Tradition is unanimous on one point at least: it is that the original Avesta comprised twenty-one _Nasks_, or books, a statement which there is no good reason to doubt. The same tradition which was acquainted with the general character of these Nasks professes also to tell exactly how many of them survived the inroad of Alexander; for although the sacred text itself was destroyed, its contents were lost only in part, the priests preserving large portions of the precious scriptures. These met with many vicissitudes in the five centuries that intervened between the conquest of Alexander and the great restoration of Zoroastrianism in the third century of our era, under the Sassanian dynasty. At this period all obtainable Zoroastrian scriptures were collected, the compilation was codified, and a detailed notice made of the contents of each of the original Nasks compared with the portions then surviving. The original Avesta was, it would appear, a sort of encyclopaedic work; not of religion alone, but of useful knowledge relating to law, to the arts, science, the professions, and to every-day life. If we may judge from the existing table of contents of these Nasks, the zealous Sassanians, even in the time of the collecting (A. D. 226-380), were able to restore but a fragment of the archetype, perhaps a fourth part of the original Avesta. Nor was this remnant destined to escape misfortune. The Mohammedan invasion, in the seventh century of our era added a final and crushing blow. Much of the religion that might otherwise have been handed down to us, despite "the accursed Iskander's" conquest, now perished through the sword and the Koran. Its loss, we must remember, is in part compensated by the Pahlavi religious literature of Sassanian days. Fragmentary and disjointed as are the remnants of the Avesta, we are fortunate in possessing even this moiety of the Bible of Zoroaster, whose compass is about one tenth that of our own sacred book. A grouping of the existing texts is here presented:--1. Yasna (including Gathas). 2. Visperad. 3. Yashts. 4. Minor Texts. 5. Vendidad. 6. Fragments. Even these texts no single manuscript in our time contains complete. The present collection is made by combining various Avestan codexes. In spite of the great antiquity of the literature, all the existing manuscripts are comparatively young. None is older than the thirteenth century of our own era, while the direct history of only one or two can be followed back to about the tenth century. This mere external circumstance has of course no bearing on the actual early age of the Zoroastrian scriptures. It must be kept in mind that Zoroaster lived at least six centuries before the birth of Christ.