A strange thing you learn about American popular music, if you look back far enough, is that for a long time it didn’t much have “genres” — it had ethnicities. Vaudeville acts, for instance, had tunes for just about every major immigrant group: the Italian number, the Yiddish number, the Irish one, the Chinese. Some were sung in a spirit of abuse; others were written or performed by members of those groups themselves. And of course there were the minstrel shows, in which people with mocking, cork-painted faces sang what they pretended were the songs of Southern former slaves. This was how we reckoned with our melting pot: crudely, obliviously, maybe with a nice tune and a beat you could dance to.