It was after Xerxes had reached Therma that the real difficulties of his land march began. Hitherto the route had traversed that north shore of the Ægean along which, between the mountains and the sea, nature has provided a strip of lowland which, though narrow, does not present great physical obstacles to the march of an army. But now all was different. It was as though, after traversing an unimpeded high road, he had to enter upon a region of enclosures, whose mountain walls, stupendous in difficulty, if not in height, forced him to take a winding pathway leading from one to the other of the narrow passages by which these walls were pierced. But this was not the only serious consideration which those in charge of the expedition had to face. Hitherto the line of march, which had lain along the immediate sea coast, had been peculiarly favourable to the plan of advance adopted,—a plan founded essentially on the co-operation of the fleet with the land forces. It may well be supposed that the dual nature of this advance was largely designed with intent to supply the commissariat of the army, as well as with the idea of military co-operation between it and the fleet. The huge numbers which Herodotus gives of the Persian army are not merely open to doubt; they are practically impossible; but there exists no evidence whatever which would justify the assumption that the numbers were aught but very large indeed. They must have amounted at the lowest estimate to several hundred thousand men. The expedition was a great effort by a great empire, by far the greatest empire of its time, and, during the short period of its vigorous life, one of the greatest that the world has ever seen. The difficulty of transporting such a force round the long circuit of the Ægean must have been enormous; and the bare fact that it was transported successfully over such a distance bears eloquent testimony to the military organization of the empire which despatched it. The tendency, which is sometimes displayed, to belittle the Persia of this time is in violent disagreement with such evidence as is extant, and is due to an attempt to read her history in the fourth century into her history of one century earlier. If human ability is to be judged by what human power can do, the mere fact that the organizers of this expedition were able to bring a force of this magnitude over hundreds of miles of difficult country, to keep it adequately supplied with provisions, and to place it upon the field of operations in an efficient state, must be taken as convincing proof of ability of no ordinary kind. #HERODOTUS AS A TOPOGRAPHER.#