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could have burned the bunched-up rats with the rightequipment, but they didn't have it and couldn't get it for years. Evenif they'd had it, the use of such equipment would endanger the crops,which they had to save if they could. It was up to the dogs.

  The rat formation came to the edge of the fields, and broke. Theycould face a common enemy and remain united, but in the presence offood, they forgot that unity and scattered--hunger was the greatdivisor. The dogs leaped joyously in pursuit. They hunted down thestarved rodents, one by one, and killed them as they ate.

  When daylight came, the rat menace had ended.

  The next week the colonists harvested and processed the food forstorage and immediately planted another crop.

  Marin sat in the lab and tried to analyze the situation. The colonywas moving from crisis to crisis, all of them involving food. Initself, each critical situation was minor, but lumped together theycould add up to failure. No matter how he looked at it, they justdidn't have the equipment they needed to colonize Glade.

  The fault seemed to lie with Biological Survey; they hadn't reportedthe presence of pests that were endangering the food supply.Regardless of what the exec thought about them, Survey knew theirbusiness. If they said there were no mice or rats on Glade, then therehadn't been any--_when the survey was made_.

  The question was: when did they come and how did they get here?

  Marin sat and stared at the wall, turning over hypotheses in his mind,discarding them when they failed to make sense.

  His gaze shifted from the wall to the cage of the omnivores, thesquirrel-size forest creature. The most numerous animal on Glade, itwas a commonplace sight to the colonists.

  And yet it was a remarkable animal, more than he had realized. Plain,insignificant in appearance, it might be the most important of anyanimal Man had encountered on the many worlds he had settled on. Thelonger he watched, the more Marin became convinced of it.

  He sat silent, observing the creature, not daring to move. He satuntil it was dark and the omnivore resumed its normal activity.

  _Normal?_ The word didn't apply on Glade.

  The interlude with the omnivore provided him with one answer. Heneeded another one; he thought he knew what it was, but he had to havemore data, additional observations.

  He set up his equipment carefully on the fringes of the settlement.There and in no other place existed the information he wanted.

  He spent time in the digger, checking his original investigations. Itadded up to a complete picture.

  When he was certain of his facts, he called on Hafner.

  The executive was congenial; it was a reflection of the smoothnesswith which the objectives of the colony were being achieved.

  "Sit down," he said affably. "Smoke?"

  The biologist sat down and took a cigarette.

  "I thought you'd like to know where the mice came from," he began.

  Hafner smiled. "They don't bother us any more."

  "I've also determined the origin of the rats."

  "They're under control. We're doing nicely."

  * * * * *

  On the contrary, thought Marin. He searched for the proper beginning.

  "Glade has an Earth-type climate and topography," he said. "Has hadfor the past twenty thousand years. Before that, about a hundredmillion years ago, it was also like Earth of the comparable period."

  He watched the look of polite interest settle on the executive's faceas he stated the obvious. Well, it _was_ obvious, up to a point. Theconclusions weren't, though.

  "Between a hundred million years and twenty thousand years ago,something happened to Glade," Marin went on. "I don't know the cause;it belongs to cosmic history and we may never find out. Anyway,whatever the cause--fluctuations in the sun, unstable equilibrium offorces within the planet, or perhaps an encounter with an interstellardust cloud of variable density--the climate on Glade changed.

  "It changed with inconceivable violence and it kept on changing. Ahundred million years ago, plus or minus, there was carboniferousforest on Glade. Giant reptiles resembling dinosaurs and tiny mammalsroamed through it. The first great change wiped out the dinosaurs, asit did on Earth. It didn't wipe out the still more primitive ancestorof the omnivore, because it could adapt to changing conditions.

  "Let me give you an idea how the conditions changed. For a few years agiven area would be a desert; after that it would turn into a jungle.Still later a glacier would begin to form. And then the cycle would berepeated, with wild variations. All this might happen--didhappen--within a span covered by the lifetime of a single omnivore.This occurred many times. For roughly a hundred million years, it wasthe norm of existence on Glade. This condition was hardly conducive tothe preservation of fossils."

  Hafner saw the significance and was concerned. "You mean theseclimatic fluctuations suddenly stopped, twenty thousand years ago? Arethey likely to begin again?"

  "I don't know," confessed the biologist. "We can probably determine itif we're interested."

  The exec nodded grimly. "We're interested, all right."

  Maybe we are, thought the biologist. He said, "The point is thatsurvival was difficult. Birds could and did fly to more suitableclimates; quite a few of them survived. Only one species of mammalsmanaged to come through."

  "Your facts are not straight," observed Hafner. "There are fourspecies, ranging in size from a squirrel to a water buffalo."

  "One species," Marin repeated doggedly. "They're the same. If the foodsupply for the largest animal increases, some of the smaller so-calledspecies grow up. Conversely, if food becomes scarce in any category,the next generation, which apparently can be produced almostinstantly, switches to a form which does have an adequate foodsupply."

  "The mice," Hafner said slowly.

  * * * * *

  Marin finished the thought for him. "The mice weren't here when we gothere. They were born of the squirrel-size omnivore."

  Hafner nodded. "And the rats?"

  "Born of the next larger size. After all, we're environment,too--perhaps the harshest the beasts have yet faced."

  Hafner was a practical man, trained to administer a colony. Conceptswere not his familiar ground. "Mutations, then? But I thought--"

  The biologist smiled. It was thin and cracked at the edges of hismouth. "On Earth, it would be mutation. Here it is merely normalevolutionary adaptation." He shook his head. "I never told you, butomnivores, though they could be mistaken for an animal from Earth,have no genes or chromosomes. Obviously they do have heredity, but howit is passed down, I don't know. However it functions, it responds toexternal conditions far faster than anything we've ever encountered."

  Hafner nodded to himself. "Then we'll never be free from pests." Heclasped and unclasped his hands. "Unless, of course, we rid theplanet of all animal life."

  "Radioactive dust?" asked the biologist. "They have survived worse."

  The exec considered alternatives. "Maybe we should leave the planetand leave it to the animals."

  "Too late," said the biologist. "They'll be on Earth, too, and all theplanets we've settled on."

  Hafner looked at him. The same pictures formed in his mind that Marinhad thought of. Three ships had been sent to colonize Glade. One hadremained with the colonists, survival insurance in case anythingunforeseen happened. Two had gone back to Earth to carry the reportthat all was well and that more supplies were needed. They had alsocarried specimens from the planet.

  The cages those creatures were kept in were secure. But a smallerspecies could get out, must already be free, inhabiting, undetected,the cargo spaces of the ships.

  There was nothing they could do to intercept those ships. And oncethey reached Earth, would the biologists suspect? Not for a long time.First a new kind of rat would appear. A mutation could account forthat. Without specific knowledge, there would be nothing to connect itwith the specimens picked up from Glade.

  "We have to stay," said the biologist. "We have to
study them and wecan do it best here."

  He thought of the vast complex of buildings on Earth. There was toomuch invested to tear them down and make them verminproof. Billions ofpeople could not be moved off the planet while the work was beingdone.

  They were committed to Glade not as a colony, but as a giganticlaboratory. They had gained one planet and lost the equivalent of ten,perhaps more when the destructive properties of the omnivores werefinally assessed.

  A rasping animal cough interrupted the biologist's thoughts. Hafnerjerked his head and glanced out the window. Lips tight, he grabbed arifle off the wall and ran out. Marin followed him.

  * * * * *

  The exec