Gerre Hancock improvising

Here is a video I ran across on Youtube. Gerre Hancock was the organist at St Thomas Episcopal Church in Fifth Avenue, New York. Hancock wrote a very fine book on organ improvisation, where he recalled a moment in his early life when he joined Nadia Boulanger's class in Paris:

At the first lesson, I was asked to improvise on a theme of Mlle. Boulanger's choosing, using a simple ABA song form. The A section, of some twenty-four measures' length, when fairly well, as did the B section, of some sixteen measures' length, and built upon an inversion of the A theme. With confidence thus renewed I began to elaborate at greater length, traveling to one key for a diminution of the A theme, back to the sonic for an augmentation of the B theme, changing to yet another key for a diminution over an augmentation, and so forth, each scheme growing in ambition and ornateness. All considerations of time and space were consequently forgotten as the young performer continued to show off, to put it bluntly. After a while, a tap on my shoulder brought the organ playing to an abrupt halt, and Mlle. Boulanger asked this simple question: "Why do you keep playing the organ when the piece was finished some time ago?" After all these years, the great lesson I learned then is as fresh as though I had learned it only yesterday; the considerations of space, balance, and proportion are essential in creating any work of art.

Gerre Hancock, Improvising: How to Master the Art (New York: Oxford University Press, 1994), ix.

Here is the piece that Hancock's improvisation was responding to: