Zend-Avesta

The old Bactrian tongue in which the Avesta was composed became practically a dead language.

But in the course of time the Avesta was subjected to many additions and interpretations, called the Zend, which show degeneracy.

There is not absolute identity, however, for those which are spoken of as good by the Vedas are treated as bad in the Avesta.

If our reasoning is sound, we cannot, I think, regard the Avesta as a trustworthy exposition of the ancient teaching of Zoroaster.