DURING THE UNUSUALLY frigid winter of 1949, a breeding pair of gray wolves crossed a frozen-over channel onto Michigan’s Isle Royale, a narrow spit of land just south of the US-Canadian maritime border in Lake Superior. Finding abundant prey, including moose, the pair had pups, starting a small lupine clan. Over the next almost 50 years, without access to the mainland, the clan grew increasingly inbred, with over half the wolves developing congenital spinal deformities and serious eye problems. As the wolf population declined — scientists even found one mother dead in her den, with seven unborn pups in her — the moose population came thundering back, gobbling up and trampling the forest’s buds and shoots. The ecosystem’s food chain now had a few broken links.

The Isle Royale wolf population was saved, however, by a lone migrant. In 1997, a male wolf made his way to the island. Within a generation — wolf generations are a little less than five years — 56 percent of the young wolves carried the newcomer’s genes. In the years since, thanks to ongoing conservation efforts, more wolves have been brought to the island to provide enough genetic diversity not only to save the wolves but preserve the ecosystem’s new balance.

This is just one of many examples of the bio-benefits of migratory species provided by Sonia Shah in her new book, The Next Great Migration. Hers is an original take on the oft-stultifying debate about immigration, most frequently argued over by unbending stalwarts on opposite extremes, or sometimes quibbled over by noncommittal centrists. There are now more displaced humans than ever — around one percent of the total human population — and the climate crises together with humanity’s ceaseless creep are driving an increasing number of nonhuman species to search for more welcoming climes. That half of the story is popularly understood: the world is on the move. What is less often acknowledged, and what Shah convincingly fills out, is its biological necessity. “Migration’s ecological function extends beyond the survival of the migrant itself,” she writes. “Wild migrants build the botanical scaffolding of entire ecosystems.” Besides spreading pollen and seeds — upon which the survival of many plants depend — migrants also transport genes, thus bringing genetic diversity. Migration is not only a human fact but a biological one.

But the understanding of migration’s critical import — whether broadly biological or specifically human — has been a long time coming.

“The idea that certain people and species belong in certain fixed places has had a long history in Western culture,” Shah writes. By its logic, “migration is by necessity a catastrophe, because it violates the natural order.” The so-called “natural order” is actually a construct that has been buoyed for millennia by a broad coalition of scientists, politicians, and other ideologically inflected cavillers. As for the word “migrant,” it didn’t even appear in the English language until the 17th century — when it was coined by Thomas Browne — and it took another hundred years before it was applied to humans. One important migrant-denialist, as Shah details, was Swedish-born naturalist Carl Linnaeus, most famous for formalizing binomial nomenclature, the modern system of classifying organisms as, say, Canis lupus or Homo sapiens.

Shah goes beyond Linnaeus’s contribution to taxonomy — which, notably, is itself subject to critique, as when essayist Anne Fadiman describes it as a “form of mental colonising and empire-building” — to illuminate his blinkered fealty to the dominant narratives of the day. More than just falling in line, he worked to cement the alleged differences between human populations — crudely exaggerating, for instance, features of “red,” “yellow,” “black,” or “white” skinned people. He sparred with competing theorists who were beginning to propose then-revolutionary ideas — for instance, that all humans originated in and migrated out of Africa. With the concept of the “Great Chain of Being,” he toadied to the reigning theological explanation for the world being as it was; this concept hierarchically categorized, in ascending order, matter, plants, animals, peasants, clergy, noblemen, kings, and, finally, God. To support his views, Linnaeus took a trip to northern Sweden where he “studied” the indigenous Sami people, all the while complaining of the climate and the locals not speaking Swedish. Robbing them of a few native costumes, he then freely fabricated stories about their culture and origins. He later tried to give credence to biological differences between Africans and Europeans by committing to the bizarre fantasy that black women had elongated labia minora, to which he referred using the Latin term sinus pudoris. The cultural backdrop to his explanations and speculations was the generally held view that migration was an anomaly, and that people and animals lived where they belonged and belonged where they lived — and always had.

Ignorance — deliberate, political, or simply true and profound — of the realities of even animal migration went so far as pushing scientists to hatch myriad far-fetched theories to explain, for example, where migratory birds went in the winter. Leading naturalists at the time explained some birds’ seasonal disappearance by claiming that they hibernated in lakes — a theory first proposed by Aristotle — or hid in remote caves. Driving such assumptions was, in part, the idea of a stable and God-created “harmony of nature.” When some thinkers began to question such fixed stability, Linneaus doubled down, insisting that animals inhabited their specific climes, and remained there. The implication for humans was not only that they had not migrated from Africa, but that Africans — as well as Asians and Native Americans — were biologically distinct. This kind of racial essentialism was an important structural component of what would morph into race science or eugenics. Linnaeus divided Homo sapiens into Homo sapiens europaeus (white, serious, strong), Homo sapiens asiaticus (yellow, melancholy, greedy), Homo sapiens americanus (red, ill-tempered, subjugated), and Homo sapiens afer (black, impassive, lazy), as well as Homo caudatus (inhabitants of the Antarctic globe), and even Homo monstrosus (pygmies and Patagonian giants).

