There was another thing to consider, though: the winds that blew all the way across the Xurdimur and furnished propulsion for the wheeled sailing craft. To have winds you must have pressure differentials, which were usually caused by heat differentials. Although the Xurdimur was ringed by mountains there were no large eminences on it for ten thousand miles, nothing to replenish the currents of air. Or so it seemed to his limited knowledge of meteorology, though he did wonder how the trade winds that swept Earth's seas managed to keep going for so many thousands of leagues, just on their original impetus. Or did they get boosts? He didn't know. What he did know was that the Xurdimur was a thing that shouldn't be. Yet, the very presence of men here was just as amazing, just as preposterous. Homo sapiens was scattered throughout the Galaxy. Everywhere that the space-traveling Earthmen had gone, they had found that about every fourth inhabitable planet was populated by men of their species. The proof lay not just in the outward physical resemblance of terrestrial and extra-terrestrial; it lay in their ability to breed. Earthman, Sirian, Albirean, Vegan, it made no difference. Their men could have children by the women of other planets. Naturally there had been many theories to account for this fact. All had as a common basis the assumption that Homo sapiens had sometime, somewhere, in the very remote past, originated on one planet and then had spread out over the Galaxy from it. And, somehow, space travel had been lost and each race had gone back to savagery, only to begin again the long hard struggle toward civilization and the re-discovery of spaceships. Why, no one knew. One could only guess. There was the problem of language. It might seem that if man had come from a common birthplace he would at least have kept a trace of his home language and that the linguists could break down the development of tongue and link one planet to another through it. But no. Every world had its own Tower of Babel, its own ten thousand languages. The terrestrial scientist might trace Russian and English and Swedish, and Lithuanian and Persian and Hindustani back to a proto-Indo-European, but he had never found on any other planet a language which he could say had also derived from the Aryan Ursprache. Green's mind wandered to the two Earthmen now imprisoned in the city of Estorya. He hoped they weren't being treated badly. They could be in horrible pain at this very moment, if the priests felt like subjecting them to a little demon-testing.