W.Y. Evans-Wentz - The Fairy-Faith in Celtic Countries - On reincarnation
Type of Spiritual Experience
A description of the experience
The Fairy-Faith in Celtic Countries, by W.Y. Evans-Wentz, 
The esoteric teaching in many of the mystic schools of antiquity was that the atoms of each human body transmigrate through all lower forms of life during the long period supposed to intervene between death and re-birth of the individuality.
This doctrine seems to be one of the main sources of the corruption which crept into the ancient reincarnation doctrines and transformed many of them into doctrines of transmigration of the human soul into animal and plant bodies; and some unscrupulous priest-hoods openly taught such corrupted doctrines as a means of making the ignorant populace submissive to ecclesiastical rule, the theological theory expounded by such priesthoods being that the evil-doer, but not the keeper of the letter of the canonical law, is condemned to expiate his sins through birth in brute bodies.
The pure form of the mystic doctrine was that after the lapse of the long period of disembodiment the individuality [Higher spirit] reconstructs its human body anew by drawing to itself the identical atoms which constituted its previous human body--these atoms, and not the individuality, having transmigrated through all the lower kingdoms.
Such an esoteric doctrine probably lies behind the exoteric Egyptian teaching that the human soul after the death of its body passes through all plant and animal bodies during a period of three thousand years, after which it returns to human embodiment.
Some scholars have held that the exoteric interpretation of this theory and its consequent literal interpretation as a transmigration doctrine led the Egyptians to mummify the bodies of their dead. Cf. Lucretius, De Rerum Natura, Book III, ll. 843-61; and Herodotus, Book II, on Egypt.