No musical group embodied the spirit of 1960s activism more than Peter, Paul and Mary. Together, they performed over a half-century, compiling a song book that includes Lemon Tree, 500 Miles, Where Have All the Flowers Gone?, Blowin’ in the Wind, Puff the Magic Dragon, Leaving on a Jet Plane, I Dig Rock and Roll Music and Day Is Done. Though Mary Travers died in 2009, Peter Yarrow and Paul Stookey [known as Noel Paul] continue to perform, both as soloists and as a duo. Yarrow makes a solo appearance at the Emerson Center in Vero Beach on Jan. 13.