Eventually, the means was tracked down from a clue in one of the oldest books. It was not found, but its existence deduced and proved to the satisfaction of the searchers. Somewhere, hidden in a forgotten corner of the solar system, was the missing tool. This fact settled, it became necessary to locate and use the missing tool. It was a pitifully small generation. Only seven bright-eyed and high-hearted young men. But that night they broke out of their prescribed boundaries. They went off reservation, and separated in seven directions. Each had a theory, and a hope to be explored. Without their ident-armbands of platinum with the old Greek letter deeply incised, they resembled any other seven youths picked at random from the teeming multitudes. They could mingle unquestioned, and their studies had prepared them for various tasks to which their forged papers entitled them. But adaptation was not easy. A single incautious moment could betray them. Even a routine brain wave check would be sufficient to identify the fugitives, for in subtle ways, the gamma-men were different. Seven young men with a noble purpose, and fanatical hopes in their hearts. And now there were two. Five of them had made fatal slips, and had been hunted down to hideous deaths. Ferris was one of the two survivors. * * * * * Four people stood paralyzed while the hammering resounded at the gate. This was no human fist demanding attention, but an odd, robot-like clanking, as if a mechanical beast nosed in determined rooting against the metal leaves of the gate. It was just such a beast. A burring whine rose into notes of shrill frustration. Metallic and electronic frustration, for the tracker was a bloodhound of vacuum tubes and relays and switches and batteries and transformers. Unerring and inexorable, its robot senses sorted a single frequency from all other brain wave patterns, and it clung to the trail with chilling efficiency. Something about its unhuman lusting numbed most quarry before the pursuers in charge of the monster could check its demonic eagerness for prey. Now, like a metallic carnivore scenting blood, the robot tracker nuzzled the gate and rebounded to nuzzle again. All four of the humans inside the compound imagined the scene outside. Pencil beams of hand radilumes glinted here and there, the questing soldiers and police squads, the glittering serpentine body of the tracker, with its scurrying treads churning clouds of dust as it whined and rooted at the gate. Bat Ferris shot a glance of uneasy calculation at his three companions. The girl was an unknown quantity. Angel, momentarily shocked, was predictable enough within limits. Pao Chung was openly an opportunist, willing to turn any situation to personal profit. Unarmed, Ferris could not even deal with them, let alone with the police outside. He frowned angrily.