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seemingly ideal conditions, hadn't developed. Why?

  He had asked Biological Controls for this assignment because it hadseemed an interesting problem. Now, apparently, he was being pressedinto service as an exterminator.

  He reached in the cage and picked up the omnivore. Mammals on Gladewere not unexpected. Parallel development took care of that. Givenroughly the same kind of environment, similar animals would usuallyevolve.

  In the Late Carboniferous forest on Earth, there had been creatureslike the omnivore, the primitive mammal from which all others hadevolved. On Glade, that kind of evolution just hadn't taken place.What had kept nature from exploiting its evolutionary potentialities?There was the real problem, not how to wipe them out.

  Marin stuck a needle in the omnivore. It squealed and then relaxed. Hedrew out the blood and set it back in the cage. He could learn a lotabout the animal from trying to kill it.

  * * * * *

  The quartermaster was shouting, though his normal voice carried quitewell.

  "How do you know it's mice?" the biologist asked him.

  "Look," said the quartermaster angrily.

  Marin looked. The evidence did indicate mice.

  Before he could speak, the quartermaster snapped, "Don't tell methey're only mice-like creatures. I know that. The question is: howcan I get rid of them?"

  "Have you tried poison?"

  "Tell me what poison to use and I'll use it."

  It wasn't the easiest question to answer. What was poisonous to ananimal he had never seen and knew nothing about? According toBiological Survey, the animal didn't exist.

  It was unexpectedly serious. The colony could live off the land, andwas expected to. But another group of colonists was due in threeyears. The colony was supposed to accumulate a surplus of food to feedthe increased numbers. If they couldn't store the food they grew anybetter than the concentrates, that surplus was going to be scanty.

  Marin went over the warehouse thoroughly. It was the usual earlyconstruction on a colonial world. Not esthetic, it was sturdy enough.Fused dirt floor, reinforced foot-thick walls, a ceiling slab of thesame. The whole was bound together with a molecular cement that madeit practically airtight. It had no windows; there were two doors.Certainly it should keep out rodents.

  A closer examination revealed an unexpected flaw. The floor was ashard as glass; no animal could gnaw through it, but, like glass, itwas also brittle. The crew that had built the warehouse had evidentlybeen in such a hurry to get back to Earth that they hadn't been ascareful as they should have been, for here and there the floor wasthin. Somewhere under the heavy equipment piled on it, the floor hadcracked. There a burrowing animal had means of entry.

  Short of building another warehouse, it was too late to do anythingabout that. Mice-like animals were inside and had to be controlledwhere they were.

  The biologist straightened up. "Catch me a few of them alive and I'llsee what I can do."

  * * * * *

  In the morning, a dozen live specimens were delivered to the lab. Theyactually did resemble mice.

  Their reactions were puzzling. No two of them were affected by thesame poison. A compound that stiffened one in a matter of minutes leftthe others hale and hearty, and the poison he had developed to controlthe omnivores was completely ineffective.

  The depredations in the warehouse went on. Black mice, white ones,gray and brown, short-tailed and long-eared, or the reverse, theycontinued to eat the concentrates and spoil what they didn't eat.

  Marin conferred with the executive, outlined the problem as he saw itand his ideas on what could be done to combat the nuisance.

  "But we can't build another warehouse," argued Hafner. "Not until theatomic generator is set up, at any rate. And then we'll have otheruses for the power." The executive rested his head in his hands. "Ilike the other solution better. Build one and see how it works."

  "I was thinking of three," said the biologist.

  "One," Hafner insisted. "We can't spare the equipment until we knowhow it works."

  At that he was probably right. They had equipment, as much as threeships could bring. But the more they brought, the more was expected ofthe colony. The net effect was that equipment was always in shortsupply.

  Marin took the authorization to the engineer. On the way, he privatelyrevised his specifications upward. If he couldn't get as many as hewanted, he might as well get a better one.

  In two days, the machine was ready.

  It was delivered in a small crate to the warehouse. The crate wasopened and the machine leaped out and stood there, poised.

  "A cat!" exclaimed the quartermaster, pleased. He stretched out hishand toward the black fuzzy robot.

  "If you've touched anything a mouse may have, get your hand away,"warned the biologist. "It reacts to smell as well as sight and sound."

  Hastily, the quartermaster withdrew his hand. The robot disappearedsilently into the maze of stored material.

  In one week, though there were still some mice in the warehouse, theywere no longer a danger.

  * * * * *

  The executive called Marin into his office, a small sturdy buildinglocated in the center of the settlement. The colony was growing,assuming an aspect of permanency. Hafner sat in his chair and lookedout over that growth with satisfaction.

  "A good job on the mouse plague," he said.

  The biologist nodded. "Not bad, except there shouldn't be any micehere. Biological Survey--"

  "Forget it," said the exec. "Everybody makes mistakes, even B. S." Heleaned back and looked seriously at the biologist. "I have a job Ineed done. Just now I'm short of men. If you have no objections...."

  The exec was always short of men, would be until the planet wasovercrowded, and he would try to find someone to do the work his ownmen should have done. Dano Marin was not directly responsible toHafner; he was on loan to the expedition from Biological Controls.Still, it was a good idea to cooperate with the executive. He sighed.

