If an immigration and an amalgamation of peoples took place, at the time of the Pyramids it had already long been buried in oblivion; the Egyptians regard themselves as autocthonous, and--with the exception of a part of the population in the lower lands of Nubia, Libya, and Asia--as a single nation, within which there can be no question of a clash of mental conceptions, and within which the proud and the humble, the lord and the bondman, have nothing to distinguish them externally. Historical presentation demands that we should treat the Egyptians throughout as one people, whatever may be the number of different tribes that settled in the Nile Valley in prehistoric time.[b] The earliest stage of man that is known in Egypt is the Palæolithic; this was contemporary with a rainy climate, which enabled at least some vegetation to grow on the high desert, for the great bulk of the worked flints are found five to fifteen hundred feet above the Nile, on a tableland which is now entirely barren desert. Water-worn palæoliths are found in the beds of the stream courses, now entirely dried up, and flaked flints of a rather later style occur in the deep beds of Nile gravels, which are twenty or thirty feet above the highest level of the present river. This type of work, however, lasted on to the age of the existing conditions, for perfectly sharp and fresh palæoliths are found on the desert as low down as the present high Nile. PREHISTORIC EGYPT The date of the change of climate is roughly shown by the depth of the Nile deposits. It is well known by a scale extending over about three thousand years, that in different parts of Egypt the rise of the Nile bed has been on an average about four inches per century, owing to the annual deposits of mud during the inundation. And in various borings that have been made, the depth of the Nile mud is only about twenty-five or thirty feet. Hence an age of about eight or nine thousand years for the cultivable land may be taken as a minimum, probably to be somewhat extended by slighter deposit in the earlier time.