Via CRS (pdf):

Figure 2 illustrates that immigration over the last few decades of the 20th century was not as dominated by three or four countries as it was earlier in the century. Although Europe was home to the countries sending the most immigrants during the early 20th century (e.g., Germany, Italy, Austria-Hungary, and the United Kingdom), Mexico has been a top sending country for most of the 20th century—largely after 1970—and into the 21st century. Other top sending countries from FY2001 through FY2010 were the Dominican Republic, El Salvador, Colombia, and Cuba (Western Hemisphere); and the Philippines, India, China, South Korea, and Vietnam (Asia). These data suggest that the per-country ceilings established in 1965 had some effect. As Figure 2 illustrates, immigrants from only three or four countries made up more than half of all LPRs prior to 1960. By the last two decades of the 20th century, immigrants from seven to eight countries comprised about half of all LPRs, and this pattern has continued into the 21st century.

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