"Where do they come from?" Split had paused in the act of checking equipment to take his turn at the telescope. If he had not done so, I might not have made a discovery. The landscape was _moving_. The long shadows that I had not noticed through the telescope were a prominent part of the picture I saw through the ship's window when I looked out across the scene with the naked eye. The shadows were moving. They were tree shadows. They were moving toward the clearing where the crowd gathered. And the reason for their movement was that the trees themselves were moving. "Notice anything?" I asked Split. "The crowd is growing. We've certainly landed on top of a city." He gazed. "They're coming from underground." Looking through the telescope, obviously he didn't catch the view of the moving trees. "Notice anything else unusual?" I persisted. "Yes. The females--I'm speaking hypothetically--but they _must_ be females--are all wearing puffy white fur ornaments around their elbows. I wonder why?" "You haven't noticed the trees?" "The females are quite attractive," said Split. I forgot about the moving trees, then, and took over the telescope. Mobile trees were not new to me. I had seen similar vegetation on other planets--"sponge-trees"--which possessed a sort of muscular quality. If these were similar, they were no doubt feeding along the surface of the slope below the rocky plateau. The people in the clearing beyond paid no attention to them. I studied the crowd of people. Only the leader wore the brilliant garb. The others were more scantily clothed. All were handsome of build. The lemon-tinted sunlight glanced off the muscular shoulders of the males and the soft curves of the females. "Those furry elbow ornaments on the females," I said to Split, "they're for protection. The caves they live in must be narrow, so they pad their elbows." "Why don't they pad their shoulders? They don't have anything on their shoulders." "Are you complaining?" We became fascinated in watching, from the seclusion of our ship. If we were to walk out, or make any sounds, we might have interrupted their meeting. Here they were in their native ritual of sunset, not knowing that people from another world watched. The tall leader must be making a speech. They sat around him in little huddles. He moved his arms in calm, graceful gestures. "They'd better break it up!" Split said suddenly. "The jungles are moving in on them." "They're spellbound," I said. "They're used to sponge-trees. Didn't you ever see moving trees?" Split said sharply, "Those trees are marching! They're an army under cover. Look!" I saw, then. The whole line of advancing vegetation was camouflage for a sneak attack. And all those natives sitting around in meeting were as innocent as a flock of sitting ducks. Split Campbell's voice was edged with alarm. "Captain! Those worshippers--how can we warn them? Oh-oh! Too late. Look!"