NAEP Civics Grade 12

"America is . . . the great Melting Pot! Here you stand, good folk, think I, when I see them at Ellis Island, here you stand in your fifty groups, with your fifty languages and histories, and your fifty blood hatreds and rivalries. But you won't be long like that, brothers . . . Into the Crucible with you all! God is making the American." -- Israel Zangwill, The Melting Pot, 1908

Which of the following phenomena does the passage celebrate?