on glowing feet and sniffed blindly at
the air with tiny glowing noses. Rats.

On and on moved the rat, its little pinkish feet pattering almost
silently on the oxidized metal surface of the rails. Its sensitive ears
picked up the movements and the squeals of other rats, but it paid them
no heed. Several times it met other rats on the rail, but most of them
sensed the alienness of _this_ rat and scuttled out of its way.

Once, it met a rat who did not give way. Hungry, perhaps, or perhaps
merely yielding to the paranoid fury that was a normal component of the
rattish mind, it squealed its defiance to the rat that was not a rat. It
advanced, baring its rodent teeth in a yellow-daggered snarl of hate.

The rat that was not a rat became suddenly motionless, its sharp little
nose pointed directly at the oncoming enemy. There came a noise, a tiny
popping hiss, like that of a very small drop of water striking hot
metal. From the left nostril of the not-rat, a tiny, glasslike needle
snapped out at bullet speed. It struck the advancing rat in the center
of the pink tongue that was visible in the open mouth. Then the not-rat
scuttled backward faster than any real rat could have moved.

For a second the real rat hesitated, and it may be that the realization
penetrated into its dim brain that rats did not fight this way. Then, as
the tiny needle dissolved in its bloodstream, it closed its eyes and
collapsed, rolling limply off the rail to the rotted wooden tie
beneath.

The rat might come to before it was found and devoured by its
fellows--or it might not. The not-rat moved on, not caring either way.
The human intelligence that looked out from the eyes of the not-rat was
only concerned with getting to the Nipe.

* * * * *

"That's how we found the Nipe," Colonel Mannheim said, "and that's how
we keep tabs on him now. We have over seven hundred of these
remote-control robots hidden in strategic spots throughout those tunnels
now, and we can put more in whenever we want, but it took time to get
everything set up this way. Now we can follow the Nipe wherever he goes,
so long as he stays in those tunnels. If he went out through the one
open-air exit up in the northern part of the island, we could have him
followed by bird-robots. But"--he shrugged wryly--"I'm afraid the
underwater problem still has us stumped. We can't get the carrier wave
for the remote-control impulses to go very far underwater.