  "It's not as bad as you think," said Hafner, interpreting the soundcorrectly. He smiled. "We've got the digger together. I want you torun it."

  Since it tied right in with his investigations, Dano Marin lookedrelieved and showed it.

  "Except for food, we have to import most of our supplies," Hafnerexplained. "It's a long haul, and we've got to make use of everythingon the planet we can. We need oil. There are going to be a lot ofwheels turning, and every one of them will have to have oil. In timewe'll set up a synthetic plant, but if we can locate a productivefield now, it's to our advantage."

  "You're assuming the geology of Glade is similar to Earth?"

  Hafner waggled his hand. "Why not? It's a nicer twin of Earth."

  Why not? Because you couldn't always tell from the surface, thoughtMarin. It seemed like Earth, but was it? Here was a good chance tofind out the history of Glade.

  Hafner stood up. "Any time you're ready, a technician will check youout on the digger. Let me know before you go."

  * * * * *

  Actually, the digger wasn't a digger. It didn't move or otherwisedisplace a gram of dirt or rock. It was a means of looking down belowthe surface, to any practical depth. A large crawler, it was bigenough for a man to live in without discomfort for a week.

  It carried an outsize ultrasonic generator and a device for directingthe beam into the planet. That was the sending apparatus. Thereceiving end began with a large sonic lens which picked up soundbeams reflected from any desired depth, converted it into electricalenergy and thence into an image which was flashed onto a screen.

  At the depth of ten miles, the image was fuzzy, though good enough todistinguish the main features of the strata. At three miles, it wasbetter. It could pick up the sound reflection of a buried coin andconvert it into a picture on which the date could be seen.

  It was to a geologist as a microscope is to a biologist. Being abiologist, Dano Marin
could appreciate the analogy.

  He started at the tip of the peninsula and zigzagged across, headingtoward the isthmus. Methodically, he covered the territory, sleepingat night in the digger. On the morning of the third day, he discoveredoil traces, and by that afternoon he had located the main field.

  He should probably have turned back at once, but now that he had foundoil, he investigated more deliberately. Starting at the top, he letthe image range downward below the top strata.

  It was the reverse of what it should have been. In the top few feet,there were plentiful fossil remains, mostly of the four species ofmammals. The squirrel-like creature and the far larger grazing animalwere the forest dwellers. Of the plains animals, there were only two,in size fitting neatly between the extremes of the forest dwellers.

  After the first few feet, which corresponded to approximately twentythousand years, he found virtually no fossils. Not until he reached adepth which he could correlate to the Late Carboniferous age on Earthdid fossils reappear. Then they were of animals appropriate to theepoch. At that depth and below, the history of Glade was quite similarto Earth's.

  Puzzled, he checked again in a dozen widely scattered localities. Theresults were always the same--fossil history for the first twentythousand years, then none for roughly a hundred million. Beyond that,it was easy to trace the thread of biological development.

  In that period of approximately one hundred million years, somethingunique had happened to Glade. What was it?

  On the fifth day his investigations were interrupted by the sound ofthe keyed-on radio.

  "Marin."

  "Yes?" He flipped on the sending switch.

  "How soon can you get back?"

  He looked at the photo-map. "Three hours. Two if I hurry."

  "Make it two. Never mind the oil."

  "I've found oil. But what's the matter?"

  "You can see it better than I can describe it. We'll discuss it whenyou get back."

  * * * * *

  Reluctantly, Marin retracted the instruments into the digger. Heturned it around and, with not too much regard for the terrain, let itroar. The treads tossed dirt high in the air. Animals fled squealingfrom in front of him. If the grove was small enough, he went aroundit, otherwise he went through and left matchsticks behind.

  He skidded the crawler ponderously to halt near the edge of thesettlement. The center of activity was the warehouse. Pickups wheeledin and out, transferring supplies to a cleared area outside. He foundHafner in a corner of the warehouse, talking to the engineer.

  Hafner turned around when he came up. "Your mice have grown, Marin."

  Marin looked down. The robot cat lay on the floor. He knelt andexamined it. The steel skeleton hadn't broken; it had been bent,badly. The tough plastic skin had been torn off and, inside, thedelicate mechanism had been chewed into an unrecognizable mass.

  Around the cat were rats, twenty or thirty of them, huge by anystandards. The cat had fought; the dead animals were headless ordisemboweled, unbelievably battered. But the robot had beenoutnumbered.

  Biological Survey had said there weren't any rats on Glade. They hadalso said that about mice. What was the key to their error?

  The biologist stood up. "What are you going to do about it?"

  "Build another warehouse, two-foot-thick fused dirt floors, monolithicconstruction. Transfer all perishables to it."

  Marin nodded. That would do it. It would take time, of course, andpower, all they could draw out of the recently set up atomicgenerator. All other construction would have to be suspended. Nowonder Hafner was disturbed.

  "Why not build more cats?" Marin suggested.

