In 1905 Antoine Meillet (1905, iii) prefaced the French translation of Brugmann's Kurze vergleichende Grammatik der indogermanischen Sprachen an account of the current state of affairs in comparative grammar (a term which clearly included historical and comparative linguistics). In spite of its length, it is worth quoting because better than any more modern account it gives a view of the type of work done and of the spread that the subject had reached.
We should not assume that Brugmann's doctrines are those of a small school: Brugmann is not a theoretician with original and personal views of his own. Also, strictly speaking, at present there are not two schools of comparative grammar. No doubt the different groups of linguists differ in the way of presenting the facts, not everyone is interested in the same matters, and the ways in which things are explained differ from one to the other. The students of Johannes Schmidt (especially W. Schulze, Kretschmer) are particularly concerned with keeping as close as possible to the facts that are philologically documented, to the material detail of things; the Göttingen group - Fick, Bezzenberger, Hoffmann, Prellwitz. Bechtel, Collitz, etc. - is more interested in etymology and consequently is more prone to relax its phonetic rigour; the Leipzig group: Brugmann, Osthoff, Leskien, Paul, G. Meyer, Hiibschmann, Thurneysen, Kluge, Bartholomae, Stolz - to quote only the first generation - was characterized by the importance attributed to analogy and by its wish to set up general laws. Consequently from this group have emerged most of the manuals which have multiplied in recent years, beginning with the Greek grammar by G. Meyer, and continuing with the works of Sommer, Berneker, Streitberg, Hirt, etc.; some of Brugmann's most remarkable pupils, Streitberg and Hirt, have started sophisticated enquiries on the accent and vocalism and have built up a whole system of complex hypotheses. In Russia Fortunatov's teaching has created a small group which has its special form of notation and its own way of working; in France Bréal's influence has partly oriented the research towards questions of meaning, and, on the other hand, Ferdinand de Saussure's systematic and rigorous doctrines have left a deep impression on a whole group. The Swedish scholars, like Johansson, Persson, Lidén, have dedicated themselves above all to finding new etymologies as the Göttingen group, but independently from them. There are other scholars, like the famous Thomsen and more recently Pedersen in Denmark, Wackernagel, Solmsen, Zupitza in Germany, V. Henry in France, Zubaty in Bohemia, Bugge and Torp in Norway, Danielsson in Sweden, who could not be easily classified in any group. But all these various scholars are only separated by nuances; all agree on the fundamental principles, all argue in the same way and in the essential points they all reach the same conclusions.