Near the start of F. Scott Fitzgerald’s The Great Gatsby, Tom Buchanan laments to Nick Carraway that white Americans, who should be “the dominant race,” are being swamped by inferior foreigners. He urges Nick to read a book — “‘The Rise of the Colored Empires’ by this man Goddard” — that explains how imperiled people of their kind are becoming.

“The idea is if we don’t look out the white race will be will be utterly submerged,” Tom declares. “It’s all scientific stuff; it’s been proved.” What science has “proved,” he says, is that “we’re Nordics” and that “we’ve produced all the things that go to make civilization — oh, science and art and all that. Do you see?”

Fitzgerald was mocking Tom’s concern about the impending demise of white “Nordics” — Americans of English and Scandinavian descent — at the hands of subpar “colored” breeds from Southern and Eastern Europe. But a century ago such anxieties about what was called “race suicide” were shared by the most influential Americans of the age. An enthusiasm for eugenics, the belief that only men and women of superior racial stock should be encouraged to reproduce, fueled a fervor to sharply restrict immigration. Beginning in the years before World War I, that fervor was communicated by Boston Brahmins, progressive politicians, Ivy League academics, and widely read journalists. Above all, it was endorsed and promoted by scientists.

As Daniel Okrent documents in The Guarded Gate, a riveting new history of the anti-immigration movement of the early 20th century, the flames of racial xenophobia were fanned by respected men with scientific credentials — experts in biology, zoology, anthropology, even paleontology and climatology. The threat posed to white Americans of “Nordic” ancestry by the unchecked influx of low-quality foreigners was not a figment of Fitzgerald’s imagination. It was a matter of scientific and scholarly consensus, set forth as incontestable fact in biology textbooks, in prestigious newspapers and magazines, and in bestselling books by activists who saw catastrophe looming.

One of those bestsellers (thinly disguised in Gatsby) was The Rising Tide of Color Against White World-Supremacy, by the Harvard historian and political scientist Lothrop Stoddard. Published in 1920, it bewailed the “headlong plunge into white race-suicide” and warned of the calamity Americans faced from the “hordes of immigrant Alpines and Mediterraneans, not to mention Asiatic elements like Levantines and Jews.” He foresaw “cataclysmic possibilities,” among them “mongrelization” and the end of “white political dominion.”

Stoddard’s book was effusively praised by Madison Grant, the nation’s most renowned conservationist and the chairman of the New York Zoological Society. Grant, to whom the media routinely turned for scientific insight, served on the National Research Council established by President Wilson to “stimulate research in the mathematical, physical, and biological sciences.” He was also the author of his own influential book of white-supremacist nativism, The Passing of the Great Race, which argued forcefully for an end to open immigration before it was too late. “Immigrant laborers are now breeding out their masters, and killing by filth and by crowding as effectively as by the sword,” he wrote.

Stoddard and Grant worked closely with Henry Fairfield Osborn, a Princeton-educated geologist and paleontologist, professor of zoology at Columbia, and longtime president of the American Museum of Natural History. In 1921, Osborn convened the Second International Eugenics Congress in New York, presiding over a glittering roster of scientific delegates that included Alexander Graham Bell. In addition to exhibits and papers on heredity and evolution, writes Okrent, the theme of immigration restriction “was inescapable.” In his welcoming address, Osborn proclaimed the urgency of “barring the entrance of those who are unfit to share the duties and responsibilities of our well-founded government.”

Again and again, the need to overturn US immigration policy — above all by excluding Italians, Russians, Jews, and Asians — was presented not as a radical political position but as a scientific imperative, something on which all educated people agreed. “Science is our polestar,” Stoddard averred. There were a few intellectual dissidents, but most skeptics eventually succumbed to the overwhelming scientific and progressive consensus.

The prominent biologist Charles Davenport, for example, disliked the idea that whole ethnic groups should be deemed undesirable. Yet by 1911, readers of his college textbook were informed that unless immigration rules were changed, “the population of the United States will, on account of the great influx of blood from Southeastern Europe, rapidly become darker in pigmentation, smaller in stature, more mercurial, more attached to music and art, [and] more given to crimes of larceny, kidnapping, assault, murder, rape and sex-immorality … than were the original English settlers.”

Leading media voices echoed and amplified the scientists’ warnings. The Atlantic Monthly, the New York Times, the Boston Globe and Boston Herald, The Century magazine, Collier’s, The Saturday Evening Post — all were on the nativist bandwagon, many invoking the authority of science in support of the anti-immigration crusade.

They had their victory. In 1924, Congress passed the harshest immigration law in US history, slamming the gates shut on virtually all non-“Nordic” immigrants. Before the law was enacted, 76 percent of newcomers were from the nations of Southern and Eastern Europe. That fell to 11 percent after the new law took effect.

The 1924 quotas remained in place for decades. Not even the rise of the Third Reich could induce Washington to lift them. In 1939, the SS St. Louis, carrying 900 refugees from Hitler’s Germany, reached the United States. But the quota for German immigrants was filled. The ship returned to Europe, where the Holocaust was waiting.

A century ago, immigration restrictionists were glad to cloak their racial bigotry in the reputable language of science. Like Tom Buchanan, they readily believed those who assured them their prejudices were “all scientific,” and had been “proved” by experts with advanced degrees. But a thing isn’t true or false just because scientists say it is. Science is no more reliable a touchstone of right from wrong, or wise from foolish, than any other form of human inquiry. Like all human beings, scientists are prone to fanaticism and confirmation bias and the lure of popular acclaim. Skepticism is always in order when sweeping changes in policy are demanded in the name of science. What Okrent calls “the corrupting potential of scientific authority” uncoiled with devastating effect in the last century. If we aren’t careful, it can do the same in this one.

(Jeff Jacoby is a columnist for The Boston Globe).