Final answer:
Walt Whitman describes America as a "nation of nations" to highlight its diversity, and he extends this theme in "I Hear America Singing" by celebrating each individual's unique contribution to the nation's harmony.
Step-by-step explanation:
When Walt Whitman calls America a "nation of nations" in the preface to Leaves of Grass, he is referring to the unique and diverse composition of the United States; it is a country made up of many different peoples, cultures, and ideas coming together, each contributing to the vibrancy and character of the nation as a whole. He develops this idea in his poem "I Hear America Singing" by celebrating the various individuals and their distinct contributions to society. Through the collective singing of these diverse voices, Whitman encapsulates the spirit of American democracy and individualism, suggesting that everyone has a valuable role to play in the tapestry of the nation.