The earliest achievement of this (of equality and the restriction on the powers of the constitutionally mandated magistrates), the most ancient opposition in Rome, consisted in the abolition of the life-tenure of the presidency of the community; in other words, in the abolition of the monarchy... Not only in Rome (but all over the Italian peninsula) ... we find the rulers for life of an earlier epoch superseded in after times by annual magistrates. In this light the reasons which led to the substitution of the consuls for kings in Rome need no explanation. The organism of the ancient Greek and Italian polity through its own action and by a sort of natural necessity produced the limitation of the life-presidency to a shortened, and for the most part an annual, term... Simple, however, as was the cause of the change, it might be brought about in various ways, resolution (of the community),.. or the rule might voluntarily abdicate; or the people might rise in rebellion against a tyrannical ruler, and expel him. It was in this latter way that the monarchy was terminated in Rome. For however much the history of the expulsion of the last Tarquinius, "the proud", may have been interwoven with anecdotes and spun out into a romance, it is not in its leading outlines to be called in question. Tradition credibly enough indicates as the causes of the revolt, that the king neglected to consult the senate and to complete its numbers; that he pronounced sentences of capital punishment and confiscation without advising with his counsellors(sic); that he accumulated immense stores of grain in his granaries, and exacted from the burgesses military labours and task-work beyond what was due... we are (in light of the ignorance of historical facts around the abolition of the monarchy) fortunately in possession of a clearer light as to the nature of the change which was made in the constitution (after the expulsion of the monarchy). The royal power was by no means abolished, as is shown by the fact that, when a vacancy occurred, a "temporary king" (Interrex) was nominated as before. The one life-king was simply replaced by two [one year] kings, who called themselves generals (praetores), or judges..., or merely colleagues (Consuls) [literally, "Those who leap or dance together"]. The collegiate principle, from which this last - and subsequently most current - name of the annual kings was derived, assumed in their case an altogether peculiar form. The supreme power was not entrusted to the two magistrates conjointly, but each consul possessed and exercised it for himself as fully and wholly as it had been possessed and exercised by the king; and, although a partition of functions doubtless took place from the first - the one consul for instance undertaking the command of the army, and the other the administration of justice - that partition was by no means binding, and each of the colleagues was legally at liberty to interfere at any time in the province of the other.
Theodor Mommsen
