nto two bands; the smaller descended southwards
into Thibet; the larger passed westwards, and after a hard struggle
dispossessed a people called 'Su' of the plains west of the river of Hi.
These latter advanced to Ferghana and the Jaxartes; and the Yue-chi not
long afterwards retreating from the Usiun, another nomadic race, passed
the 'Su' on the north and occupied the tracts between the Oxus and the
Caspian. The Su were thus in the vicinity of the Bactrian Greeks; the
Yue-chi in the neighborhood of the Parthians. 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. The expeditions of Cyrus against the Massagetse, of
Darius Hystaspis against the European Scyths, of Alexander against the
Getee, of Trajan and Probus across the Danube, were designed to check
and intimidate the northern nations, to break their power, and diminish
the likelihood of their taking the offensive. It was now more than four
centuries since in this part of Asia any such effort had been made; and
the northern ba