  The executive smiled nastily. "You weren't here when we opened thedoors. The warehouse was swarming with rats. How many robot cats wouldwe need--five, fifteen? I don't know. Anyway the engineer tells me wehave enough parts to build three more cats. The one lying there can'tbe salvaged."

  It didn't take an engineer to see that, thought Marin.

  Hafner continued, "If we need more, we'll have to rob the computer inthe spaceship. I refuse to permit that."

  Obviously he would. The spaceship was the only link with Earth untilthe next expedition brought more colonists. No exec in his right mindwould permit the ship to be crippled.

  But why had Hafner called him back? Merely to keep him informed of thesituation?

  * * * * *

  Hafner seemed to guess his thoughts. "At night we'll floodlight thesupplies we remove from the warehouse. We'll post a guard armed withdecharged rifles until we can move the food into the new warehouse.That'll take about ten days. Meanwhile, our fast crops are ripening.It's my guess the rats will turn to them for food. In order to protectour future food supply, you'll have to activate your animals."

  The biologist started. "But it's against regulations to loose anyanimal on a planet until a complete investigation of the possible illeffects is made."

  "That takes ten or twenty years. This is an emergency and I'll beresponsible--in writing, if you want."

  The biologist was effectively countermanded. Another rabbit-infestedAustralia or the planet that the snails took over might be in themaking, but there was nothing he could do about it.

  "I hardly think they'll be of any use against rats this size," heprotested.

  "You've got hormones. Apply them." The executive turned and begandiscussing construction with the engineer.

  * * * * *

  Marin had the dead rats gathered up and placed in the freezer forfurther study.

  After that, he retired to the laboratory and worked out a course oftreatment for the domesticated animals that the colonists had broughtwith them. He gave them the first injections and watched themcarefully until they were safely through the initial shock phase ofgrowth. As soon as he saw they were going to survive, he bred them.

  Next he turned to the rats. Of note was the wide variation in size.Internally, the same thing was true. They had the usual organs, butthe proportions of each varied greatly, more than is normal. Nor weretheir teeth uniform. Some carried huge fangs set in delicate jaws;others had tiny teeth that didn't match the massive bone structure. Asa species, they were the most scrambled the biologist had everencountered.

  He turned the microscope on their tissues and tabulated the results.There was less difference here between individual specimens, but itwas enough to set him pondering. The reproductive cells wereespecially baffling.

  Late in the day, he felt rather than heard the soundless whoosh of theconstruction machinery. He looked out of the laboratory and saw smokerolling upward. As soon as the vegetation was charred, the smokeceased and heat waves danced into the sky.

  They were building on a hill. The little creatures that crept andcrawled in the brush attacked in the most vulnerable spot, the foodsupply. There was no brush, not a blade of grass, on the hill when thecolonists finished.

  * * * * *

  Terriers. In the past, they were the hunting dogs of the agriculturalera. What they lacked in size they made up in ferocity toward rodents.They had earned their keep originally in granaries and fields, and,for a brief time, they were doing it again on colonial worlds whereconditions were repeated.

  The dogs the colonists brought had been terriers. They were still asfast, still with the same anti-rodent disposition, but they were nolonger small. It had been a difficult job, yet Marin had done it well,for the dogs had lost none of their skill and speed in growing to thesize of a great dane.

  The rats moved in on the fields of fast crops. Fast crops were made toorder for a colonial world. They could be planted, grown, andharvested in a matter of weeks. After four such plantings, thefertility of the soil was destroyed, but that meant nothing in theearly years of a colonial planet, for land was plentiful.

  The rat tide grew in the fast crops, and the dogs were loosed on therats. They ranged through the fields, hunting. A rush, a s
nap of theirjaws, the shake of a head, and the rat was tossed aside, its backbroken. The dogs went on to the next.

  Until they could not see, the dogs prowled and slaughtered. At nightthey came in bloody, most of it not their own, and exhausted. Marinpumped them full of antibiotics, bandaged their wounds, fed themthrough their veins, and shot them into sleep. In the morning heawakened them with an injection of stimulant and sent them tinglinginto battle.

  It took the rats two days to learn they could not feed during the day.Not so numerous, they came at night. They climbed on the vines andnibbled the fruit. They gnawed growing grain and ravaged vegetables.

  The next day the colonists set up lights. The dogs were with them,discouraging the few rats who were still foolish enough to foragewhile the sun was overhead.

  An hour before dusk, Marin called the dogs in and gave them anenforced rest. He brought them out of it after dark and took them tothe fields, staggering. The scent of rats revived them; they were aseager as ever, if not quite so fast.

  The rats came from the surrounding meadows, not singly, or in twos andthrees, as they had before; this time they came together. Squealingand rustling the grass, they moved toward the fields. It was dark, andthough he could not see them, Marin could hear them. He ordered thegreat lights turned on in the area of the fields.

  The rats stopped under the glare, milling around uneasily. The dogsquivered and whined. Marin held them back. The rats resumed theirmarch, and Marin released the dogs.

  The dogs charged in to attack, but didn't dare brave the main mass.They picked off the stragglers and forced the rats into a tighterformation. After that the rats were virtually unassailable.

  The colonists