Every picture by Brazilian-born expatriate Sebastião Salgado tells part of a complicated story, and those stories make up a dynamic portrait of the modern world that’s so unfamiliar to our Western eyes, we don’t recognize it as contemporary. Salgado’s overarching story is the world — principally the Third World — in transition. The subjects of his pictures are migrants and refugees — whom he calls the " most visible victims " of social, political, and natural upheaval — as they flee poverty, hardship, isolation, famine, and war for " better " lives, often of squalor and second-class citizenship.

Between 1993 and 1999, Salgado, whose own family fled military rule in Brazil to Europe in 1969 and who now lives in Paris, photographed major human migrations and their aftermaths in Asia, Africa, Eastern Europe, and Latin America. His work was supported by Kodak Professional, a division of Eastman Kodak. Selections from the resulting 400-plus black-and-white documentary photographs have been organized by the Aperture Foundation into a traveling exhibit, which will be on view in Maine through March 23. Unfortunately, even the sponsoring Portland Museum of Art’s temporary-exhibition space is too small to accommodate the entire show, so the work has been apportioned among six venues as far flung as Bangor, Rockport, and Gorham (see " Where To See What " ).

The exhibition coincides with Southern Maine’s emergence as a major destination point for immigrants to the US, and there are important socio-political lessons to be learned from Salgado’s often painful overview of the causes and consequences of cultural and economic displacement. But the six-venue scheme gives those messages a lot to overcome. The unprecedented (and welcome) collaboration among museums was orchestrated by former PMA curator of prints, drawings, and photographs Aprile Gallant, who has since moved on to the Smith College Museum of Art. Her successor, Susan Danly, suggests that the scattering of the exhibition won’t dilute Salgado’s themes because, she says, the problems of refugee populations addressed in each set of photos are similar. Indeed, there is overlap, and the entire exhibit would be overwhelming in a single dose, but the globe-spanning size of end-of-the-century human migration is a major part of the point Salgado’s making. And that’s diffracted by the show’s diaspora.

There are, in fact, differing statements being made by the larger essay’s subsets. Salgado’s extraordinarily powerful African series confronts migrations that have resulted from famine and war. He shows us refugee camps, refugees in hiding, the grim ravages of cholera. The theme of the Asia photographs is the discretionary migration of rural families to the already overcrowded Asian cities. His Latin America collection mixes the rural-exodus theme with its local causes, such as brutal land wars between corporate agriculture and peasants, and the plight of formerly isolated indigenous people contaminated by exploration and development.

Salgado’s full essay has been published, by Aperture, in an exquisite show catalogue, Migrations: Humanity in Transition, a steal at $65 and available at the PMA (though not all photos in this show are in the book and not all photos in the book are in this show). A satellite exhibit, " The Children, " a collection of refugee-children portraits that Salgado shot as a side project and that is now on display at the University of New England, is also available in book form.

The PMA took the largest set of prints: 280 photos shot in Asia (defined to include Turkey and Egypt). Salgado is an accomplished documentary photographer whose location work incorporates basic compositional elements of photo art, such as depth and layering, with tech-based visual approaches (e.g., there’s an unusually large amount of telephoto work in this show) and a journalist’s eye for iconography and representative example. And " Asia " is visually impressive. Super-large-format untitled prints range from a conventional shot of traditional hillside agriculture to dramatic cityscapes (a panorama of Shanghai’s financial district looming in the murky atmosphere across a river dividing its modern towers from a diminutive group of traditional dancers) to unbelievable crowd scenes (squatters amid unimaginable squalor along a railroad right-of-way).

Salgado’s prowess as a photo-documentarian comes across, but he didn’t compile this collection to showcase his art. The events he covered were not widely reported in the West, and few Americans can recall the political context that created tens of thousands of refugees in Rwanda and Angola. Neither are we close to the economic and cultural forces that entice Asian farmers to the slums and squatter camps of unwelcoming, unmanageable cities. These pictures need explanations. Migrations includes a 32-page index with beautifully written and detailed captions for each picture; these increase the essay’s power by several hundred percent.

