SLUMS and shantytowns are usually seen as blights on the cities they surround. In Doug Saunders's provocative new book, Arrival City, these peripheral places are recast as immigrant gateways, portals through which peasants pass in order to join the city-dwelling elite.
Far from being capitalism's victims, he argues, the slums' entrepreneurial inhabitants can become its biggest beneficiaries: they are "the best of the best from the villages" and possible future members of the middle class. Arrival City addresses the great neglected trend of the 21st century: urbanisation.
Travelling across the globe, from Rio de Janeiro's favelas to Nairobi's slums and Berlin's Turkish enclave, Saunders weaves the tales of individual migrants through his vast story, that of the current, great human movement - involving one-third of our species - from the countryside to the city.
As one Tower Hamlets teenager jokes to her sister, who longs to live on the family's farm in Bangladesh: "You're welcome to it. By the time you get there, everyone else in the village will have left to come here."
Saunders argues that it will be those countries that welcome migrants to the metropolis - offering citizenship, property rights, education and transportation - which will prosper.
These "arrival cities", if allowed to flourish, will help to reduce poverty and end population growth as citizens swap large agrarian families for the two or fewer children common in the city. Ignore these migrants or, worse, bulldoze their homes, however, and social unrest and religious extremism can result. History is Saunders's evidence: the Parisian crowd who stormed the Bastille were largely the disgruntled residents of a failed arrival city.
Though a powerful work, Arrival City occasionally slips into repetition and Saunders sometimes states as fact what he needs to argue for. Moreover, his liberal leanings perhaps encourage him to overlook some of the problems of urbanisation and the losers it creates.
The poor and unemployed in the developed world are ignored as he emphasises the benefits to the West of an influx of unskilled rural-to-urban migrants from developing countries. He also tells us little of the fate of the village and the relatives, including children, left behind.
The segments where Saunders explains a quirk of human behaviour, in a similar vein to Freakonomics, inject lighter moments into the book. He reveals why cable TV comes to the favelas before running water, a postal service or a sewerage system, for example, and why the property market is as great an obsession for many slum-dwellers as it is for the British bourgeoisie.
But Arrival City is above all a warning. Migration is changing our world, and Saunders believes our reaction to it now will determine whether it can help eliminate poverty or whether it will cause catastrophe.
"This is our opportunity, now, to turn this final migration into a force of lasting progress," he concludes. "It will work only if we stop ignoring those awkward neighbourhoods on the edge of town."