The _acies_ and _agmen_ are then, in their simplest form, the same thing looked at from two different points of view. The thin line drawn up to face the enemy may be imagined turning to the right or left and marching off. Of course it is not meant to be implied that such, and such only, were actually the primitive methods. Just as a mechanical problem is solved by assuming the absence of friction, a condition which in fact can never be realised, and correcting the result afterwards on account of friction, so one may for the moment leave out of sight all subsidiary things, in order to bring out in its simplicity the fundamental idea of an order of battle. Historically, no doubt, by the time men had advanced far enough to comprehend the value of combining to form a line, they had attained also to diversity of weapons, which would tend at once to interfere with this bare simplicity. Every fresh change, especially the introduction of war-chariots or of horse-soldiers, would further complicate the _acies_. So too, as soon as an army carries anything with it, the simple idea of the _agmen_ is encroached on. Nevertheless both _acies_ and _agmen_ are rooted, so to speak, in the nature of things: the former can be traced in every battle, the latter in every march. Some of the departures from the principle of the line are rather apparent than real. A reserve is no exception, even when it becomes a whole second, or even third line: for the reserve ex _hypothesi_ is not fighting: when it is wanted to fight it is brought up to the front, and ceases to be in reserve. Foot-soldiers standing on the defensive, especially as against horsemen, present the largest amount of front in the safest way by forming a closed figure, the ring of the Northmen and of the Scots, or the familiar square of modern infantry in the days before the rifle. Nevertheless modifications are liable to be introduced, so to speak, from both sides. The order of battle is deepened, with the idea of giving greater impetus to a charge, from the weight of men behind backing up the front ranks. Epaminondas, using this device unexpectedly at Leuctra, defeated the Spartans, whose superior discipline and physique made them invincible so long as both sides used the same formation. His success led to the adoption of the Macedonian phalanx, and the abandonment of the line for the time being, until the Romans reverted successfully to the natural order. The order of march, for a real journey, cannot well be modified, because roads do not allow it. But for short marches, over open ground, there was much to be gained by massing men more closely together. They could hear orders better, and could be moved in any direction with more ease and precision. Hence arose the column, which is strictly speaking a series of short lines ranged one behind the other, and which, as military evolutions were developed, became the natural formation for manœuvring, as distinguished from fighting.