Among the grammarians who rendered important service in the editing of the older classics was Callimachus, whose name also appears in the list of poets. This is the same Callimachus whose report concerning the number of the books contained in the library is quoted by Tzetzes. Very few of the other names of the Alexandrian editors have been preserved, their editions having in most cases been modestly sent forth with the names of the authors only. The publishers of Alexandria must also have been modest, for not a single firm has sent its name down to posterity. There are many references in later literature to the existence in Alexandria of great book-producing concerns, and, as Birt remarks, an active production of literature must have necessitated an effective machinery for the distribution of literature. Strabo speaks of the excellent organization of the book scribes of Alexandria, and states that Roman methods of bookmaking were derived from Alexandria. The fact that for a number of centuries the entire supply of the most important of the materials required was derived from Egypt, gave an enormous advantage to the development of publishers in Alexandria. Even after the perfection of the methods for the preparation of parchment, papyrus retained its place in the preference of writers, Greek and Roman, and until about the fourth century A.D. the use of parchment continued very inconsiderable. But the papyrus was produced only in Egypt. It was therefore a serious blow at the literary undertakings of the kings of Pergamum when Philadelphus, in pursuance of his policy of concentrating in Alexandria the production of literature, prohibited for some years the export from Egypt of papyrus. It was this embargo that gave a temporary stimulus in Pergamum to the production of dressed skins, and the special interest taken by Pergamum in this industry caused the most carefully finished of the skins (very different in their appearance from the old time διφθέραι) to bear the name of parchment, _pergamentum_. With the removal of the embargo, however, the writers in Asia Minor appear in the main to have speedily gone back to the use of the more convenient papyrus; the production of parchment languished, and when in the latter Empire, parchment again came into vogue, as its manufacture could as well be carried on in many other places, it did not remain an important product of Pergamum.