There are two ways to tell the story of the twentieth century. You can describe a series of wars, revolutions, crises, epidemics, financial calamities. Or you can point to the gentle but inexorable rise in the quality of life of almost everybody on the planet: the swelling of income, the conquest of disease, the disappearance of parasites, the retreat of want, the increasing persistence of peace, the lengthening of life, the advances in technology.