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Redemption of first-born
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requiring a company of men to be exclusively devoted to this ministry, the primitive office of the first-born was superseded by that of the Levites (Numbers 3:11-13), and it was ordained that the first-born of man and of unclean animals should henceforth be redeemed (18:15).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 (Leviticus 27:11-13, 27). The first-born of an ass, if not redeemed, was to be put to death (Exodus 13:13; 34:20).