The continuity and similarity of the course of linguistic history in all its stages, and the competency of linguistic correspondences, wherever we find them, to prove unity of origin and community of tradition, are truths which we need to bear in mind as we proceed with our inquiries into language. If we meet in different tongues with words which are clearly the same word, notwithstanding differences of form and meaning which they may exhibit, we cannot help concluding that they are common representatives of a single original, once formed and adopted by a single community, and that from this they have come down by the ordinary and still subsisting processes of linguistic tradition, which always and everywhere involve liability to alteration in outer shape and inner content. It is true that there are found in language accidental resemblances between words of wholly different origin: of such we shall have to take more particular notice in a later lecture (the tenth): but exceptions like these do not make void the rule; the possibility of their occurrence only imposes upon the etymologist the necessity of greater care and circumspection in his comparisons, of studying more thoroughly the history of the words with which he has to deal. It is also true that real historical correspondences may exist between isolated words in two languages without implying the original identity of those languages, or anything more than a borrowing by the one out of the stores of expression belonging to the other. Our own tongue, for instance, aside from its wholesale composition out of the tongues of two different races, draws more or less of its material from nearly every one of the languages of Europe, and from not a few of those of Asia, Africa, and America. Yet it is evident that such borrowing has its limits, both of degree and of kind, and that it may be within the power of the linguistic student readily to distinguish its results from the effects of a genuine community of linguistic tradition.
The method by which we are to proceed in grouping and classifying the languages spoken by mankind, now and in former times, results with necessary consequence from the principles which we have laid down. We have seen that no given form of speech remains permanently the same: each
