The two last-named writers set forth an elaboration of an opinion first held by the Pythagoreans and worked out in detail by Crates of Mallos in the second century before Christ, which gained great ascendancy over the minds of map makers and writers of the Middle Ages. They explained that the _oikoumene_ is one of four similar inhabited bodies of land on the surface of the globe. These bodies of land are separated from one another by two oceans which encircle the earth, one running east and west in the fiery equatorial regions, and one running north and south at right angles to the equatorial ocean. This idea, which we shall call the “Cratesian” theory after its foremost expositor, did not pass unchallenged either in antiquity or in the Christian period. Involving as it did the doctrine of the antipodes—people dwelling in quarters absolutely inaccessible to men of our race, eternally cut off from our _oikoumene_ by the fires of the equator and the terrors of the meridional ocean—the Cratesian theory provoked the indignation of the Fathers of the Church as containing the seeds of heresy.[66] EXTENT OF THE “OIKOUMENE” Aristotle, although he had derived from the Pythagoreans the theory of an uninhabitable torrid belt,[67] believed in a greater southward extension of our _oikoumene_ than would be possible in accordance with the Cratesian theory. He harbored no idea of the existence of another _oikoumene_ in the same latitude as ours. He says very clearly in the _De caelo_[68] that there is no great distance between India and Spain and hinted at the same opinion in the _Meteorology_.[69] Seneca[70] held similar views. The opposite theory—which has been called the continental as opposed to the oceanic hypothesis[71]—that Africa and Asia extended unknown distances south and east and that the Atlantic and Indian Oceans, like the Caspian Sea, were enclosed basins—also had its adherents, among them Herodotus, Hipparchus, and, most significant of all, Ptolemy. But Ptolemy’s _Geography_, though its content was reflected in Arabic notions of the earth’s surface, had almost no readers in the Christian West until the fifteenth century, and the works of Herodotus and Hipparchus were unknown. _PHYSICAL GEOGRAPHY_ ARISTOTLE, SENECA, AND PLINY