With reference to the growth of American languages, it was Humboldt’s view that they manifest the utmost refractoriness both to external influence and to internal modifications. They reveal a marvellous tenacity of traditional words and forms, not only in dialects, but even in particular classes of the community, men having different expressions from women, the old from the young, the higher from the lower classes. These are maintained with scrupulous exactitude through generations, and except by the introduction of words, three centuries of daily commingling with the white race, have not at all altered the grammer[TN-2] and scarcely the phonetics of many of their languages. Nor is this referable to the contrast between an Aryan and an American language. The same immiscibility is shown between themselves. “Even where many radically different languages are located closely together, as in Mexico, I have not found a single example where one exercised a constructive or formative influence on the other. But it is by the encounter of great and contrasted differences that languages gain strength, riches, and completeness. Only thus are the perceptive powers, the imagination and the feelings impelled to enrich and extend the means of expression, which, if left to the labors of the understanding alone, are liable to be but meagre and arid.”[16-†] § 9. INTERNAL FORM OF LANGUAGES. Besides the grammatical form of a language, Humboldt recognized another which he called its _internal form_. This is that subtle something not expressed in words, which even more than the formal parts of speech, reveals the linguistic genius of a nation. It may be defined as the impression which the language bears of the clearness of the conceptions of those speaking it, and of their native gift of speech. He illustrates it by instancing the absence of a developed mode in Sanscrit, and maintains that in the creators of that tongue the conception of modality was never truly felt and distinguished from tense. In this respect its inner form was greatly inferior to the Greek, in the mind of which nation the ideally perfect construction of the verb unfolded itself with far more clearness. The study of this inner form of a language belongs to the highest realm of linguistic investigation, and is that which throws the most light on the national character and capacities.[17-*] § 10. CRITERIA OF RANK IN LANGUAGES.