Moreover, at the bottom of this supposed or imagined unity of history, in consequence of which the _processus_ of the different societies would form one single series of progress, there is another fact, which has offered motive and occasion for so many fantastic ideologies. If all nations have not progressed equally, still more, if some have stopped and have followed a backward route, if the _processus_ of social development has not always, in every place and in all times, the same rhythm and the same intensity, it is nevertheless certain that, with the passage of the decisive activity from one people to another people in the course of history, the useful products, already acquired by those who were in decadence, have been transmitted to those who were growing and rising. That is not so true of the products of sentiment and imagination, which nevertheless are themselves preserved and perpetuated in literary tradition, as of the results of thought, and especially of the discovery and of the production of technical means, which, once found, are communicated and transmitted directly. Need we remind the reader that writing was never lost, although the peoples who invented it have disappeared from historic continuity? Need we recall again that we all have in our pockets, engraved on our watches, the Babylonian dial, and that we make use of algebra, which was introduced by those Arabs, whose historical activity has since been dispersed like the sands of the desert? It is useless to multiply these examples, because it is sufficient to think of technology and the history of discoveries in the broad sense of the word, for which the almost continuous transmission of the instruments of labor and production is evident. And after all, the provisional summaries which are called universal histories, although they always reveal, in their aim and in their execution, something forced and artificial, would never have been attempted if human events had not offered to the empiricism of the narrators a certain thread, even though subtle, of continuity.