Cat and Mouse

The Warden needed to have a certain very obnoxious pest eliminated ... and he knew just the pest-eradicator he needed....
The Harn first came to the Warden's attention through its effect on the game population of an area in World 7 of the Warden's sector. A natural ecology was being maintained on World 7 as a control for experimental seedings of intelligent life-forms in other similar worlds. How the Harn got there, the Warden never knew. In its free-moving larval state, the Harn was a ticklike creature which might have sifted through a natural inter-dimensional rift; or it might have come through as a hitchhiker on some legitimate traveler, possibly even the Warden himself.
In any event, it was there now. Free of natural enemies and competition, it had expanded enormously. So far, the effect in the control world was localized, but this would not be the case when the Harn seeded. Prompt action was indicated.
The Warden's inclination and training was in the direction of avoiding direct intervention in the ecology of the worlds under his jurisdiction, even in the field of predator control. He considered introduction of natural enemies of the Harn from its own world, and decided against it. That cure was as bad, if not worse, than the disease itself.
There was, however, in one adjacent world, a life-form not normally associated with the Harn; but which analysis indicated would be inimical to it, and reasonably amenable to control.
It was worth trying, anyway.
October 3rd, Ed Brown got up to the base cabin of his trap line with his winter's outfit.
He hung an N. C. Company calendar on the wall and started marking off the days.
October 8th, the hole into the other world opened.
In the meantime, of course, Ed had not been idle. All summer the cabin had stood empty. He got his bedding, stove, and other cabin gear down from the cache and made the place livable. The mice were thick, a good fur sign, but a nuisance otherwise. Down in the cellar hole, when he went to clear it out for the new spud crop, he found burrowings everywhere.