The extent of the knowledge which the ancients thus had of western Africa is well illustrated in the map representing the geographical theories of Pomponius Mela, whose book was written about A. D. 50. Of the eastern coast and the interior Mela knew less than Ptolemy a century later, but of the Atlantic coast he knew more than Ptolemy. The fact that the former geographer was a native of Spain and the latter a native of Egypt no doubt had something to do with this. Mela had profited by the Carthaginian discoveries. His general conception of the earth was substantially that of Eratosthenes. It was what has been styled the "oceanic" theory, in contrast with the "continental" theory of Ptolemy. In the unvisited regions on all sides of the known world Eratosthenes imagined vast oceans, Ptolemy imagined vast deserts or impenetrable swamps. The former doctrine was of course much more favourable to maritime enterprise than the latter. The works of Ptolemy exercised over the mediaeval mind an almost despotic sway, which, in spite of their many merits, was in some respects a hindrance to progress; so that, inasmuch as the splendid work of Strabo, the most eminent follower of Eratosthenes, was unknown to mediaeval Europe until about 1450, it was fortunate that the Latin treatise of Mela was generally read and highly esteemed. People in those days were such uncritical readers that very likely the antagonism between Ptolemy and Mela may have failed to excite comment,[359] especially in view of the lack of suitable maps such as emphasize that antagonism to our modern minds. But in the fifteenth century, when men were getting their first inklings of critical scholarship, and when the practical question of an ocean voyage to Asia was pressing for solution, such a point could no longer fail to attract attention; and it happened fortunately that the wet theory, no less than the dry theory, had a popular advocate among those classical authors to whose authority so much deference was paid. [Footnote 359: Just as our grandfathers used to read the Bible without noticing such points as the divergences between the books of Kings and Chronicles, the contradictions between the genealogies of Jesus in Matthew and Luke, the radically different theories of Christ's personality and career in the Fourth Gospel as compared with the three Synoptics, etc.] [Sidenote: Ancient theory of the five zones.] [Sidenote: The Inhabited World and the Antipodes.]