The concept of migration is a derivative of the prehistoric belief in the Cycle of Life. The Celts believed that when a person died his soul would, after a set amount of time, simply transfer to another body.
Transmigration was not involved with the concept of karma; an individual's deeds in one life did not affect his promotion or demotion in the next. Instead it was believed that a soul would find a body most suited to it, so a good warrior would find his way to a warrior's family while a farmer would find his way back to the land. The belief in transmigration was so strong among the ancient Celts that they were occasionally known to makes bets and take loans that would be paid in the next life.