dominant media of communication in the lands where they ruled, with the
result that most of the languages of Europe to-day are of the Aryan or
Indo-European type. It does not, however, follow necessarily from this
that the early religious ideas or the artistic civilisation of countries
now Aryan in speech, came necessarily from the conquerors rather than the
conquered. In the last century it was long held that in countries of
Aryan speech the essential features of their civilisation, their
religious ideas, their social institutions, nay, more, their inhabitants
themselves, were of Aryan origin.

A more critical investigation has, however, enabled us to distinguish
clearly between the development of various factors of human life which in
their evolution can follow and often have followed more or less
independent lines. The physical history of race, for instance, forms a
problem by itself and must be studied by anthropological and ethnological
methods. Language, again, has often spread along lines other than those
of race, and its investigation appertains to the sphere of the
philologist. Material civilisation, too, has not of necessity followed
the lines either of racial or of linguistic development, and the search
for its ancient trade-routes may be safely left to the archaeologist.
Similarly the spread of ideas in religion and thought is one which has
advanced on lines of its own, and its investigation must be conducted by
the methods and along the lines of the comparative study of religions.

In the wide sense, then, in which the word 'Celtic religion' will be used
in this work, it will cover the modes of religious thought prevalent in
the countries and districts, which, in course of time, were mainly
characterised by their Celtic speech. To the sum-total of these
religious ideas contributions have been made from many sources. It would
be rash to affirm that the various streams of Aryan Celtic conquest made