Earth,
have no genes or chromosomes. Obviously they do have heredity, but how
it is passed down, I don't know. However it functions, it responds to
external conditions far faster than anything we've ever encountered."

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

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

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

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

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

The cages those creatures were kept in were secure. But a smaller
species 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 once
they reached Earth, would the biologists suspect? Not for a long time.
First a new kind of rat would appear. A mutation could account for
that. Without specific knowledge, there would be nothing to connect it
with the specimens picked up from Glade.

[Illustration]

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

He thought of the vast complex of buildings on Earth. There was too
much invested to tear them down and make them verminproof. Billions of
people could not be moved off the planet while the work was being
done.

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

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

* * * * *

The exec headed toward the fields where the second fast crop was
maturing. On top of a knoll, he stopped and knelt. He flipped the dial
to _extreme charge_, aim