"For ages, it seems, these globe-men have dwelt on the planets of Alto. First they had inhabited the outermost planet, billions of miles from Alto itself, but which was yet warm enough for existence because of their sun's titanic size and immense heat. There they had risen to greatness, had built up their science and civilization to undreamed-of heights. But as the ages passed, that outermost world of theirs was growing colder and colder, since Alto, like all other suns, was slowly but steadily cooling, shrinking and dying, radiating less and less heat. At last there came a time when the planet of the globe-men was fast becoming too cold for existence there, and then their scientists stirred themselves to find a way out. Spurred on by necessity, they hit upon the invention of the gravity screen and with it constructed their first interplanetary space-ships. These they made in vast numbers, and in them the globe-people moved _en masse_ to the next innermost planet, which still received enough heat from Alto to support life. There they settled, and there their civilization endured for further ages. "But slowly, surely, their sun continued to cool and die, and with the terrible, machinelike inevitability of natural laws there came a day when again their world had grown too cold for their existence. This time, though, they had the remedy for their situation at hand, and again there took place a great migration from their cold planet to a warmer inner one. And so, as the ages passed, they escaped extinction by migrating from planet to planet, moving ever sunward as their sun waned in size and splendor, creeping closer and closer toward its dying fires. "At last, though, after long ages, there drew down toward them the doom which they had averted for so long. Alto was still shrinking, cooling, and now they were settled upon its warmest, inmost planet, and had no warmer world to which to flee. But a short time longer, as they measured time, and their planet would become a frozen, lifeless world, for their sun would inevitably cool still further until it was one of the countless dark stars, dead and burned-out suns, which throng the heavens. It seemed, indeed, that this time there was to be no escape.