Mostly south of the equator, there is another immigration wave

Ouesseni Kaboréq was once a butcher in Burkina Faso, a poor, landlocked west African country. Encouraged by an uncle who was flourishing abroad, he left his country in search of better-paying work. He has done so well that he now employs 41 people. All but two are immigrants like him. The natives cannot bear to get their hands dirty, he says.

But Kaboréq did not migrate to Paris or New Jersey. Instead he crossed just one land border, into neighboring Ivory Coast.

He works in the large meat market in Port Bouët, on the outskirts of Abidjan, near a store that demonstrates its classiness with a picture of Barack Obama on the awning.

Kaboréq is not the kind of immigrant whom economists obsess over, nor the kind who irks voters and brings populists to power in the West. But his kind is already extremely common, and is set to become more so.

International migration can be divided into four types. The most important is the familiar one, from developing countries to developed ones. About 120 million people alive today have made such a move, calculates the McKinsey Global Institute, an arm of the consultancy — from Mexican grape-pickers in California to Senegalese street vendors in France.

But the second-largest flow is between developing countries. Between 2000 and 2015 Asia, including the Middle East, added more immigrants than Europe or North America.

Some are war refugees, like the Syrians who live in Jordan and the Somalis in Ethiopia and Kenya. But many developing-world migrants are like Kaboréq: people who leave a poor country for a somewhat less poor neighboring one in search of higher wages.

Ivory Coast is still very poor — about as poor as Bangladesh. It is, however, better off than Burkina Faso. Batien Mamadou, a farm laborer who works 70 miles northwest of Abidjan, said wages are at least twice as high. And Ivory Coast is a much better place to start a business. The contrast between the two countries is like the difference between a grand African home and the White House, said Bernard Bonane, who fled Burkina Faso following a coup in 1987 and now runs a security firm.

Bonane, who lives in a stylish house in a street crawling with guards, says that few of his neighbors are immigrants. That, he thinks, is because most new arrivals send money home rather than splashing out on property.

The World Bank estimates that $343 million in remittances flowed from Ivory Coast to Burkina Faso in 2015. The exact amount is unknowable, not least because the two countries share a currency, meaning money can easily be moved across the border in ways that officials do not notice.

But the importance of these short-range remittances is plain. Ivory Coast is thought to account for fully 87 percent of all remittances to Burkina Faso.

Rather little of the cash that flows out of the world's richest countries ends up in the poorest ones. Gulf states such as Dubai and Saudi Arabia take in millions of remittance workers from lower-middle-income countries such as India, but hardly any from really poor ones such as Chad and Malawi. The world's poorest people cannot afford to travel to the West or the Gulf.

Whereas fairly poor countries like Nigeria can send many people to the West, households in very poor countries like Mali depend on workers who have migrated within west Africa.

West Africa, however, cannot match the mighty human rivers of Asia. In November, India's home-affairs minister, Kiren Rijiju, declared that about 20 million people from Bangladesh were living illegally in India. Sanjeev Tripathi, the former head of India's Research and Analysis Wing, thinks that an overstatement. His estimate, based on census data, is that more than 15 million Bangladeshis are living in India. If either is right, the Bangladesh-to-India migration corridor is the largest in the world.

It is also one of the most fraught. Immigration from Bangladesh not only raises anxieties about national security; it also suggests to those who worry about such things that a predominantly Hindu society is being diluted.

In the 1980s, students in Assam, a state that touches Bangladesh, led a revolt against mass migration and forced the national government to introduce tougher laws. Nationalist politicians still make hay out of the issue. Narendra Modi, India's prime minister, has accused Bangladeshis of "destroying" Assam and has insinuated that rhinos are being killed to make space for immigrants. In fact, Bangladeshis are spread across India.

The World Bank estimates that more money is remitted to Bangladesh from India — $4.5 billion in 2015 — than from any other country. As in west Africa, this is an economic lifeline.