So, last month I mused over some comments C. S. Lewis made about THE HOBBIT and THE LORD OF THE RINGS in a 1958 letter, during the course of which he said
[CSL:] "THE HOBBIT is merely a fragment of his myth, detached, and adapted for children, and losing much by the adaptation."
The significance of this, I argued, was its revealing that CSL felt that
[JDR:] "THE HOBBIT originated as part of the legendarium, not as an independent work later incorporated within it. And this from the point of view of someone who read Silmarillion texts before reading THE HOBBIT as well as the first person outside the immediate family to read THE HOBBIT as soon as Tolkien finished it. A good witness to have on the side of those of us who emphasize THE HOBBIT's connections to the legendarium versus those who stress the stand-alone nature of the work."
The next day, I got a comment which asked
['Ardamir':] "Of course THE HOBBIT, as it stands today, is 'merely a fragment of his myth, detached'. But I am not sure if the statement tells us anything about the thoughts C. S. Lewis would have had about it when it was in the early stages of composition. Would you care to elaborate a bit why you think this is a comment 'that THE HOBBIT originated as part of the legendarium, not as an independent work later incorporated within it?"
In the hurry of getting ready for my Wheaton trip, I didn't have time to revisit this, but would like to do so now.
It essentially comes down to Lewis's word choice. A 'fragment' might just be an unfinished work, like Tolkien's LOST ROAD or Lewis's own DARK TOWER. But a work would only be described as 'detached' if it was once part of a whole and has now been removed from its original context, like a leaf torn out of a book. Taken together with 'adapted . . . and losing much by the adaptation', it's clear that Lewis felt THE HOBBIT was essentially part of the legendarium in inception, rather than an add-on or later addition.
This is borne out by another piece by Lewis, the TIMES obituary,* one passage from which reads
"Thus the private language and its offshoot, the private mythology, were directly connected with some of the most highly practical results he achieved [in scholarship and in academia], while they continued in private to burgeon into tales and poems which seldom reached print, though they might have won him fame in almost any period but the twentieth century.
"THE HOBBIT (1937) was in origin a fragment from this cycle adapted for juvenile tastes but with one all important novelty, the Hobbits themselves . . .
"They soon demanded to be united with his heroic myth on a far deeper level than THE HOBBIT had allowed, and by 1936 he was at work on his great romance THE LORD OF THE RINGS, published in three volumes . . . "
If anything, the use of 'from' rather than 'of' strengthens the case. So, from both these statements, I put Lewis down firmly in the Hobbit-originated-as-part-of-the-legendarium school. A well-informed witness to help bolster that case, though of course not the last word.
*I have taken my text here from that reprinted as the first item in Salu & Farrell's memorial festschrift, TOLKIEN: SCHOLAR AND STORYTELLER , page 14; emphasis mine.