In the case of the more popular block-books which went through many issues and editions[10] we can trace the gradual substitution of later characteristics for earlier ones. At what intervals of time these changes were made we have bibliographically no adequate grounds even for guessing. Analogies from books printed with movable types may be quoted on both sides. On the one hand, we find the blocks for book-illustrations enjoying an amazingly long life. Thus blocks cut at Venice and Florence between 1490 and 1500 continued in use for fifteen or twenty years, were then laid aside, and reappear between 1550 and 1560, certainly the worse for wear, but yet capable by a lucky chance of yielding quite a fair impression. The fact that one issue of a block-book can be positively assigned to 1470 or 1473, thus does not of itself forbid an earlier issue being placed as far back in the fifteenth century as any one may please to propose. On the other hand, when a printed book was a popular success editions succeeded each other with great rapidity, and one centre of printing vied with another in producing copies of it. The chief reason for the current disinclination to assume a date earlier than 1450 or 1460 for any extant block-book is the total absence of any evidence demanding it. If such evidence were forthcoming, there would be no inherent impossibility to set against it. But in the absence of such evidence twenty years seems an ample time to allow for the vogue of the block-books, and (despite the neatness of the a priori theory of development mentioned at the beginning of this chapter) this fits in better with the history both of printing and of book-illustration than any longer period. The first attempt to describe the extant block-books was made by Carl Heinrich von Heinecken in 1771, in his _Idee generale d'une collection d'estampes_. This held the field until the publication in 1858 of Samuel Legh Sotheby's _Principia Typographica: the block-books issued in Holland, Flanders and Germany, during the fifteenth century_, a painstaking and well-illustrated work in three folio volumes. The most recent and probably the final treatment of the subject is that by Dr. W. L. Schreiber, in Vol. IV of his _Manuel de l'Amateur de la Gravure sur bois et sur metal au xv^e siecle_, published in 1902 (facsimiles in Vols. VII and VIII, 1895-1900). Dr. Schreiber enumerates no fewer than thirty-three works as existing in the form of block-books, the number of extant issues and editions of them amounting to over one hundred. Here it must suffice to offer brief notes on some of the more important. _BIBLIA PAUPERUM_