On the particulars of this account, which come from the Chinese historians, we cannot perhaps altogether depend; but there is no reason to doubt the main fact, attested by a writer who visited the Yue-chi in B.C.139, that they had migrated about the period mentioned from the interior of Asia, and had established themselves sixty years later in the Caspian region.

Such a movement would necessarily have thrown the entire previous population of those parts into commotion, and would probably have precipitated them upon their neighbors.

It accounts satisfactorily for the pressure of the northern hordes at this period on the Parthians, Bactrians, and even the Indians; and it completely explains the crisis in Parthian history, which we have now reached, and the necessity which lay upon the nation of meeting and, if possible, overcoming, an entirely new danger.
In fact, one of those occasions of peril had arisen, to which in ancient times the civilized world was always liable from an outburst of northern barbarism.

Whether the peril has altogether passed away or not we need not here inquire; but certainly in the old world there was always a chance that civilization, art, refinement, luxury, might suddenly and almost without warning be swept away by an overwhelming influx of savage hordes from the unpolished North.

From the reign of Oyaxares, when the evil first showed itself, the danger was patent to all wise and far-seeing governors both in Europe and Asia, and was from time to time guarded against.