This explains the circumstance that a number of monuments and objects of Chaldæan origin are found in the ruins of Susa among others indigenous to Elam. The capital of Elam appears to have been built once more after the departure of the Assyrians, for a cylinder of Nebuchadnezzar informs us that this prince built many temples there, as well as at Babylon. Susa never again really recovered her ancient splendour until the time of the second dynasty of the Achæmenid kings of Persia. Darius, son of Hystaspes, made it the capital of his realms, and until the rise of Alexandria, Susa remained the most important centre of art, and of Persian civilization (see Chapter V.). At this period, Susa was once more the theatre of events similar to those which so many centuries before had agitated her existence. When the entire East, Susa, as well as Babylon, and even Sardis, had fallen into the power of the Persian Achæmenids, and when Darius and, later, Xerxes, invaded Greece in B.C. 492 and 480, the Hellenic sanctuaries were pillaged in their turn. The Persians carried away their treasures, statues, and ex-votos across Asia as far as Susa, and there placed them in their own temples as trophies of their victories. When, in his turn, Alexander in B.C. 331 invaded the East, as avenger of the Hellenic race, he laid a heavy hand on the treasures of Susa; in that capital he discovered the great works of art of Greece, more especially the bronze statues of the Tyrannicides, Harmodius and Aristogiton, which Xerxes had carried away from Athens: these the conquering Macedonians were delighted to restore to the Athenians. The temple of Didyma near Miletus, like all others, had been pillaged by Darius, and its treasures carried off. What must have been the astonishment of M. de Morgan when he discovered in the course of his excavations a huge bronze knuckle-bone, weighing more than 152 lb. and bearing a Greek inscription of the seventh century B.C., recording that this singular object was dedicated by a dweller in Miletus to Apollo Didymæus. Thus we find an ex-voto from the Temple of Didyma carried to Susa by the Persians under Darius, and which has now found its way to the banks of the Seine, an object of astonishment to visitors to the Louvre. This enormous knuckle-bone is provided with two handles to facilitate transport. The upper one, worn through by long-continued friction, shows traces of the iron bar or hook passed through it for its long journey from Miletus to Susa. Thus history--Proteus with his thousand forms--repeats itself unceasingly under its many transformations; even modern times furnish us with numerous episodes similar to those just related. § III. THE PRINCIPLES OF BUILDING.