Along with this identity of plan, there coëxists the utmost independence of expression. An American language is usually perfectly transparent. Nothing is easier than to reduce it to its ultimate elements, its fundamental radicals. These are few in numbers and interjectional in character. The Athapascan, the Algonkin, whose wide extension I have referred to, have been reduced to half a dozen particles or sounds expressive of the simplest conceptions.[23] Upon these, by combination, repetition, imitation and other such processes, the astonishing structure of the tongue has been erected, every portion of it displaying the mechanism of its origin. It is this transparency which renders these tongues so attractive to the philosophic student of human expression, and so valuable to him who would obtain from them the record of the progress of the nation. A thorough study of such a language would embrace its material, its formal and its psychologic contents. Its material elements include the peculiarities of its vocabulary: for example, its numerals and the system they indicate, its words for weights and measures, for color and direction, for relations of consanguinity and affinity, for articles of use and ornament, for social and domestic conditions, and the like. Few studies of American languages go beyond this material or lexicographic limit; but in truth these are merely the externalities of a tongue, and have nothing to do with linguistic science proper. This concerns itself with the forms of the language, with the relation of parts of speech to each other and to the sentence, and with the historical development of the grammatical categories. Beyond this, again, is the determination of the psychical character of the tribe through the forms instinctively adopted for the expression of its thoughts, and reciprocally the reaction exerted by these forms on the later intellectual growth of those who were taught them as their only means of articulate expression. These are data of the highest value in the study of prehistoric time; but so far as America is concerned, I could name very few scholars who have pursued this promising line of research.