In the case of Egypt, the center of civilization moved northward and southward, along the banks of the Nile. First of all, as I told you, people from all over Africa and western Asia moved into the valley and settled down. Thereupon they formed small villages and townships and accepted the rule of a Commander-in-Chief, who was called Pharaoh, and who had his capital in Memphis, in the lower part of Egypt. After a couple of thousand years, the rulers of this ancient house became too weak to maintain themselves. A new family from the town of Thebes, 350 miles towards the south in Upper Egypt, tried to make itself master of the entire valley. In the year 2400 B.C. they succeeded. As rulers of both Upper and Lower Egypt, they set forth to conquer the rest of the world. They marched towards the sources of the Nile (which they never reached) and conquered black Ethiopia. Next they crossed the desert of Sinai and invaded Syria where they made their name feared by the Babylonians and Assyrians. The possession of these outlying districts assured the safety of Egypt and they could set to work to turn the valley into a happy home, for as many of the people as could find room there. They built many new dikes and dams and a vast reservoir in the desert which they filled with water from the Nile to be kept and used in case of a prolonged drought. They encouraged people to devote themselves to the study of mathematics and astronomy so that they might determine the time when the floods of the Nile were to be expected. Since for this purpose it was necessary to have a handy method by which time could be measured, they established the year of 365 days, which they divided into twelve months. Contrary to the old tradition which made the Egyptians keep away from all things foreign, they allowed the exchange of Egyptian merchandise for goods which had been carried to their harbors from elsewhere. They traded with the Greeks of Crete and with the Arabs of western Asia and they got spices from the Indies and they imported gold and silk from China. But all human institutions are subject to certain definite laws of progress and decline and a State or a dynasty is no exception. After four hundred years of prosperity, these mighty kings showed signs of growing tired. Rather than ride a camel at the head of their army, the rulers of the great Egyptian Empire stayed within the gates of their palace and listened to the music of the harp or the flute. One day there came rumors to the town of Thebes that wild tribes of horsemen had been pillaging along the frontiers. An army was sent to drive them away. This army moved into the desert. To the last man it was killed by the fierce Arabs, who now marched towards the Nile, bringing their flocks of sheep and their household goods. Another army was told to stop their progress. The battle was disastrous for the Egyptians and the valley of the Nile was open to the invaders.