The intention of Darius to cross the Ister existed apparently from the beginning of the campaign. He sent the fleet thither with orders to prepare a bridge; but the intention not to proceed far beyond that river is almost equally clearly shown by his leaving his fleet at the point of crossing. Whether this second intention was part of the original design is another question. The tale of the countermanding of the orders given to the Greeks at the river, absurd though it is, may rest on a remote foundation of fact; and the fact may have been that Darius did actually modify his original plans on his arrival at the Ister. The expedition north of that point seems to have been either of the nature of a reconnaissance in force, on the results of which the king decided against further advance and attempt at conquest, or a display intended to strike awe into the tribes beyond the newly won territory. Whether any attempt at conquest in these northern regions formed part of the original programme may be doubted: one thing is certain, that the expedition in the form it was made was not anything of the kind. It is noticeable that at this period the great river formed a very marked and striking ethnographical boundary between the tribes of Thracian stock and the Scyths of Europe. It has already been pointed out that Darius had within his dominions in Asia Minor—mainly in the region of Bithynia—people of Thracian origin. May he not have undertaken the expedition with intent to minimize the chances of disturbance on the weak imperial frontier formed by the Propontis and its two straits by subduing the free kinsmen of his Thracian subjects? It is a precaution such as the rulers of empires at every time in history and in every part of the world have frequently had to take. It may be, then, that Darius was in search of an ethnic frontier, and when he had found it on the Ister he sought not for further conquest, but confined himself to measures intended to secure what he had won. _The Ethnic Frontier._ It is perhaps unnecessary to insist on the importance of an ethnic frontier. It is, of course, frequently coincident with a physical frontier. Where such a coincidence exists, a “scientific” frontier is attained. But in cases where the two are not coincident, history has shown again and again that the ethnic is the more “scientific” of the two. Regarding the question from another point of view,—it is infinitely more dangerous for a state to have on its borders a race who have kin immediately beyond the border in the enjoyment of a form of government which assures them their independence, or in which they are paramount, than to have on its frontier a race alien to their neighbours beyond the border.