Music: Hip-Hop Nation

Music mixes with memory. As we think back over the 20th century, every decade has a melody, a rhythm, a sound track. The years and the sounds bleed together as we scan through them in our recollections, a car radio searching for a clear station. The century starts off blue: Robert Johnson selling his soul to the devil at the crossroads. Then the jazz age: Louis Armstrong, Duke Ellington and, later on, Benny Goodman and "Strange fruit hanging from the poplar trees." Midcentury, things start to rock with Chuck Berry, "Wop-bop-a-loo-bop a-lop bam boom!" the Beatles, Aretha Franklin, "a hard rain's...