In 1924, a new American law ended the wave of immigration to this country that had begun in the 19th century.  Hundreds of thousands of southern- and eastern-European immigrants had entered the United States each year before the law, but after 1924, those numbers were reduced to a trickle.  Daniel Okrent is the author of a remarkable history of the bigotry and sham science that lay at the heart of the Immigration Act of 1924.