From the beginning the office of the priesthood in each family belonged to the eldest son. But when the extensive plan of sacrificial worship was introduced, requiring a company of men to be exclusively devoted to this ministry, the primitive office of the first-born was superseded by that of the Levites (Num. 3:11-13), and it was ordained that the first-born of man and of unclean animals should henceforth be redeemed (18:15).
But the first-born of unclean animals was either to be redeemed or sold and the price given to the priest (Lev. 27:11-13, 27). The first-born of an ass, if not redeemed, was to be put to death (Ex. 13:13; 34:20).