In the beginning of the 19th century these sporadic hints were worked up in a more systematic way. The chief impulses came about the same time from three men of genius, the Dane, Rasmus Rask, and the two Germans, Franz Bopp and Jacob Grimm. Rask saw the importance of sound laws and formulated the Ger manic consonant shift before Grimm ; he classified not only Aryan, but also Finno-Ugric and Dravidian languages and gave an excellent exposition of the methodological principles of the new science. Bopp started from Sanskrit, but successively submitted a greater and greater number of the Aryan languages to penetrat ing study and wrote the first comprehensive comparative grammar of all these languages—a work which exercised an enormous in fluence on linguistic studies all over Europe. Grimm was chiefly occupied with the Germanic group and brought out a very im portant grammar of all the languages of that group with the main stress laid on historic development. Bopp and Grimm had a great number of followers, chiefly in Germany—of the elder generation only one scholar can be mentioned here : Pott, whose etymological investigations cleared much new ground. Chairs of comparative philology were founded in many universities, and periodicals were devoted to the science, the most important for a long time being Kuhn's Zeitschrift fur vergleichende Sprachforsch ung. The new points of view (comparative and historical) soon affected all branches of linguistics, and a number of special "phi lologies" came into existence with similar objects and methods, Romanic philology headed by Diez, Slavic by Schleicher and Miklosich, Keltic by Zeuss. For the classical languages Georg Curtius's researches were of very great importance.
The chief leader in Aryan philology about 1860 was August Schleicher, whose Compendium der vergleichenden Grammatik (1st ed. 1861) had far-reaching influence, especially through its insistence on regularity of sound-changes and through its bold attempts at reconstructing theoretical forms of primitive Aryan, which he took as the starting points for his explanations of forms actually existing. But the system was too rigid and rectilinear to do justice to the complexity of linguistic life; the whole was vitiated by the dogmatic assumption that primitive Aryan had a very simple structure with only three vowels (i, a, u), and that the whole evolution had taken place in prehistoric times through three periods, a root-period, a period of agglutination, and finally one of inflection, while the historic period had wit nessed only decay.
The most important discovery of that period was the "palatal law," by which certain consonantal peculiarities were used to prove that Sanskrit in prehistoric times had had a vocalic system similar to that of Greek, with e and o in definite cases where Sanskrit as we know it has uniformly a. The belief in the ab solute primitivity of Sanskrit, which had been a dogma in the first period, could thus be maintained no longer, and some feat ures of European languages, which had been supposed to be later developments, were now seen to be remnants of the oldest acces sible form of the Aryan languages. The theory of the vowel alter nation found, for instance, in Greek Xdrco, MXotra, EX troy and still surviving in English drink, drank, drunk (ablaut, gradation or apophony) was completely reversed, the grade lip being now considered a weakening of leip, instead of being taken as the original stage, of which leip was an expansion. Karl Verner dis covered the astonishing fact that differences in Germanic con sonants, like that between German t in V ater and d in Bruder, found their explanation in primeval accent-conditions preserved in their oldest form in Sanskrit only, and that the difference be tween s in English was and r in were was connected with the fact that the singulars of perfect tenses in Sanskrit are stressed on the root, but plurals on the ending. All this opened new vistas and gave fresh impulses to Aryan philology. A codification of Aryan grammar according to the new views was given in Brug mann's Grundriss der vergleichenden Grammatik der indoger rnanischen Sprachen (2nd ed. 1897-1916), while the theoretical basis was worked out in H. Paul's Prinzipien der Sprachgeschichte (4th ed. 1909) and B. DelbrUck's Einleitung in das Sprach studium (5th ed. 1908) ; DelbrUck also wrote the first compre hensive Aryan syntax (Vergleichende Syntax der indoger manischen Sprachen, 1893-1900).