first great change wiped out the dinosaurs, as
it did on Earth. It didn't wipe out the still more primitive ancestor
of the omnivore, because it could adapt to changing conditions.

"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.

* * * * *

Marin finished the thought for him. "The mice weren't here when we got
here. 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. Concepts
were not his familiar ground. "Mutations, then? But I thought--"

The biologist smiled. It was thin and cracked at the edges of his
mouth. "On Earth, it would be mutation. Here it is merely normal
evolutionary adaptation." He shook his head. "I never told you, but
omnivores, though they could be mistaken for an animal from