After the Pliocene Epoch, when man first began to appear on the scene, there was another lowering of the level of the Nile valley in Egypt, and the Mediterranean extended its waters perhaps to the vicinity of Assiut. At this time the Mediterranean was almost certainly connected with the Red Sea across the Isthmus of Suez. The Nile stream was probably rapid in its descent towards the extended Mediterranean, and cut a deeper and deeper channel. Then another rise of the land took place, separating the Mediterranean from the Red Sea, and sending back the Mediterranean to something like its present limits; the upheaval indeed may have made what is now the Delta of the Nile higher than it is at the present day, while the river cut its way through a channel many feet deeper than the existing bed. Gradually, however, the Lower Nile became more sluggish as the land near the Mediterranean rose, and, losing its rapidity, it deposited more and more thickly the detritus brought down from Equatorial Africa, Abyssinia, and Nubia, and so raised its bed to a higher level. It has also enlarged its delta by the deposit of mud, though the fatuity of its work in this direction (in the presence of earth waves) is shown by the fluctuations which have occurred even within the last three thousand years. Not more than a thousand years ago Lake Menzaleh was a fertile and richly cultivated district. [Illustration: IN THE LIBYAN DESERT.] Some time after man penetrated into Egypt (probably from the east) the countries to the west of the Lower Nile began to rise above the sea, for much of the Libyan Desert was under the Mediterranean in the Tertiary Epoch. This retreat of the sea coupled with other conditions not clearly known to us brought about a marked change in the climate of Africa north of the fifteenth degree of north latitude. The aridity of the Sahara Desert and of Arabia began to exercise a potent influence over the fate of northern Africa. Many of the lands, which as late as the human period were still covered with plentiful vegetation and were traversable by the apes, the elephants, and the antelopes of to-day, began to dry up into their present condition, an aridity which, from all we know, is increasing and extending. Only Egypt was kept alive by the beneficent stream which, so abundantly nurtured by the snow-mountains and equatorial rain-belt of eastern Africa, survived even its passage of a thousand miles through the blazing desert, and covered the narrow ribbon of Upper Egypt and the tassel of the Delta with an ever fruitful soil of finely triturated mud.