>Humor must not professedly teach, and it must not professedly preach, but
>must do both if it would live forever.....
>p. 202 "Mark Twain in Eruption." (1940ed.)
>    Scott, I think this must be the quote you were referring to.
>Gretchen Sharlow

I'm also pretty sure there's also something in the Autobiography, along
similar lines but more detailed.  If someone else doesn't find it first,
try to remember to look it up and post it.