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Liz, Sarah and Marina's turning (a relatively orderly) "tree of evolution" into a totally meandering "river of discussion"--stable because of (rather than despite) change--put me in mind of something Mark Twain said in Chapter 10 of Life on the Mississippi. The chapter is entitled "Completing My Education":

piloting becomes another matter when you apply it to vast streams like the Mississippi...whose alluvial banks cave and change constantly, whose snags are always hunting up new quarters, whose sand-bars are never at rest, whose channels are for ever dodging and shirking, and whose obstructions must be confronted in all nights and all weathers..

Paul and I have heard this analogy applied before to the ways in which our classes operate; participants in a summer institute, a few years ago, described it as "the Mississippi River, also changing but always still .... "focused", coherent in some way."