Every generation, it seems, can hold two contradictory ideas very firmly in its grasp. We are, we say, a nation of immigrants, yet we are never quite sure about these new guys. They're suspiciously un-American, or so novel and un-American and, golly, just so interesting. Today, we have doubts about illegal Latino immigrants. In the 19th century, it was the Irish, the Germans, the Jews and the Italians. Before that it was African-Americans and Native Americans. Who will it be next?