The organisation of the priesthood needed for these divisions was naturally brought about by the fact that those entrusted with the office of high priest supposed themselves to be descendants of Aaron, and that even in David's reign these had been joined by the priests who claimed to be of the same origin. These families, the descendants of Eleazar and Ithamar, retained the essential arrangements of the sacrifice and the expiation, the priesthood in the stricter sense. Even the families, who side by side with these are said to have belonged to the race of Aaron, which, like Aaron, are said to have sprung from the branch of Kohath, were not any longer admitted to this service. The priestly families of this and other origin, which are first found at a later date in Jerusalem, who retained their dwelling outside Jerusalem, were united with the races of Gershom and Merari, and to them, as to the families of the race of Kohath which did not come through Aaron, were transferred the lesser services in the worship and in the very complicated ritual. Those men of these races who were acquainted with music and singing, together with such musicians as were not of priestly blood, were also divided into sections. They had to accompany the sacrifice and acts of religious worship with sacred songs and the harp. Others were made overseers of the sacred vessels and the dedicatory offerings, others set apart for the purification of the sanctuary and for door-keepers. All these services were hereditary in the combinations of families allotted to them. This organisation of the priesthood cannot have come into existence, as the tradition tells us, immediately after the completion of the temple; it can only have taken place as the effects of a splendid centre of worship in the metropolis of the kingdom became more widely felt, and was finally brought to completion under the guidance of the priests attending on the sacred ark.[366]