The study of the verbal behavior of speaker and listener, as well as of the practices of the verbal environment which generates such behavior, may not contribute directly to historical or descriptive linguistics, but it is enough for our present purposes to be able to say that a verbal environment could have arisen from nonverbal sources and, in its transmission from generation to generation, would have been subject to influences which might account for the multiplication of forms and controlling relations and the increasing effectiveness of verbal behavior as a whole. (p. 470)
