How did Madison Grant’s book The Passing of the Great Race promote eugenic ideas?

Madison Grant’s 1916 book, The Passing of the Great Race, was a highly influential work that promoted eugenic ideas by framing the preservation of "good stock" as essential to national strength.1

Grant, a prominent figure among American leaders, argued that the political and economic stability of the United States was threatened by the dilution of its population with what he characterized as "socially undesirable" traits. By popularizing these views, he helped foster a climate of fear regarding immigration, suggesting that the influx of individuals from specific regions—particularly Italy, Greece, and Eastern Europe—was contributing to a decline in the nation's genetic quality.1

His work provided a pseudo-scientific justification for discriminatory policies, directly influencing the legislative landscape of the era. The arguments presented in his book helped build support for the Immigration Act of 1924, which severely restricted immigration from countries deemed to be diluting the American population. Ultimately, Grant’s promotion of these eugenic ideologies reinforced the dangerous and unethical belief that society should exert reproductive control over those labeled as "unfit" to protect the perceived superiority of a specific racial group.1