I n d i a was at that time almost a land of mystery. The great Darius had caused the valley of the Indus to be explored by Scylax of Caryanda, and was able to annex part of the country; but for a long time the frontier of the Persian Empire was at the Paropamisos, and the valley of the Ganges, separated from that of the Indus by a vast desert, had always been practically unknown. There were Indian elephants and troops in the Persian army ; but the soldiers probably came from the mountains bordering on the Empire.