The Dewey Decimal System moves from the general to the specific; so, for example, history (the 900s) starts with the human record of the entire globe, then, when it gets to the present, goes back in time and begins again, narrowing its focus geographically on the second pass: continents, countries, states, and towns, etc. (asleep yet?). Recent history is better represented, of course, because it's still fresh to the people writing it, and they write lots of it, and there is a huge audience for it because we lived it. Hundreds of years from now the plethora of books of contemporary history will have been culled and condensed, like Ken Burns has done with the Civil War, but for now, the history of the 20th century takes up most of the library’s history shelves.