If, therefore, we are led to consider the monarchy as not purely an elective office, and substitute for election the principle of nomination, we must consider that it was the right, and probably the duty, of the king of Rome to nominate his successor. If there had been no due nomination during his lifetime, and consequently no distinctly marked out successor to the monarchy, the duty of providing such a successor lapsed to the Senate, from which body the _interrex_ was appointed. The _interregnum_ is said by tradition to have dated from the first vacancy in the regal office, after the death of Romulus.[178] When such a vacancy had occurred, the auspices, under which the state had been founded, and which were the mark of divine acceptance of the kingly rule, “returned to the _patres_,”[179] and we are told that this was from the first interpreted to mean, not to the _comitia curiata_, but to the patrician Senate. The earliest _interregnum_ is represented as an exercise of collective rule by the Senate; but, on the analogy of the sole magistracy, it took the form of a creation of a succession of _interreges_. The first step was the division of the Senate into _decuriae_;[180] each decury had fifty days of government allotted to it; within this period each individual member of the _decuria_ exercised rule for five days, and, according to one account, the succession of the decuries was determined by lot (_sortitio_).[181] The rule is represented as collegiate, the whole decury possessing the _imperium_, while the individual who ruled for five days had the _fasces_ and the external emblems of the royal power.[182] In later times we shall see that, though the _interregnum_ was retained, the whole procedure was simplified by the abandonment of the collegiate principle. If it ever existed, we must suppose that, as soon as ever the resolution of the Senate was taken, the collective rule could be interrupted by any _interrex_, except the first, nominating the king.[183] The _interregnum_, although represented by our authorities as an invariable part of the procedure in the appointment of a king, was probably from the first a makeshift, only resorted to when the ordinary procedure had been interrupted through unforeseen causes, and there was no definitely designated successor.[184] Yet, though the monarchy was not strictly elective, certain quasi-elective processes were connected by tradition with the appointment of the king, on the part both of Senate and people.