This is by no means an impossible demand. The total population of the world is, on the most generous estimate, not more than 1,500,000,000, and the effectual indexing of this number of people, the record of their movement hither and thither, the entry of various material facts, such as marriage, parentage, criminal convictions and the like, the entry of the new-born and the elimination of the dead, colossal task though it would be, is still not so great as to be immeasurably beyond comparison with the work of the post-offices in the world of to-day, or the cataloguing of such libraries as that of the British Museum, or such collections as that of the insects in Cromwell Road. Such an index could be housed quite comfortably on one side of Northumberland Avenue, for example. It is only a reasonable tribute to the distinctive lucidity of the French mind to suppose the central index housed in a vast series of buildings at or near Paris. The index would be classified primarily by some unchanging physical characteristic, such as we are told the thumb-mark and finger-mark afford, and to these would be added any other physical traits that were of material value. The classification of thumb-marks and of inalterable physical characteristics goes on steadily, and there is every reason for assuming it possible that each human being could be given a distinct formula, a number or "scientific name," under which he or she could be docketed. [Footnote: It is quite possible that the actual thumb-mark may play only a small part in the work of identification, but it is an obvious convenience to our thread of story to assume that it is the one sufficient feature.] About the buildings in which this great main index would be gathered, would be a system of other indices with cross references to the main one, arranged under names, under professional qualifications, under diseases, crimes and the like.