"They were beings of an age dead for hundreds of millions of years, I learnt, creatures of the Mesozoic age, that period of the earth's history which we call the age of reptiles. For in that age the races of mammals had hardly begun to arise, and the great and smaller reptiles and lizard-races were the rulers of all earth. And just as man, the creature of dominant intelligence, was to develop later from the races of mammals, so had these lizard-men, the dominant intelligence of their own age, developed from the races of reptiles. They had spread out in great numbers over what is now North America, the most habitable portion of earth during the Mesozoic age. They had built strange cities, had developed their knowledge and science in myriad ways, and had learned how to conquer and subjugate the great reptilian creatures who swarmed then on earth, to make servants of them. The great brontosaurs, more tractable than the rest, they used as mounts and beasts of burden; the fiercer tyrannosaurs and allosaurs were their beasts of war; and on the mighty pterodactyls they soared into the upper air and flitted across earth's surface. Great indeed was their power, and through that power and through their terrible, giant servants they ruled all the habitable parts of earth unquestioningly. "At last, though, there came that great convulsion of earth which was to mark the end of the Mesozoic age, that vast world-cataclysm in which continents sank beneath the seas and new lands rose from the oceans' depths. In such convulsions and mighty quakes the cities of the lizard-men were shaken down and annihilated, and across all their world was wild confusion. They knew, then, that they must find some other place of refuge or perish, and so they hit upon the plan of descending to one of the great cavernous spaces which lie scores of miles down in earth's interior. They had discovered long before that such great caverns exist inside earth's crust, and so they pierced a shaft down to one of them and descended into it to investigate. "They found it a place large enough to hold all their numbers, and one quite habitable. It was lit perpetually with crimson light, too, since the molten fires of earth's heart had pressed up close to the walls of the cavern, and through an opening in those walls there poured down eternally a raging Niagara of molten rock and flame, that titanic fall of living fire whose blazing radiance illuminates all this cavern-world. So beneath this fall of fire the lizard-men constructed a canal which conducted it into a great stone basin which lay directly beneath the opening of their shaft in the cavern's roof, and from this basin the molten fires were able to seep gradually into crevices beneath the cavern.