Parmenides may have been the first to conceive of zones upon the earth’s surface corresponding to the zones into which the astronomers had divided the heavens. Eratosthenes is said to have been the first to place the theory of terrestrial zones upon a firmly scientific footing, “by determining exactly upon the sphere the position of the fixed circles which mark the limits of each zone” (Thalamas).[61] Ancient geographers set the number of terrestrial zones at five, though they differed as to the character of the climates within them. The general opinion—one which was shared by Aristotle—was that the polar caps and the equatorial regions were incapable of sustaining life, the first on account of cold, the second on account of heat. Despite the fact that the notion of the existence of a fiery belt between the tropics was challenged by Polybius and Posidonius, who had heard reports from expeditions in these regions, this notion persisted in the writings of Martianus Capella, Macrobius, and many others and exerted an extremely restrictive effect on the subsequent development of geographical knowledge and enterprise. The majority of the ancient writers whose works were read in Christendom before 1300 also thought that the _oikoumene_, or portion of the earth inhabited by men of our kind, is completely surrounded by an ocean. This is a belief common to many early peoples.[62] In the Greek world we can trace it back to the Homeric and Hesiodic Ocean Stream and to the conceptions of early Ionian philosophers, who had gone so far as to maintain that the earth had been created out of water,[63] or at least that it was originally submerged beneath the ocean and had been brought forth through the evaporation of the water by sun and stars.[64] The theory of an encircling ocean was certainly held by Aristotle, Pliny,[65] Seneca, Macrobius, and Martianus Capella. CRATES’ THEORY OF FOUR LAND MASSES