I’ve thought a lot about D&C 107’s claim that the Melchizedek Priesthood was originally designed to be passed from father and son to the chosen seed. I’ve wondered how this is or isn’t connected to other promises about a chosen seed, such as ones to Adam, Enoch, Noah, Abraham, Issac, Jacob, and Joseph. Lehi tells his son Joseph that Joseph of old was promised that his seed would never die out, and would be alive to the end of the earth. Not only that, he was promised that his see would be the seed which was promised to be alive to the end of the earth.
D&C 107 says that the Melchizedek Priesthood — well, the keys of it — “rightly belongs” to the chosen seed. It’s stated something like “it should go to them, and belongs to them, but since they aren’t around right now, we’re going to let you guys help out too.” It sounds similar to the way thing work with the Aaronic priesthood keys. It rightly belongs to the descendants of Aaron, but in the meanwhile, anyone with the Melchizedek priesthood can fill in.
Today I found again that D&C 86 describes Joseph Smith of being in the chosen lineage. That is so very, very fascinating and I have to do some more thinking about that. If things work as it sounds like, that would mean he would be a part of the one, chosen, special line promised to prophets of old. It could also be that there are many many chosen lines, since Abraham was promised seed as numerous as the sands of the sea.
Either way, the promise of this chosen seed is that they have the responsibility to preach the gospel, administer ordinances, and bless all the families of the earth. Which is certainly what Joseph Smith did.
10 Therefore your life and the priesthood have remained, and must needs remain through you and your lineage until the restoration of all things spoken by the mouths of all the holy prophets since the world began.