For the show, however, Aperture has included brief summarized or excerpted versions of the captions in small type in the lower left of each mat. They tell you something about the photo’s subject, but not enough. A photo of Southern Indian peasant children gathered around a low bonfire is a picture of poverty — but how much sadder is it when you know that the peasants are illegally processing coal scavenged from the open mines that destroyed their farmland. The captioning format, coupled with the fact that the mostly large-format prints are hung uncomfortably low, makes viewing the PMA installation something of a back-wrenching athletic experience.

The gargantuan format of many of the prints raises another challenge. When you compare the museum exhibit to the Aperture catalogue, it becomes clear that some pictures suffer from being supersized while others benefit from it.

A bird’s-eye interior of a Jakarta mosque or the dramatic panorama of a construction worker perched in a cage of steel rods above a burgeoning modern city can’t be big enough. But a compositionally striking view of a cluttered Bombay slum — a field of rubble bisected by the smooth unblemished industrial-age pathway of a large-diameter water pipeline along which slum dwellers tightrope their way through the shantytown chaos — has been blown up too much. The details are clouded by the print’s exaggerated grain pattern, and the shot’s power and irony are diluted.

On the other hand, Salgado’s complex group portrait taken in Bombay’s Dharavy district — one of Asia’s two largest slums, where residents live on scavenged garbage — improves greatly in large format. Viewers are impressed by the shot’s jigsaw-puzzle of humanity — adults and children perched, looming, posed, and juxtaposed — before they realize they’ve noticed only the foreground. Offered small in the show’s catalogue, that picture loses its depth and the scene loses its dynamism as the elements cease to compete for your attention.

All of which is to say that " Migrations " is a valuable and important but demanding exhibition. Study some prints at close range; step back as much as 20 feet to feel the impact of others. The Aperture catalogue and index are required companions. Make an effort to see and understand what you’re seeing. The message is worth that effort.

ACCORDING TO THE STATISTICS accompanying the Salgado show, there are more than 26 million refugees in the world today, and 100 million migrants. Each year, 40 million people leave their rural homes for cities. During the six years he worked on the " Migrations " project, Sebastião Salgado photographed in more than 35 countries, and he chronicled the often overlapping diasporas as a global phenomenon: the exodus from Latin American to the US; the overcrowding of Latin American and Asian cities by rural migrants; refugee populations fleeing unbridled civil warfare in Africa and Eastern Europe; Jews escaping the former Soviet Union; Palestinians in exile in Lebanon.

Recurring themes and unimagined consequences alike emerge as Salgado documents plight upon plight in disparate contexts. Yugoslavian refugees are crowded behind coiled barbed wire. A surrealist rooftop in São Paulo is packed with crawling infants who’ve been abandoned to foster care by their families. Starving and maimed African refugees struggle along corpse-lined paths between camps. Desperate people subsist on rubbish. Shantytown slums are littered with refuse. Salgado shows us the bewildered hopelessness of displaced people, the dissolution of the family, the inevitability of disease, the squalor and poverty that greet migrants in urban centers, and the sometimes inexplicable tenacity with which people hold on to their traditions in hostile environments.

" Migrations " doesn’t offer many pretty pictures. The world Salgado toured is a nightmare defined by barbed wire, mud, dust, smog, trash, crowds, and suffering. Mobbed Indian railway stations vie for your amazement with vast acres of Rwandan refugee camps in Zaire. Evidence of Rwandan genocide — a former schoolroom heaped with the skeletons of massacred Tutsi — competes with a less gruesome but powerfully disturbing image of young boys hiding from forced conscription into the civil war in southern Sudan.

Salgado’s African series — especially the photos he shot in Zaire and Rwanda — are the best work in " Migrations. " The horror here is so close to the surface, the sheer number of victims so overpowering, that even we Western viewers can’t help realizing how sheltered and naive we’ve been.

" Migrations " is a wake-up call worth experiencing. If you can’t manage the current show’s six-venue circuit, see as much as you can and buy the catalogue. Not only is this an exquisite documentary-photography project of astounding scope, it’s a global civics lesson you won’t learn on TV. At this point in the 21st century, Salgado’s subjects may be too unfamiliar to resonate with American audiences, but as the human transitions build upon themselves and become inescapable through their cultural and economic momentum, " Migrations " is destined to become a classic body of documentary work. Its politics are a reality we can’t afford to ignore for much longer. The transitory world Salgado shows us is now bigger than our own.