Interesting, as the Persians also had their own Priestly class that seems to have served a role nearly identical to that of the Druids. The Magi of the ancient Persians were a professional class of people that held the positions of priests, magistrates, and healers. Despite arguments to the contrary there is evidence that this social class was not hereditary. In the Gathas, the earliest of the Iranian sacred texts, there are no commandments that specifically identify the priesthood as a closed caste. Nor are there any in the later parts of the Avesta.
There are commandments in the Hordad Yashtthat specifically warn not to teach spells to any person other than a father, full-blood brother or an occult priest. The occult priest was a special category of priest who dealt with magic and not with the other elements of the priestly profession.
However, during this time it was very common for children to follow in his parent’s footsteps and there are a number of passages in the Avesta which indicate that a person was free to choose his profession including the priesthood. In the Vendidad it says “let him who wants knowledge, be taught the holy word… (during regular parts of day and night) … until he learns all the words taught by former teachers (aethrapaitis)” (Vd 4:44-45).
Zarathushtra himself prays that King Vishtaspa, who had previously been a warrior, have ten sons, three to become priests, three to become warriors, three to become pastoralists, and one to succeed the King (Vishtasp Yasht 1:3).
Further evidence that the priestly caste was not hereditary can be found in Haoma’s curse on those that do not offer him his rightful portion of the sacrifice (the jaw, the tongue, and the left eye). He curses these sinners to have no priest born into the household. This implies that any household could produce a priest.
Therefore, it seems that, at least among the Persians, it may have been possible to become a Priest regardless of one's upbringing. This indicates to me that the castes may not have been as rigid as Dumezil proposes and that perhaps too much attention has been paid to this particular theory.
Ellis, Peter Berresford. The Druids. Grand Rapids: William B. Eerdmans Pub Co. 1995. 157. Ibid. 189.
 Ibid. 247.
 Ibid. 199.
 Ibid. 212.
 Ellis, Peter Berresford. The Druids. Grand Rapids: William B. Eerdmans Pub Co. 1995. 157.
The Encyclopedia Britanica: A Dictionary of Arts, Science, Literature and General Inforation. 11th . Vol. III. Cambridge: Cambridge University Press, 1910. 597.
 Pliny qtd in Ellis, Peter Berresford. The Druids. Grand Rapids: William B. Eerdmans Pub Co. 1995. 247.
 Yasht 4:10. Retrieved 8/8/2008. <http://www.avesta.org/ka/yt4_bi.htm>.
 Yasht 1:3. Retrieved 8/05/2008. <http://www.avesta.org/vendidad/vd4sbe.htm>.
 Vishtasp Yasht 1:3. Retrieved 8/05/2008. <http://www.avesta.org/fragment/ vytsbe.htm>.
 Yasna 11:4. Retrieved 8/05/2008.<http://www.avesta.org/yasna/y9to11s.htm>.
 Yasna 11:5-6. Retrieved 8/08/2008.<http://www.avesta.org/yasna/y9to11s.htm>.