Willard nodded. "The Ancients died of the Silver Plague, indirectly. They tried to escape by changing their bodies. The library told me that."
"Their bodies? How?"
"Well—you've seen the robots in Chahnn and here. Originally they were the servants of the Ancients."
"Intelligent?"
"No—not in the way you mean. They could be conditioned to perform certain tasks, but usually they were controlled telepathically by the Ancients, who wore specialized helmet-transmitters for the purpose. The robots had radioatomic brains that reacted to telepathic commands. Then when the Silver Plague struck, the Ancients tried to escape by transplanting, not their physical brains, but their minds. I don't quite know how it was done. But the thought-patterns, the individual mental matrix, of each Ancient was somehow impressed on the radioatomic brain of a robot. Their minds were put into the robots' brains—and controlled the metal bodies. So they escaped the Plague. But they died anyway. Human, intelligent minds can't be transplanted successfully into artificial bodies that way. So—in a hundred years—they were dead, all of them."
So that was the secret of the Ancients' disappearance from Ganymede. They had taken new bodies—and those bodies had killed them through their sheer alienage.
Willard crushed out his cigaret-stub. "All the knowledge of the Ancients at my finger-tips, Ed. You can imagine what research I've done!"
"I should have thought you'd have looked for a weapon against the Zarno," Garth said practically. "The Ancients were able to conquer them."
"I did—first of all, after I'd learned how to work the recording-machines. A certain ray will destroy them—a vibrationary beam that shakes them to pieces, disrupts their molecular structure. The only trouble is—" Willard grinned sardonically. "It takes a damn good machine shop to build such a projector."
"Oh. We couldn't—"