In a prior post, I said it was not clear whether the Speaker of the House of Commons was required to be a member. A couple of scholars have now supplied evidence that strongly suggests that the Speaker was required to be a member.
Andrew Hyman in a comment to my prior post noted the following from a 1708 law dictionary:
Speaker of the Parliament, is an officer in that High Court, who is, as it were, the common mouth of the rest: and as that honorable assembly consists of two houses, so there are two speakers, the one termed the Lord Speaker of the House of Peers, and is most commonly the Lord Chancellor, or Lord Keeper of the Great Seal of England. The other (being a member of the House of Commons), is called the Speaker of the House of Commons; both whose duties you have particularly described in a book entitled, The Order and Usage of Keeping the Parliament.
Also, Seth Barrett Tillman writes with the following evidence:
At the opening of a new Parliament “the lord chancellor confers first with his majesty, and then in his name commands the commons to assemble in their house, and to choose one of their members to be their speaker.”
Henry Elsynge, The Manner of Holding Parliaments in England 155 (London, printed by Richardson and Clark for Tho. Payne 1768) (First print 1660). And:
At the opening of a new Parliament “the Lord Chancellor confers first with his Majesty, and then in his Name, commands the Commons to assemble in their House, and to choose one of their Members to be their Speaker.”
George Petyt, Lex Parliamentaria: or, A Treatise of the Law and Custom of Parliaments 265 (London, Henry Lintot, Third Edition, 1748).
To make the historical point as strong as possible, however, one would also want evidence that the Speakers of the legislative houses in the states during the period from 1776 to 1787 were also required to be members. But assume that was shown as well.
Then the question is whether the term Speaker, in a legal context, would have been understood as containing this provision. When a term historically included certain characteristics, one may understand it as including those characteristics. That is especially the case in a short document like the U.S. Constitution.
But terms do not necessarily include all of the historical characteristics. Even if Speakers were required to wear wigs in England, that does not mean that the Constitution required it. Rather, to determine whether the historical characteristics are required, one would normally consider the whether they accord with the purpose of the provision. As I noted previously, it is not clear that the context and purpose of the Speaker provision was to deny the House the discretion to decide whom to appoint.