odified.
The same words in the same form are repeated from generation to
generation, so that lexic and grammatic elements have a life that
changes very slowly. This is especially true where the habitat of the
tribe is unchanged. Migration introduces a potent agency of mutation,
but a new environment impresses its characteristics upon a language more
by a change in the semantic content or meaning of words than by change
in their forms. There is another agency of change of profound influence,
namely, association with other tongues. When peoples are absorbed by
peaceful or militant agencies new materials are brought into their
language, and the affiliation of such matter seems to be the chief
factor in the differentiation of languages within the same stock. In
the presence of opinions that have slowly grown in this direction, the
author is inclined to think that some of the groups herein recognized as
families will ultimately be divided, as the common materials of such
languages, when they are more thoroughly studied, will be seen to have
been borrowed.

In the studies which have been made as preliminary to this paper, I have
had great assistance from Mr. James C. Pilling and Mr. Henry W. Henshaw.
Mr. Pilling began by preparing a list of papers used by me, but his work
has developed until it assumes the proportions of a great bibliographic
research, and already he has published five bibliographies, amounting in
all to about 1,200 pages. He is publishing this bibliographic material
by linguistic families, as classified by myself in this paper. Scholars
in this field of research will find their labors greatly abridged by the
work of Mr. Pilling. Mr. Henshaw began the preparation of the list of
tribes, but his work also has developed into an elaborate system of
research into the synonymy of the North American tribes, and when his
work is published it will constitute a great and valuable contribution
to the subject. The present paper is but a preface to the works of Mr.
Pilling and Mr. Henshaw, and would have been published in form as such
had not their publications assumed such proportions as to preclude it.
And finally, it is needful to say that I could not have found the time
to make this classification, imperfect as it is, except with the aid of
the great labors of the gentlemen mentioned, for they have gathered the
literature and brought it ready to my hand. For the classification
itself, however, I am wholly responsible.