At some such period as ten thousand years ago northeast Africa was repeatedly invaded from Arabia by a branch of the Caucasian race--the Hamites--which in Arabia had absorbed a certain proportion of early Negro and Dravidian[13] blood. About the same time in Egypt itself there were invasions of other Caucasian immigrants; some perhaps of the Dravidian stock still met with in Baluchistan and India, and others of Libyan (Iberian, Algerian) race. There had also been early minglings between the big black Negroes on the Central Nile and Hamite invaders which had resulted in further hybrids such as the Nubians or “Ethiopians.” These Ethiopians constantly invaded and raided Egypt, thus mingling with the Caucasian Egyptians, but also at other times acted as middlemen between civilised Egypt and the utterly barbarous countries of the Bahr-al-Ghazal and the Sudan; they brought to Egypt knowledge of the Pygmies and such of the bigger beasts of Africa as had become extinct in Egypt before the arrival of intelligent man. Through these Nubians the Egyptians occasionally had glimmering ideas as to the sources of the White Nile. 2. The Egyptians kept up a fairly constant communication with Abyssinia and Somaliland by sea and overland. They had a fair knowledge of the geographical features of Abyssinia and of the origin and source of the Blue Nile. Moreover, through the ancestors of the Galas and Somalis they came slightly into contact with the peoples of Lake Rudolf and the Victoria Nyanza. 3. The Greeks, who began to travel in Egypt five hundred years before Christ, expressed some curiosity about the origin of the Nile, and communicated this inquiring spirit to the Romans. This resulted for a time in the knowledge of the White Nile as far south as Fashoda. 4. The Arabs of western and southern Arabia very early in the history of civilisation developed a culture scarcely inferior to that of the Egyptians, and entered into trading and colonising relations with Abyssinia and Somaliland, and with the East African coastlands as far south as the modern Rhodesia. From their settlements on the Zanzibar coast (such as Mombasa) they probably journeyed inland on trading expeditions, or else the natives, who came to trade with them at the coast, gave them geographical information. In one or other way they learnt the existence of great lakes and snow-mountains. These stories the Arabs passed on to inquiring Greeks as far back as two thousand years ago; and an account which was an uncommonly near guess at the truth was given to the reading world during the first two centuries after the birth of Christ by writers on geography like Marinus of Tyre and Claudius Ptolemæus of Alexandria.