That portion of Europe, Asia, and Africa known to the ancients, whether regarded as an island, or as separated from the rest of the world by climatic conditions merely, or by ignorance, formed a distinct concept and was known by a particular name, _ἡ οἰκουμένη_. Originally supposed to be circular, it was later thought to be oblong and as having a length more than double its width. Those who believed in its insularity likened its shape to a sling, or to an outspread chlamys or military cloak, and assumed that it lay wholly within the northern hemisphere. In absolute figures, the length of the known world was placed by Eratosthenes at 77,800 stadia, and by Strabo at 70,000. The latter figure remained the common estimate until Marinus of Tyre, in the second century a.d., receiving direct information from the silk-traders of a caravan route to China, substituted the portentous exaggeration of 90,000 stadia on the parallel of Rhodes, or 225°. Ptolemy, who followed Marinus in many things, shrank from the naïveté whereby the Tyrian had interpreted a seven months’ caravan journey to represent seven months’ travelling in a direct line at the rate of twenty miles a day, and cut down his figures to 180°, or 72,000 stadia.[282] It appears, therefore, that Strabo considered the known world as occupying not much over one third of the circuit of the temperate zone, while Marinus, who adopted 180,000 stadia as the measure of the earth, claimed a knowledge of two thirds of that zone, and supposed that land extended indefinitely eastward beyond the limit of knowledge. What did the ancients picture to themselves of this unknown portion of the globe? The more imaginative found there a home for ancient myth and modern fable; the geographers, severely practical, excluded it from the scope of their survey; philosophers and physicists could easily supply from theory what they did not know as fact. Pythagoras, it is said, had taught that the whole surface of the earth was inhabited. Aristotle demonstrated that the southern hemisphere must have its temperate zone, where winds similar to our own prevailed; his successors elaborated the hint into a systematized nomenclature, whereby the inhabitants of the earth were divided into four classes, according to their location upon the surface of the earth with relation to one another.[283]