Among the Greek merchants of the first century trading with India was a certain Diogenes, who may have supplied the unknown Alexandrian author of the Periplus with some of his information. Diogenes, returning from a voyage to India in about 50 A.C., landed at Rhapta or Rhaptum. From some such point—Rhaptum in this instance may be distinct from Rhapta and equivalent to Pangani, a trading-post at the mouth of the Rufu River, opposite Pemba Island—Diogenes travelled inland for twenty-five days—so, at least, he stated—and arrived in the vicinity of two great lakes and a snowy range of mountains whence the Nile drew its twin sources. Twenty-five days’ journey might have brought a Greek traveller easily within sight of Kilimanjaro, but certainly not of the Victoria Nyanza. It is more likely that Diogenes saw Kilimanjaro and added to his impressions, of that mighty dome of snow and ice the statements of the Arab traders who may at that period have penetrated inland as far as the Victoria Nyanza and even ascertained the existence of Ruwenzori and the Albert Nyanza. If pre-historic Arab trade permeated these countries at that time, it was no doubt afterwards driven back to the coast by the tumultuous movements of the Bantu and Nilotic negroes. Though the information of Diogenes may have reached the author of the Periplus, it was, so far as the semi-legendary history goes, told to a Syrian geographer, Marinus of Tyre, who published it at Alexandria about the same time that the Periplus was being written. The writings of Marinus disappeared with the dispersal of the Alexandrian Library. But that portion dealing with the sources of the Nile was quoted almost word for word by a later writer, Claudius Ptolemaeus, a latinized Egyptian-Greek who resided in Alexandria. Ptolemy (as he is commonly called in English) wrote his works about the year 150 A.C.; and to him is commonly attributed the first clearly expressed theory as to the main origin of the White Nile. He believed that this mysterious river found its ultimate source in two great lakes, the waters of which were derived from a great snowy range called the Mountains of the Moon. It is, however, clear from the writings of Eratosthenes (an African Greek who published his geographical works about 200 B.C.) and Pliny the Elder (Caius Plinius Secundus, whose principal book was published in 77 A.C.) that before the Christian era a glimmering of the geography of the Upper Nile basin had already reached Greek Egypt. Perhaps earlier still it had come to the knowledge of the Persian rulers of Egypt, and may have been brought to them by Ethiopian slave and ivory traders, akin to the modern Abyssinians and Galas, who at that period seem to have freely penetrated through the lands of the negro savages.