The duration of the Parthian was about five centuries of the first Persian, less than two and a half; of the Median, at the utmost, one and a half; of the later Babylonian, less than one.

The only monarchy existing under conditions at all similar to Assyria, whereto an equally long--or rather a still longer--duration has been assigned with some show of reason, is Egypt.

But there it is admitted that the continuity was interrupted by the long foreign domination of the Hyksos, and by at least one other foreign conquest--that of the Ethiopian Sabacos or Shebeks.

According to Ctesias, one and the same dynasty occupied the Assyrian throne during the whole period, of thirteen hundred years.

Sardanapalus, the last king in his list, being the descendant and legitimate successor of Ninus.
There can be no doubt that a monarchy lasting about six centuries and a half, and ruled by at least two or three different dynasties, is per se a thing far more probable than one ruled by one and the same dynasty for more than thirteen centuries.