"What happened to the natives who lived here?" Billy Kasker asked. The instructor frowned. "Oh, they were killed." At first he was a little irritated at the question, then again satisfaction came back into his voice. "They got what was coming to them for trying to resist our sky ships," Joe Buckner said. "Oh, yes, they deserved their fate." The instructor hitched the _Thor_ gun a little higher on his hip. Billy Kasker was silent. "We will go next to the fields, then to the factory section--such of as there is--then to that part of the city which we have allowed the natives to rebuild. Come." The class moved out of the city. Here they saw their first natives. Clad mostly in rags--many of them bent and stooped, some of them showing the marks of hunger--they were a quiet people who kept strictly out of the way of the class group. But except for the clothing and the marks of hunger, they were identical in appearance with their conquerors. "Why, they look just like us!" Joe Buckner said indignantly. He sounded outraged at the resemblance. "There are many differences," the instructor said quickly. "Note their clothing, how poorly made it is. They make it themselves out of the wool of some kind of animal--deer, I believe, or bear." "Sheep," Billy Kasker corrected. "Oh, yes, sheep is the name of the animal. Thank you, Billy." "You're welcome, sir." "But they oughtn't to look like _us_!" Joe Buckner continued. "There are chemical differences," Susan Sidwell said. "Once, in the laboratory, we analyzed their blood. The color was different for one thing. They also have a much different metabolism." "But suppose one of them escaped from the museum and got into our part of the world. How would we know he wasn't one of us, if he put on our clothes?" Joe Buckner sounded outraged. "That is one purpose our bracelets serve," the instructor answered. "A very good question, Joe. As you know, each of us receives a bracelet at birth, which is slipped over the hand and onto the wrist. Made of _plasticum_, which cannot be cut by any method, the bracelet has the unique property of expanding in size as the wearer grows. It cannot be removed except by cutting off the arm of the wearer." He laughed as if he had made a good joke. "But I am sure no one would ever think of doing that. The bracelet carries the serial number assigned to each of us." He held up his arm, exhibiting the gleaming circle of _plasticum_ on his wrist. To him--to all of them--it was a badge of honor, a mark that proved one belonged to a superior race. "If one of the natives escaped, the absence of a bracelet would disclose his identity at once. We would take measures to have him eliminated." "I see," Joe Buckner said. He sounded mollified. "How would we eliminate him?" "I believe it is customary to use a _Thor_ gun in such cases--a large caliber which will disintegrate him instantly. The model I have will only blast a hole a few inches in diameter."