"Let me give you an idea how the conditions changed. For a few years a given 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 be repeated, with wild variations. All this might happen—did happen—within a span covered by the lifetime of a single omnivore. This occurred many times. For roughly a hundred million years, it was the norm of existence on Glade. This condition was hardly conducive to the preservation of fossils."
Hafner saw the significance and was concerned. "You mean these climatic fluctuations suddenly stopped, twenty thousand years ago? Are they likely to begin again?"
"I don't know," confessed the biologist. "We can probably determine it if we're interested."
The exec nodded grimly. "We're interested, all right."
Maybe we are, thought the biologist. He said, "The point is that survival was difficult. Birds could and did fly to more suitable climates; quite a few of them survived. Only one species of mammals managed to come through."
"Your facts are not straight," observed Hafner. "There are four species, ranging in size from a squirrel to a water buffalo."
"One species," Marin repeated doggedly. "They're the same. If the food supply for the largest animal increases, some of the smaller so-called species grow up. Conversely, if food becomes scarce in any category, the next generation, which apparently can be produced almost instantly, switches to a form which does have an adequate food supply."
"The mice," Hafner said slowly.
