With Revolver, the Beatles made the Great Leap Forward, reaching a previously unheard-of level of sophistication and fearless experimentation. Sgt. Pepper, in many ways, refines that breakthrough, as the Beatles consciously synthesized such disparate influences as psychedelia, art-song, classical music, rock & roll, and music hall, often in the course of one song. Not once does the diversity seem forced -- the genius of the record is how the vaudevillian "When I'm 64" seems like a logical extension of "Within You Without You" and how it provides a gateway to the chiming guitars of "Lovely Rita." There's no discounting the individual contributions of each member or their producer, George Martin, but the preponderance of whimsy and self-conscious art gives the impression that Paul McCartney is the leader of the Lonely Hearts Club Band. He dominates the album in terms of compositions, setting the tone for the album with his unabashed melodicism and deviously clever arrangements. In comparison, Lennon's contributions seem fewer, and a couple of them are a little slight but his major statements are stunning. "With a Little Help From My Friends" is the ideal Ringo tune, a rolling, friendly pop song that hides genuine Lennon anguish, à la "Help!"; "Lucy in the Sky With Diamonds" remains one of the touchstones of British psychedelia; and he's the mastermind behind the bulk of "A Day in the Life," a haunting number that skillfully blends Lennon's verse and chorus with McCartney's bridge. It's possible to argue that there are better Beatles albums, yet no album is as historically important as this. After Sgt. Pepper, there were no rules to follow -- rock and pop bands could try anything, for better or worse. Ironically, few tried to achieve the sweeping, all-encompassing embrace of music as the Beatles did here.