In speech, as in everything else, the forest Indian is confined within the narrow limits of his immediate surroundings. Unlike the nomadic Indian of the plains, he passes his entire existence in an area little larger than an English parish. He has almost no commercial dealings with his neighbours. The only fresh blood that penetrates his tribe is brought in by the immature children taken prisoners in war. Like the landscape his imagination owns no perspective, no horizon. In the Amazonian bush an Indian may live and die without ever having gazed upon a terrestrial object at the distance of a mile. His mode of life, a community within a single house, under a single roof, makes of household words a dialect, and with the passing of a generation makes that dialect a language.

In a society where each tribe is complete in itself and at deadly enmity with all its neighbours, and where writing is unknown, language must naturally undergo very rapid, very definite change. Moreover Indians will not voluntarily speak the language of other Indians. Thus the Amazonian natives use no common tongue, and there is little in the vocabularies so far collected to explain either the origin or the relationship of the existing dialects. Tribes divided by the breadth of a narrow river speak languages that are mutually unintelligible.