But when the names of continents emerge in Greek usage they afford the necessary distinction between the lands on either side of the Ægean-Mediterranean. They so emerge in the 6th–5th centuries B.C., and the distinction appears by that time to have been perfectly familiar, though the precise application, as will be seen, was a matter of controversy. The poet Æschylus (525–456 B.C.), who, by the way, was also a traveller, possibly to Thrace, certainly to Sicily, was acquainted with the distinction, as appears, for example, from passages in the historical drama of the _Persæ_, which deals with the failure of Xerxes’s invasion of Europe from Asia, and his retreat across the Hellespont. The distinction would hardly have been introduced into a stage-play if it had not been commonly recognized. In _Prometheus Unbound_, again, Æschylus refers to the river Phasis (Aras) as the boundary between Europe and Asia. Finally, Herodotus states in an early chapter of his work that the Persians appropriate to themselves Asia and the barbarian races inhabiting it, while they consider as separate Europe and the Greek race, and he does not find it necessary to offer any explanation of the names here. At a later stage the continental distinction appears to have been based on or associated with a distinction between temperate and hot lands. [Illustration: Fig. 2.--The World as supposed to have been conceived by Hecatæus.] Hecatæus of Miletus (_c._ 500 B.C.) has been hailed as the father of geography on the ground of his authorship of a Periodos, or circuit of the earth, the first attempt at a systematic description of the known world and its inhabitants. But even if he wrote such a work, evidence has been adduced that the extant fragments of it belong to a later forgery. However, he was a Miletan and a traveller, besides a statesman. The map which is supposed to have accompanied his work maintained the old popular idea of the earth as a circular disc, encircled by the ocean. Greece was the centre of the world, and the great sanctuary of Delphi was the centre of Greece. If this Periodos is taken as a forgery, there is a parallel case in the Periplus of the Mediterranean attributed to Scylax of Caryanda, a contemporary of Hecatæus. If Scylax wrote any such work, in its extant form it is a century and a half later than his time. He is said to have explored the Indus at the command of Darius Hystaspis, and to have returned by the Indian Ocean to the Red Sea.