A hundred years ago, the largest city in the world was London, with a population of 6.5 million. Today it is dwarfed by Tokyo. With barely a quarter the population of London a century ago, The Tokyo metropolitan area has since mushroomed to 35 million, propelling it to first place in the global city league table. Tokyo’s phenomenal growth is largely due to a single factor: migration from the countryside to the city. It is just one of many to have overtaken London, which with a population of 7.5 million today doesn’t even make the top 20.