Excerpts from Books and Wikipedia
"In the hundred years after 1815, some 35 million people entered the United States and what Hugh Brogan has called 'the largest peaceful migration in recorded history.' Americans began to feel threatened and demanded stricter controls. The 1907 Act of Congress created a new Border District in Arizona, New Mexico, and Texas as a barrier to Mexicans, and authorized refusal to laborers from China and Japan. The 1890 census showed 9 million foreign-born in a total population of 63 million. . . . The 1907 Act provided for the exclusion of those with any mental or physical defect that would prevent them from working, and of unaccompanied children. . . . The Act of 1917 required immigrants to be able to read and write in their own languages. In the 1920s a quota system limited numbers of each nationality allowed in by reference to its numbers already settled in the United States." [Furtado: 1001 Days]