"The killing agent developed by the opposing side evidently had been very thoroughly mastered. And again we can't say how they did it. It can be described as a large protein molecule, but its properties can be imagined only as arising out of a very complex organization, theoretically impossible at that level of life. It is confined to water, but its method of dispersion within that medium is not understood at all. At one instant, it is here; at the next, it apparently will have moved to a point perhaps several hundred miles away. It is life which has no existence, and cannot exist, except as a weapon. Unlike a parasite, its purpose is simply to kill, quickly and efficiently, and go on at once to another victim. Having exhausted the store of victims--a short process, obviously, even in an area of planetary dimensions--it dies of something like starvation within days. "That, of course, was as practical a limitation to those employing it as the one that it attacks only Tareegs. They did not want to be barred indefinitely from an area which had been cleansed of their enemies, and neither did they want food animals in that area to be destroyed. They...." His voice trailed off, and Troy stirred restlessly. Dr. Clingman was slumped farther down in his chair now, and the pale, protruding eyes had begun to blink drowsily. He seemed about to go to sleep. Troy said, "If the thing killed the Tareegs on Cassa One inside an hour after they'd gone into the sea, then they couldn't have had the time to start the interstellar drones back towards the twin worlds." Clingman's head turned to him again. "No," Clingman said. "Of course not." "And even," Troy went on, "if they had been able to ship a couple of loads of infected water back, it would have been harmless long before it reached their worlds." * * * * * Clingman nodded. "Quite harmless. As harmless as the new ocean on Cassa One would be by this time to Tareegs who entered it." He paused. "We'd thought, Gordon ... as you might be thinking now ... of sending the drones back instead with a load of asteroid ice containing the inert agent. That, of course, would not have reduced its effectiveness. Nevertheless, the scheme wouldn't have worked." "Why not?" Troy asked. "Because the drones, in the Tareeg view, were sacred messengers. They could be used only to announce in a certain prescribed manner that the Tareeg interstellar expeditionary force had discovered a water planet and taken possession of it, again with the required ceremony, for the twin worlds. The transmission of lumps of interplanetary ice would never have fitted that picture, would, in fact, have been an immediate warning that something very much out of order had occurred.