This movement of D'ERLON's Corps admits of being satisfactorily explained. NAPOLEON, having received information that D'ERLON had been left in reserve in front of Gosselies, and inferring, perhaps, from this circumstance that NEY was sufficiently strong to be able to hold his ground at Quatre Bras, without further aid than what he had at hand, resolved upon employing this Corps upon the Prussian Right Flank; but in the meantime, D'ERLON had, in pursuance of instructions from NEY, continued his march towards Quatre Bras; and having himself proceeded in advance, had reached Frasne, at which place Colonel LAURENT found him, and communicated to him the Emperor's Order for the march of his Corps upon St Amand; adding that on coming up with the head of his Column, he had taken upon himself to change its direction of march into that of St Amand. D'ERLON hastened to comply with NAPOLEON's wishes, and despatched General D'ELCAMBRE, his Chief of the Staff, to make known the movement to Marshal NEY. His route from Frasne towards St Amand, the point prescribed by the Order, lay through Villers Perruin, and the movement was altogether one of a retrograde nature. Hence the direction of the Column, as seen in the distance, was well calculated to alarm the troops of the French extreme Left; as also to excite surprise in the mind of NAPOLEON, who having formed no expectation of the arrival of any French troops in the Field by any other direction than that from Gosselies upon St Amand, or perhaps from Quatre Bras upon Bry, also participated in the opinion that the Column in question, under its attendant circumstances and general disposition, could be no other than that of an Enemy. As D'ERLON debouched from Villers Perruin, and advanced upon the prescribed point, St Amand, he threw out his Cavalry (JAQUINOT's) to his Left, for the protection of this Flank; and it was before this Cavalry that the Prussian Brigade, under Colonel MARWITZ, retired in the manner already explained, a movement which fully restored confidence to GIRARD's Division. All at once this Column was observed to halt, to indicate an indecision in its intentions, and finally to withdraw from the Field. D'ERLON had in fact just received from NEY a peremptory Order to join him without delay, with which he resolved to comply, probably concluding that he was bound to do so from the circumstance of his having been in the first instance placed under the Marshal's immediate command; having ascertained also from the Emperor's Aide de Camp that he was not the bearer of any instructions whatever from NAPOLEON as to his future movements, and that the appearance of his Corps upon that part of the Field of Battle had been quite unexpected. This pressing Order had been despatched by NEY immediately previous to the arrival of Colonel LAURENT on the Heights of Gemioncourt.