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