In the wide plains of Northern Asia, extending from the Arctic Ocean to the Thian Chan mountains and the Jaxartes, there had been nurtured from a remote antiquity a nomadic population, at no time very numerous in proportion to the area over which it was spread, but liable on occasions to accumulate, owing to a combination of circumstances, in this or that portion of the region occupied, and at such times causing trouble to its neighbors. From about the close of the third century B.C. symptoms of such an accumulation had begun to display themselves in the tract immediately north of the Jaxartes, and the inhabitants of the countries south of that river had suffered from a succession of raids and inroads, which were not regarded as dangerous, but which gave constant annoyance. Crossing the great desert of Kharesm by forced marches, some of the hordes invaded the green valleys of Hyrcania and Parthia, and carried desolation over those fair and flourishing districts. About the same time other tribes entered the Bactrian territory and caused alarm to the Greek kingdom recently established in that province. It appears that the Parthian monarchs, unable to save their country from incursions, consented to pay a sort of black-mail to their invaders, by allowing them the use of their pasture grounds at certain fixed times--probably during some months of each year. The Bactrian princes had to pay a heavier penalty. Province after province of their kingdom was swallowed up by the northern hordes, who gradually occupied Sogdiana, or the tract between the lower Jaxartes and the lower Oxus, whence they proceeded to make inroads into Bactria itself. The rich land on the Polytimetus, or Ak Su, the river of Samarkand, and even the highlands between the upper Jaxartes and upper Oxus, were permanently occupied by the invaders; and if the Bactrians had not compensated themselves for their losses by acquisitions of territory in Afghanistan and India, they would soon have had no kingdom left. The hordes were always increasing in strength through the influx of fresh immigrants, and in lieu of Bactria a power now stood arrayed on the north-eastern frontier of the Parthians, which was reasonably regarded with the most serious alarm and suspicion.