To explain how it happened that this one satrapy was charged with a sum equal to two-fifths of the aggregate charge on the other nineteen, Herodotus dwells upon the vast population, the extensive territory, and the abundant produce in gold, among those whom he calls Indians,—the easternmost inhabitants of the earth, since beyond them there was nothing but uninhabitable sand,—reaching, as far as we can make it out, from Baktria southward along the Indus to its mouth, but how far eastward we cannot determine. Darius is said to have undertaken an expedition against them and subdued them: moreover, he is affirmed to have constructed and despatched vessels down the Indus, from the city of Kaspatyri and the territory of the Paktyes, in its upper regions, all the way down to its mouth: then into the Indian ocean, round the peninsula of Arabia, and up the Red Sea to Egypt. The ships were commanded by Skylax,—a Greek of Karyanda on the south-western coast of Asia Minor;[425] who, if this statement be correct, executed a scheme of nautical enterprise not only one hundred and seventy years earlier, but also far more extensive, than the famous voyage of Nearchus, admiral of Alexander the Great,—since the latter only went from the Indus to the Persian gulf. The eastern portions of the Persian empire remained so unknown and unvisited until the Macedonian invasion, that we are unable to criticize these isolated statements of Herodotus. None of the Persian kings subsequent to Darius appear to have visited them, and whether the prodigious sum demandable from them according to the Persian rent-roll was ever regularly levied, may reasonably be doubted. At the same time, we may reasonably believe that the mountains in the northern parts of Persian India—Cabul and Little Thibet—were at that time extremely productive in gold, and that quantities of that metal, such as now appear almost fabulous, may have been often obtained. It appears that the produce of gold in all parts of the earth, as far as hitherto known, is obtained exclusively near the surface; so that a country once rich in that metal may well have been exhausted of its whole supply, and left at a later period without any gold at all. [425] Herodot. iii, 102; iv, 44. See the two Excursus of Bähr on these two chapters, vol. ii, pp. 648-671 of his edit. of Herodotus. It certainly is singular that neither Nearchus, nor Ptolemy, nor Aristobulus, nor Arrian, take any notice of this remarkable voyage distinctly asserted by Herodotus to have been accomplished. Such silence, however, affords no sufficient reason for calling the narrative in question. The attention of the Persian kings, successors to Darius, came to be far more occupied with the western than with the eastern portions of their empire.