The 1924 Immigration Act struck a delicate balance between racial and national distinctions—between admitting the workers American capitalists needed and avoiding the “racial indigestion” Cubberley observed. The law instituted a quota system, ranking Europeans by national-racial preference and admitting them accordingly, concocting a finely tuned racial diet for smooth white assimilation. Countries with racially unassimilable populations were assigned the minimum quota of 100 souls, but there remained a problem. What stopped a person of Chinese descent from immigrating through Mexico or Canada, which, along with the rest of the Western Hemisphere, were exempt from quotas? What stopped 100,000 of them? The 1924 act solved the problem by banning the immigration of anyone who was racially ineligible for citizenship—that is, Asians. “Congress thus created the oddity of immigration quotas for non-Chinese persons of China, non-Japanese persons of Japan, non-Indian persons of India, and so on,” writes Mae Ngai in Impossible Subjects: Illegal Aliens and the Making of Modern America.45 West of the Ural Mountains, people could become Americans, and it was up to the nation’s scientists and policymakers to cook up the right ratios. The law separated whites into national groups, but only in order to incorporate them in proper proportion as white Americans. Black Americans secured their citizenship by force in the Civil War, and so the bill’s logic required the annual admission of 200 black Africans: 100 each from the free nations of Ethiopia and Liberia. As for the rest of the racially undesirable world, European colonialism ensured that the most desirable whites—defined as desirable in large part thanks to their success in colonization—controlled those quota slots.