"I knew, with never a doubt, that this great open space at the nebula's heart had been formed because the denser portions of its interior had contracted faster than the outer portions. As you know, all nebulæ contract with the passage of time, their fiery gases condensing to form great blazing stars, the eon-old cycle of stellar evolution, from fiery nebula to flaming sun. In this cycle this great nebula followed, but because of its vast size the inner, denser portions had contracted with much greater speed than the outer parts, forming a great solid world, in time, while the outer parts were still but fiery gas. This solid world spun at the center of the great space formerly occupied by the gases that had contracted to form it, and it was warmed and lit eternally by the encircling fires of the nebula all around it, and shut off from the outside universe by those fires. "Light and warmth had this world in plenty, therefore, and with time life had risen on it, crude forms asecnding through the channels of evolutionary change into a myriad different species, of which one species, the nebula-creatures we have seen, was the most intelligent. In time they ruled this strange world, wiping out all other species, and climbed to greater and greater science and power with the passage of time, their existence never suspected by any in the universe outside. Back and forth through the Galaxy went the great star-cruisers of the federated suns, but none ever dreamed of the strange race that had grown to power on this world at the fiery nebula's heart. "But slowly, inexorably, destruction began to creep upon that race. As I have said, all nebulæ contract always, and this one was still doing so, still growing smaller and smaller, its encircling fires closing steadily in upon the spinning world at their heart. Hotter and hotter it became on that world until life was hardly possible on it for the nebula-creatures, accustomed as they were to a milder temperature. They must escape that heat or perish, and since they could not escape to outer space through the prisoning fires around them they did the last thing available, hollowed out vast caverns in the interior of their world and descended into those caverns to live. The whole surface of their world they sheathed with smooth, heat-reflecting metal, and then descended in all their hordes into the countless mighty caverns that honeycombed all their great world, taking up their life again in those cool depths, safe from the nebula's heat.