To Caesar it is the general name for the non-military professional class, whether priests, seers, teachers, lawyers, or judges.
To others the Druids are pre-eminently the philosophers and teachers of the Gauls, and are distinguished from the seers designated _vates_.

To others again, such as Pliny, they were the priests of the oak-ritual, whence their name was derived.

In view of the variety of grades of civilisation then co-existing in Gaul and Britain, it is not improbable that the development of the non-military professional class varied very considerably in different districts, and that all the aspects of Druidism which the ancient writers specify found their appropriate places in the social system of the Celts.

In Gaul and Britain, as elsewhere, the office of the primitive tribal medicine-man was capable of indefinite development, and all the forms of its evolution could not have proceeded _pari passu_ where the sociological conditions found such scope for variation.

It may well be that the oak and mistletoe ceremonies, for example, lingered in remote agricultural districts long after they had ceased to interest men along the main routes of Celtic civilisation.