One striking result of this broadening of regional knowledge was the lesson it taught in regard to the countries south of the Tropic of Cancer. The progress of exploration in Upper Egypt and in India showed that these countries were not only habitable but thickly settled. Adherents of what we have called the Cratesian theory were obliged to acknowledge that the tropic could not be taken as the beginning of the burning zone. Eratosthenes pushed the limit of the _oikoumene_ as far south as latitude 11½° N.[170] HELLENISTIC REGIONAL KNOWLEDGE While Greek military enterprise had been opening up the Orient and exploratory enterprise penetrating the tropics, an important advance was made in the direction of the northwestern seas and the British Isles. The voyage of Pytheas of Marseilles, about 330 B. C., had brought within the scope of ancient knowledge Britain, Scandinavia, Thule, and the frozen ocean beyond. Thus, in the Hellenistic period the frontiers of knowledge included the Orkneys, Shetlands, Faroes—or whatever of the northern isles was meant by Thule—the Canaries, tropical Africa, and Ceylon. No further notable extension of these borders seems to have been made until the first century after Christ, except that vague rumors of a people called “Seres” and of the use of silk had crept into the Roman world in Virgil’s time. This may have indicated acquaintance with China, although Horace took the Seres to be a tribe of Central Asia.[171] The Scythian invasions which overwhelmed the Greek kingdom of Bactria and the conquest by the newly risen power of Parthia of the provinces of the Seleucids east of the Euphrates tended to cut all communication with the interior and farther parts of the Asiatic continent; but the Mithridatic wars, as described by Theophanes, familiarized the public with the local geography of Armenia, Pontus, and the Caucasus. Similarly Caesar’s campaigns in Gaul, Germany, and Britain opened Western Europe to the Roman world. REGIONAL KNOWLEDGE OF MELA AND PLINY