Clan adoption

[MacMillan of Rathdown]

Hmm... So, bizarre example here, but for the sake of argument, if I were to change my legal last name to "MacDonald" in a court of law (in my corner of the world), does that make me a member of the clan?

I am of the opinion than proving descent from some ancient ancestor is really far too restrictive-- if it has ever actually existed as an historical requirement of clan membership. I believe that legally changing your name to "M'Donald" would fullfill part of the requirement; the rest would come down to acknowledging the chief, wearing his badge, and following Scottish heraldic custom.

Followup question: Has anyone even heard of someone offering allegiance to a clan chief in the past half-century? Like the concept of manrent, this seems like a concept of eras and centuries gone by.

Oh Yes. It isn't an uncommon occurrence at all. I have been present when clansmen, in Scotland, have sworn fealty at the inauguration of their chief. Although this is beyond the ken of some, many Scots still take "clanship" very seriously. They may not flaunt it publicly, but like their religious faith, it is something deeply held.