For many years now historians have preferred to turn their attention to long periods, as if, beneath the shifts and changes of political events, they were trying to reveal the stable, almost indestructible system of checks and balances, the irreversible processes, the constant readjustments, the underlying tendencies that gather force, and are then suddenly reversed after centuries of continuity, the movements of accumulation and slow saturation, the great silent, motionless bases that traditional history has covered with a thick layer of events. The tools that enable historians to carry out this work of analysis are partly inherited and partly of their own making: models of economic growth, quantitative analysis of market movements, accounts of demographic expansion and contraction, the study of climate and its long-term changes, the fixing of sociological constants, the description of technological adjustments and of their spread and continuity. These tools have enabled workers in the historical field to distinguish various sedimentary strata; linear successions, which for so long had been the object of research, have given way to discoveries in depth. From the political mobility at the surface down to the slow movements of material civilization, ever more levels of analysis have been established: each has its own peculiar discontinuities and patterns; and as one descends to the deepest levels, the rhythms become broader. Beneath the rapidly changing history of governments, wars, and famines, there emerge other, apparently unmoving histories: the history of sea routes, the history of corn or of gold-mining, the history of drought and of irrigation, the history of crop rotation, the history of the balance achieved by the human species between hunger and abundance. The old questions of the traditional analysis (What link should be made between disparate events? How can a causal succession be established between them? What continuity or overall significance do they possess? Is it possible to define a totality, or must one be content with reconstituting connexions?) are now being replaced by questions of another type: which strata should be isolated from others? What types of series should be established? What criteria of periodization should be adopted for each of them? What system of relations (hierarchy, dominance, stratification, univocal determination, circular causality) may be established between them? What series of series may be established? And in what large-scale chronological table may distinct series of events be determined?