"Get it?" he resumed. At their silence, he proceeded, "I see you don't. Leroy figured it out, but whether rightly or wrongly, I don't know. He thinks that the barrels and Tweel's race have a reciprocal arrangement like--well, like bees and flowers on earth. The flowers give honey for the bees; the bees carry the pollen for the flowers. See? The barrels tend the works and Tweel's people build the canal system. The Xanthus city must have been a boosting station; that explains the mysterious machines I saw. And Leroy believes further that it isn't an intelligent arrangement--not on the part of the barrels, at least--but that it's been done for so many thousands of generations that it's become instinctive--a tropism--just like the actions of ants and bees. The creatures have been bred to it!" "Nuts!" observed Harrison. "Let's hear you explain the reason for that big empty city, then." "Sure. Tweel's civilization is decadent, that's the reason. It's a dying race, and out of all the millions that must once have lived there, Tweel's couple of hundred companions are the remnant. They're an outpost, left to tend the source of the water at the polar cap; probably there are still a few respectable cities left somewhere on the canal system, most likely near the tropics. It's the last gasp of a race--and a race that reached a higher peak of culture than Man!" "Huh?" said Harrison. "Then why are they dying? Lack of water?" "I don't think so," responded the chemist. "If my guess at the city's age is right, fifteen thousand years wouldn't make enough difference in the water supply--nor a hundred thousand, for that matter. It's something else, though the water's doubtless a factor." "_Das wasser_," cut in Putz. "Vere goes dot?" "Even a chemist knows that!" scoffed Jarvis. "At least on earth. Here I'm not so sure, but on earth, every time there's a lightning flash, it electrolyzes some water vapor into hydrogen and oxygen, and then the hydrogen escapes into space, because terrestrial gravitation won't hold it permanently. And every time there's an earthquake, some water is lost to the interior. Slow--but damned certain." He turned to Harrison. "Right, Cap?" "Right," conceded the captain. "But here, of course--no earthquakes, no thunderstorms--the loss must be very slow. Then why is the race dying?" "The sun-power plant answers that," countered Jarvis. "Lack of fuel! Lack of power! No oil left, no coal left--if Mars ever had a Carboniferous Age--and no water-power--just the driblets of energy they can get from the sun. That's why they're dying." "With the limitless energy of the atom?" exploded Harrison. "They don't know about atomic energy. Probably never did. Must have used some other principle in their space-ship." "Then," snapped the captain, "what makes you rate their intelligence above the human? We've finally cracked open the atom!"