Instruction on Priesthood, circa April 1835 [D&C 107]

3 The office of an elder comes under the priesthood of Mel chizedek. The Melchisedek priesthood holds the right of  presidency, and has power and authority over all the offices in  the church, in all ages of the world, to administer in spiritual  things.
4 The presidency of the high priesthood, after the order of  Melchizedek, have a right to officiate in all the offices in the  church.
5 High priests, after the order of the Melchizedek priesthood,  have a right to officiate in their own standing, under the direc tion of the presidency, in administering spiritual things, and  also in the office of an elder, priest, (of the Levitical order,)  teacher, deacon and member.
6 An elder has a right to officiate in his stead when the high  priest is not present.
7 The high priest, and elder, are to administer in spiritual  things, agreeably to the covenants and commandments of the  church; and they have a right to officiate in all these offices of  the church when there are no higher authorities present.
8 The second priesthood is called the priesthood of Aaron,  because it was conferred upon Aaron and his seed, throughout  all their generations. Why it is called the lesser priesthood,  is because it is an appendage to the greater, or the Melchize dek priesthood, and has power in administering outward ordi nances. The bishopric is the presidency of this priesthood  and holds the keys, or authority of the same. No man has a  legal right to this office, to hold the keys of this priesthood,  except he be a litteral descendant of Aaron. But as a high  priest, of the Melchizedek priesthood, has authority to officiate  in all the lesser offices, he may officiate in the office of bishop  when no literal descendant of Aaron can be found; provided  he is called and set apart and ordained unto this power by the  hands of the presidency of the Melchizedek priesthood.
9 The power and authority of the higher or Melchizedek  priesthood, is to hold the keys of all the spiritual blessings of the  church—to have the privilege of receiving the mysteries of the  kingdom of heaven—to have the heavens opened unto them— to commune with the general assembly and church of the first  born, and to enjoy the communion and presence of God the  Father, and Jesus the Mediator of the new covenant.
10 The power and authority of the lesser, or Aaronic priest hood, is, to hold the keys of the ministring of angels, and to  administer in outward ordinances—the letter of the gospel— the baptism of repentance for the remission of sins, agreeably  to the covenants and commandments. [p. 83]
Instruction on Priesthood, Kirtland, OH, ca. Apr. 1835; in Doctrine and Covenants, 1835 ed., pp. 82–89.