A lot of sweat was dabbed from relieved foreheads in the Corps when a secondary roborocket, sent into a short one-week orbit, had picked up the animals' pulses with ease as soon as springtime was upon the land. The odds against their being thus missed were fantastic, astronomically unlikely. But it _had_ happened, despite the odds against it, and the Corps was forcibly reminded that in a universe of planets, there is infinite room for even the unlikely to occur. The only other oversight had been years later, when a just-settling colony had been half-destroyed by a herd of immense beasts similar to the buffalo of Earth, but viciously carnivorous. There had been no indication, in the six-month scanning period, that such a species even existed on the planet, the third planet of Syrinx Gamma, the sun of a newly discovered system beyond the Coalsack. The reason was maddeningly simple. The herds were migratory. Their migrations had corresponded in scope around the oceanless planet with the sweep of the scanner-beam in such a way that the roborocket was scanning either where the herd had just been or where it had not yet arrived. Again, the odds were fantastic against the occurrence. Yet, again, it _had_ happened. Other than these two events, though, there had been no further error on the part of scanners for nearly a decade. Precautions had been taken against recurrence. Roborockets were now sent to scan a planet only at a time when there would be an overlap of seasonal climes, so that the beam would inspect the surface throughout both the mild and the rigorous weathers, thus obviating a repeat of the brown bear incident. And the sweep of the beam had been extended, so that no animal with migratory movement at speeds less than that of a supersonic plane could have avoided being duly detected and catalogued. That, they thought, should prevent any more such incidents. All that Jerry knew. * * * * * And yet, here he was, descending through the black vacuum of space toward an already-colonized planet, the second planet of Sirius, a planet supposedly already scanned, catalogued, and long-since ready for inhabitation. And now, after the colonials had been there for nearly five years, something was starting to wipe them out. Some unsuspected alien thing was present on the planet, a thing that a hastily lofted roborocket had located in a matter of hours, and yet had missed in its original six-month orbital check, before the settlers came. It was impossible. Incredible. And yet, again, it _had_ happened--_was_ happening--and had to be stopped. A frantic appeal had been beamed to Earth through sub-space, an appeal for a Space Zoologist to find the alien, learn its weaknesses, and recommend its mode of destruction.