“Scientific ideas that cast migration as a form of disorder were not obscure theoretical concerns confined to esoteric academic journals,” but, Shah writes, “theoretical ballast for today’s generation of anti-immigration lobbyists and policy makers.”

Here Shah dredges up more vile fantasies, like that of the “Malphigian layer” in the late 17th century, which claimed that Africans had an extra layer of skin consisting of “a thick, fatty black liquid of unknown provenance.” While the Malphigian layer has been roundly dismissed, such invented differences between peoples continue to bedevil medical treatment: even today, black people are presumed to be able to tolerate more pain, and so it’s perhaps hardly surprising that more black women die in childbirth.

The idea was “that people who lived on different continents were biologically foreign to one another, a claim that would fuel centuries of xenophobia and generations of racial violence.” Or, put more simply, Linnaeus and other believed: “We belong here. They belong there.

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“The classifications of species as either ‘native’ or ‘alien’ is one of the organizing principles of conservation,” Shah writes, quoting a 2007 scientific study in Progress in Human Geography. The implications of that dichotomous classification are harmful to humans and nonhumans alike, setting the stage for xenophobia and white anthropomorphism. As a case in point, the son of author and conservationist Aldo Leopold recommended in 1963, that US national parks “preserve, or where necessary […] recreate the ecologic scene as viewed by the first European visitors.” The idea of a pristine, pre-colonial era presumes an ahistorical falsehood: that humans and others left no trace, or that those traces could be undone and the ecologic scene returned to a static Eden. While many indigenous cultures certainly live less disruptively within their environment, in the case of both the Americas and Australia for example, the arrival of the first Homo sapiens heralded the swift extinction of scores of native species — in the Americas, woolly mammoths, giant sloths, saber-toothed tigers, camelops, and the dire wolf. Yet the pull toward preservation persists.

In 1999, Bill Clinton established the National Invasive Species Council, which was tasked with repelling “alien species.” This move was an outgrowth of the relatively recently created disciplines of conservation biology, restoration biology, and even invasion biology. I recall being a boy in northern Ohio and hearing of the horror and devastation promised by the zebra mussel’s inexorable encroachment into the ecosystems of the Great Lakes. One invasion biologist, writes Shah, “calculated that wild species moving freely across the planet would ravage large swaths of ecosystems. The number of land animals would drop by 65 percent, land birds by 47 percent, butterflies by 35 percent, and ocean life by 58 percent.” And while the globe is certainly losing species to extinction, blaming mobility or migration is missing the mark, and buoying up the old “myth of a sedentary planet,” as she puts it.

For millennia, humans had hardly any idea of how some species could spread. They had neither the perspective nor technology to understand that creepy-crawlies have creeped and crawled vast distances and always been on the move, which is not, in the big picture, a bad thing. Zebra mussels, for example, were not the only, or even the greatest, threat to native clams in the Great Lakes. Besides disrupting the local ecosystems, they also contributed to those ecosystems by filtering water and becoming a new source of food for native fish and fowl. Shah notes that Canadian ecologist Mark Vellend has found that “wild newcomers generally increase species richness on a local and regional level.” Since the introduction of European species to the Americas 400 years ago, biodiversity has actually increased by 18 percent. In other words, Shah writes, “nature transgresses borders all the time.”

In her last chapter, “The Wall,” she tackles the immunological implications of migration. While first acknowledging that certain dangers do uncontrovertibly exist, such as Europeans bringing smallpox to the Americas, or Rome spreading malaria to the outer regions of its empire, she metaphorizes xenophobia as a fever dream. To be sure, wariness of foreign pathogens may make sense, but to guide foreign policy on such grounds or let wariness morph into discrimination or violent backlash becomes, like a fever that climbs beyond what the host organism needs, “a self-destructive reaction, leading to seizures, delirium, and collapse.” It’s like a cytokine storm in the COVID-19 era. As Shah told me, “the reflexive solution to contagion — border closures, isolation, immobility — is in fact antithetical to biological resilience on a changing planet.”

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In 2017, a solo Mexican wolf loped through the Chihuahuan Desert, heading north, following a path that other wolves, as well as humans, have traveled for thousands of years. Scientists were especially interested in this lone wolf, known as M1425, because he represented a waning population of endangered Mexican wolves dispersing genes from a tiny population in Mexico to a slightly more robust population in the United States.

Like the Isle Royale wolves, “[i]f the two wild populations of Mexican gray wolves can find and mate with each other, the exchange of genetic material could boost recovery efforts for both populations,” a New Mexico magazine reported. But the area where M1425 crossed the international boundary is now closed off by a border wall, and the Center for Biological Diversity counts 93 species directly threatened by the proposed expansion of the wall. This is what we should be worried about.

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John B. Washington writes about immigration and border politics. His first book, The Dispossessed — a narrative take on asylum policy and its ancient history — was published in May by Verso Books.