudged Arnold. "Hey!" Arnold awoke with a groan and looked.
A moment ago, there had been an extremely busy cat. Now, abruptly, there was only a collar, suspended half a foot above the floor, its little bell tinkling merrily.
"She's become invisible!" Arnold cried. "But how? Why?"
"It must be something she ate," Gregor said wildly, watching the collar dart across the floor,
"All she's eaten is sleg."
They looked at each other with sudden comprehension.
"Then sleg invisibility is not mutational!" Gregor said. "I told you so all along. Not if it can be transmitted that way. The slegs must have eaten something, too!"
Arnold nodded, "I suspected it. I suppose, after the cat digests a certain amount of sleg, the stuff takes hold. The cat becomes invisible."
From the bedlam in the room, they could tell that the invisible cat was still devouring invisible slegs.