Writers of bibliographies of bibliographies have usually preferred a classification according to subjects to an alphabetical arrangement of titles with subject indexes. Joris Vorstius defends their preference eloquently and with good arguments.[153] There is, however, something to be said against it. Convenient as a classified bibliography is as first issued, it cannot be easily revised or enlarged.[154] When library cataloguers adopt new methods, when new categories are set up in science, theology, law, and literary history, a classified bibliography of bibliographies becomes difficult to use. In the history of bibliographies of bibliographies we can look back to at least three occasions when men discarded the classified bibliographies made by their predecessors. Men of the seventeenth century seem to have made little use of Gesner's _Pandectae_, men of the eighteenth century found as little use for the difficult classification employed in Labbé's and Teissier's books, and few of us can use Petzholdt's categories easily. The lesson is that each age must create its own bibliography of bibliographies. Petzholdt's _Bibliotheca bibliographica_ is a classified bibliography that shows signs of obsolescence. The organization of knowledge and the categories that seemed suitable to Julius Petzholdt in 1866 are often confusing rather than helpful today. Keenly interested as he was in the theory of classification, no one was more competent than he to select the right headings. But a modern scholar who consults the _Bibliotheca bibliographica_ must put himself in the place of a man who lived almost a century ago. For example, he must remember that Hungary was associated politically with Austria and Austrian cataloguers and dealers listed and sold Hungarian books. Consequently, Petzholdt cites (pp. 320-321) bibliographies of Hungarian books along with bibliographies of German books and makes no entry in the table of contents for Hungarian bibliographies. I do not say that he was wrong, but I do say that a modern reader must remember the political situation of 1866 to use Petzholdt's book.