While the earlier ages of Babylonian history afford a striking picture of gradual growth and development, the periods succeeding the Dynasty of Akkad are marked by a certain retrograde movement, or reversion to earlier ideals. The stimulus, which produced the empire and the art of Akkad, may be traced to the influx of fresh racial elements into Northern Babylonia and their fusion with the older and more highly cultured elements in the south. When the impulse was exhausted and the dynasties to which it had given rise had run their course, little further development along these lines took place. Both in art and politics a Sumerian reaction followed the period of Semitic power, and the establishment of the Dynasty of Ur was significant of more than a shifting of political influence southwards. It would appear that a systematic attempt was made to return to the earlier standards. But the influence of Akkad and her monarchs, though deliberately ignored and combated, was far from ineffective. As the sculptures of Gudea owe much to the period of Narâm-Sin, so the empire of Dungi was inevitably influenced by Shar-Gani-sharri's conquests. There was no sudden arrest either of the political or of the cultural development of the country. A recovery of power by the Sumerians merely changed the direction in which further development was to take place. Although, when viewed from a general standpoint, there is no break of continuity between the epoch of Akkad and that of Ur, there is some lack of information with regard to events in the intervening period. There is every indication that between the reign of Narâm-Sin and that of Ur-Engur, the founder of the Dynasty of Ur, we have to count in generations rather than in centuries, but the total length of the period is still unknown. The close of the Dynasty of Akkad, as we have already seen, is wrapped in mystery, but the gap in our knowledge may fortunately to some extent be bridged. At this point the city of Lagash once more comes to our assistance, and, by supplying the names of a number of her patesis, enables us to arrange a sequence of rulers, and thereby to form some estimate of the length of the period involved.