The fault seemed to lie with Biological Survey; they hadn't reported the presence of pests that were endangering the food supply. Regardless of what the exec thought about them, Survey knew their business. If they said there were no mice or rats on Glade, then there hadn'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, the squirrel-size forest creature. The most numerous animal on Glade, it was 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 any animal Man had encountered on the many worlds he had settled on. The longer he watched, the more Marin became convinced of it. He sat silent, observing the creature, not daring to move. He sat until 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. He needed another one; he thought he knew what it was, but he had to have more 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. It added 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 smoothness with 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 had for the past twenty thousand years. Before that, about a hundred million years ago, it was also like Earth of the comparable period." He watched the look of polite interest settle on the executive's face as he stated the obvious. Well, it _was_ obvious, up to a point. The conclusions 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 of forces within the planet, or perhaps an encounter with an interstellar dust cloud of variable density--the climate on Glade changed.