Our first victim is from the slow movement of Mozart's B-flat Violin Sonata, K. 378, which Melissa and I have played together. The sumptuously scored second subject is a dead ringer for the CRUSADER'S HYMN tune, which is most often associated with Fairest Lord Jesus.
It's not a particularly original melodic structure, but the tunes are so similar that the hymn jumped to mind the very first time I heard it on a Heifetz disc years ago. Note that even where the violin melody becomes more ornate, it still traces the same descent from do (1) down to re (2). [That Heifetz recording always amused me because, in the first movement, he plays the clearly accompanimental violin line at the beginning as if it were the main tune, while poor Brooks Smith is off in some distant sonic space. Listen to the difference in balance when the piano has the tune and then when Jascha takes it.]
Melissa also mentions that she hears the big tune from Rhapsody in Blue during this tender little sequence from the 2nd mvt. of the Franck Violin Sonata:
It's not exact [but see Update below] because the Franck is really in the minor and starts on the 1st scale degree whereas the Gershwin begins on the 3rd. (As with previous posts, I've notated all examples in C Major to make the comparisons easier.) Thus, Franck begins whole-step/half-step and the Gershwin vice versa, but each three-note stepwise ascent is followed by the characteristic octave drop. After that they go they're separate ways and inhabit very different worlds. Still, I think it's a pretty cool pairing, especially because the contexts are otherwise so different - kind of like some of the "How Dry I Am" variations in Bernstein's talk.
I also just remembered that from the very first time I heard the exquisite Countess/Susanna duet from the Marriage of Figaro, it sounded to me like the old folk-song "O Dear, What Can the Matter Be?":
Clearly they're not exactly the same, but those shapely second measures have an obvious kinship. Most notably, each outlines a triad by starting in the middle, going down and then up so that the high note is in the most unstressed rhythmic position; each then continues with a triad going down. This is one of those connections I wish I didn't hear - it took me a while to learn to sit back and enjoy the Mozart for what it is. In fact, I'm embarrassed to say it may have taken Stephen King, Andy Dufresne, and The Shawshank Redemption to open my ears. Isn't that the miracle of Mozart, though? He can create the most sublime moments out of seemingly mundane materials.
UPDATE (the next morning): I just realized I cheated Melissa's Franck/Gershwin parallel. If you listen to the climactic phrase of the big blue Rhapsody tune, you get something much closer to Franck. Not only do they each begin now "in the minor," but they each have a wandering, modulating quality since they occur in transitional spots. Apologies for not having picked up on this sooner. (I've renotated the Franck in A Minor to match the Gerswhin.)
Recording Sources: Mozart Violin, Fairest Lord Jesus, Franck, Gershwin, Mozart Duet, O dear