884 THE PULSE OF ASIA
supposed outlet was from the Aral gulf of the enlarged sea along what has now become a line of lakes leading toward the Irtysh River. The hypothesis lacks confirmation, but the marked physical features upon which it is based may have given rise to the Alexandrine idea of a northern passage leading to the Arctic Ocean. It is not likely, however, that such an idea would prevail unless the Caspian had stood at such a level that it almost or quite coalesced with the Sea of Aral.
Another explanation of the Alexandrine error is possible. When the Greeks inquired as to the northern shores of the Caspian Sea, they would hear that the water stretched away indefinitely to the north, where a narrow channel, the almost motionless stream of the mighty Volga, up which ships can sail for nearly two thousand miles, led far toward the Arctic regions. Under the prevailing ideas as to the symmetrical distribution of sea and land, the geographers would naturally jump to the conclusion that the Caspian Sea was a gulf of the ocean corresponding to the Persian gulf on the opposite side. Such an error would be even more likely to spring up, if the Caspian extended far to the north over the plains of Russia, as the account of Herodotus suggests, discharging, perhaps, through the " Manych " to the Black Sea, as it must have done if it were so expanded eastward that the Oxus and Jaxartes rivers both entered it.
The Alexandrine idea of a northern outlet of the Caspian Sea remained almost unchallenged for nearly five centuries, until the days of Ptolemy. Diodorus of Sicily, B. c. 60, is the only known author who, during this period, speaks of the complete isolation of the sea, and he does so only inci-
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