The opening line of the passage, "Immigration has been a key factor underlying the growth of the American economy," gives some indication that it is the history of immigration to America that the author wants us to focus on. This idea is supported by the fact that he uses the word "has" rather than "will" when discussing the future effects of immigration on the country. He also focuses on how immigration has affected the economy, rather than the environment or native-born Americans. Thus, we can conclude that the most important part of the passage for understanding its theme is the last sentence: "Thus, the economic impact of immigration is positive." This example shows that even though this passage is only four sentences long, it still contains many details regarding the effect of immigration on America.