In his introduction to a collection of essays ""primarily intended to present the thought that conductors themselves have had about their art,"" Carl Bamberger shows how music has developed so as to require the artist behind the baton. It is essentially a matter of time (""Music is the only art which develops abstract material in time"") and an ongoing complexity of musical language along with the widening chronological distance between the interpreter and the composer today. Thus, as music has changed, the conductor has emerged from the original instrumentalist-cum-time-keeper, evolved through the virtuoso stage, still with us, to the present tendency to specialized conducting in an increasingly varied field. Because they have written most frankly, Mr. Bamberger draws most heavily on the German practitioners of the art, he says, but actually his list includes most of the best known names and those of several composers as well. Among the twenty-five men quoted: Berlioz, Robert Schumann, Liszt, Wagner, Richard Strauss, Hindemith, Schuller; Weingartner, Koussevitzky, Walter, Barbirolli, Bernstein. Among the missing: Toscanini and Karajan.