On October 7th, 1899, Max Beerbohm ended his review of Shaw's The Doctor's Dilemma with two quoted lines of poetry:
"O most presumptuous! lay aside the pipe
Of that sweet elder shepherd."
Where do those lines come from? Does anyone know? My source is Beerbohm's volume of collected theatre reviews, Around Theatres (Hart-Davis, 1953, p. 41.)