vironment, similar animals would usually
evolve.

In the Late Carboniferous forest on Earth, there had been creatures
like the omnivore, the primitive mammal from which all others had
evolved. On Glade, that kind of evolution just hadn't taken place.
What had kept nature from exploiting its evolutionary potentialities?
There was the real problem, not how to wipe them out.

Marin stuck a needle in the omnivore. It squealed and then relaxed. He
drew out the blood and set it back in the cage. He could learn a lot
about the animal from trying to kill it.

* * * * *

The quartermaster was shouting, though his normal voice carried quite
well.

"How do you know it's mice?" the biologist asked him.

"Look," said the quartermaster angrily.

Marin looked. The evidence did indicate mice.

Before he could speak, the quartermaster snapped, "Don't tell me
they're only mice-like creatures. I know that. The question is: how
can I get rid of them?"

"Have you tried poison?"

"Tell me what poison to use and I'll use it."

It wasn't the easiest question to answer. What was poisonous to an
animal he had never seen and knew nothing about? According to
Biological Survey, the animal didn't exist.

It was unexpectedly serious. The colony could live off the land, and
was expected to. But another group of colonists was due in three
years. The colony was supposed to accumulate a surplus of food to feed
the increased numbers. If they couldn't store the food they grew any
better than the concentrates, that surplus was going to be scanty.

Marin went over the warehouse thoroughly. It was the usual early
construction on a colonial world. Not esthetic, it was sturdy enough.
Fused dirt floor, reinforced foot-thick walls, a ceiling slab of the
same. The whole was bound together with a molecular cement that made
it practically airtight. It had no windows; there were two doors.
Certainly it should keep out rodents.

A closer examination revealed an unexpected flaw. The floor was as
hard as glass; no animal could gnaw through it, but, like glass, it
was also brittle. The crew that had built the warehouse had evidently
been in such a hurry to get back to Earth that they hadn't been as
careful as they should have been, for here and there the floor was
thin. Somewhere under the heavy equipment piled on it, the floor had
cracked. There a burrowing animal had means of entry.

Short of building another warehouse