The origin of the state of things here described is to be sought, according to the best authorities, in certain movements which took place about B.C. 200, in a remote region of inner Asia. At that time a Turanian people called the Yue-chi were expelled from their territory on the west of Chen-si by the Hiong-nu, whom some identified with the Huns. The Yue-chi separated into 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.