We can trace two schools of thought in respect to the configuration of this unknown region, both represented in the primitive conception of the earth, and both conditioned by a more fundamental postulate. It was a near thought, if the earth was a sphere, to transfer to it the systems of circles which had already been applied to the heavens. The suggestion is attributed to Thales, to Pythagoras, and to Parmenides; and it is certain that the earth was very early conceived as divided by the polar and solstitial circles into five zones, whereof two only, the temperate in either sphere, so the Greeks believed, were capable of supporting life; of the others, the polar were uninhabitable from intense cold, as was the torrid from its parching heat. This theory, which excluded from knowledge the whole southern hemisphere and a large portion of the northern, was approved by Aristotle and the Homeric school of geographers, and by the minor physicists. As knowledge grew, its truth was doubted. Polybius wrote a monograph, maintaining that the middle portion of the torrid zone had a temperate climate, and his view was adopted by Posidonius and Geminus, if not by Eratosthenes. Marinus and Ptolemy, who knew that commerce was carried on along the east coast of Africa far below the equator, cannot have fallen into the ancient error, but the error long persisted; it was always in favor with the compilers, and thus perhaps obtained that currency in Rome which enabled it to exert a restrictive and pernicious check upon maritime endeavor deep into the Middle Ages.[276]