Roman Battle Formations Mid Republic to Late Rep.

[Crispvs]

I think that the flexibility of the Polybian model lies in the ability to move different troop types into the front line of battle, replacing the troops who were previously fighting at the front.
How they did this though is surely the thing which has vexed us. Polybius gives us the checkerboard model but exactly how this worked in relation to the front line of battle is still open interpretation.
Personally I doubt that there would ever be any gaps along the front line, as this would instantly mean the edges of the gaps would be outflanked. If you had a four legion consular army, consisting of forty cohorts of infantry, arrayed for battle with a gap between each cohort, you would end up with as many as forty one cases of being outflanked, which is surely a good way to lose a battle very quickly and not a mistake you would wish to repeat.
The question therefore remains of exactly what method was used to withdraw units and replace them with new units without the front line being compromised in the process. Obviously the answer to this lies both in what the men were trained to do and in how their units were arranged prior to battle being met. We can never do any more than guess at the former, but by analysing what sources we have for the latter, as Macedon is trying to do can only lead us closer to reconstructing how it might have been done. What exactly was the utility of the checkerboard formation in facilitating changes of lines without compromising the fighting line? Let that be the question we are trying to answer rather than being distracted by other things.

One caveat which occurs to me though in trying to extend any possible solutions past the so-called Marian reforms is the disappearance of velites. Surly that would suggest a general change in tactics in the late second century BC. Caesar's mention of his three lines being traditional need not be a long tradition. Sixty years is ample time for a novel concept to become a traditional certainty.

Macedon, thanks for your detailed analysis of my proposal.

Crispvs