tion of the ideal. It is the form to which the coming generations have to conform. Deviation from it means a fall, a degeneration of the religious life. This second period I have termed Gathic.
Decay soon begins in the language in which Zoroaster composed his immortal hymns, and his successors now write in the Avestan dialect, which replaces the Gathic. The Avestan language remains the written language of the Zoroastrians from now onward to probably the last days of the Parthians, when the Pahlavi language becomes the court language of the Sasanians and supersedes the Avestan. The most extensive literature on Zoroastrianism is written in Avestan. This period, which I have called Later Avestan period, extends to the early part of the Pahlavi era and goes even beyond it. When the two periods thus overlap each other, it often becomes difficult to determine whether a certain phase of religious thought is on one side or the other of the dividing line between them. The Avestan works, in the form in which they were written in the Avestan period, no longer exist. They were scattered by the storm that swept over Persia when Alexander conquered the country, and shook her religious edifice to its base. The form in which the Avestan texts have reached us is that which was given them during the Pahlavi period. The artists employed to restore the broken edifice belong to the Pahlavi period, but the materials used come down from the Avestan sources.
The Pahlavi period ranks fourth in the arrangement of the present work, and it covers a period of about eight centuries Although It is most productive under the Sasanian rule, it does not close with the collapse of this, the last of the Zoroastrian empires, but survives it by at least three centuries in Moslem Persia. Though Pahlavi had replaced Avestan, the early works written in the ancient language had not yet ceased to influence the Pahlavi writers. In fact, some of the most important of the Pahlavi works are either versions of some Avestan works now lost to us, or draw their thought from the Avestan sources. Thus, the Pahlavi Bundahishn is the epitome of the Avestan Damdat Nask, which is subsequently lost. Similarly, not a few of the Pahlavi works written two or three centuries after the conquest of Persia by the Arabs tenaciously preserve the tradition handed down by Sasanian Persia. These are characterized by two layers of thought, one traditional, and the other representing new
