In the 19th century, following up on Jones’s discovery, language scholars began to develop the study of comparative grammar.
1. Scholars, particularly in Germany, began to propose lines of descent among different languages, introducing the metaphor of “the language tree” (one proto-language splitting into various daughter languages, some of those then splitting again into further languages). The authorship of the theory of “the language tree” belongs to A. Schleicher, due to whom all languages originate from one proto-language, forming a language type (tree) divided into language families (branches).
The theory was strongly criticized by F.Shmidt refuting its main theses about a common proto-language and chance of its reconstruction as integrity. Due to him, relation of languages is based upon their geographical closeness, thus every language is a transitional link from one to another neighbouring language. The theory got the name “The Theory of Waves”. Shmidt’s scientific views got application in studies of Indo-European dialects by means of the Linguistic Geography Method.
2. The comparative method got developed out of many attempts to reconstruct the proto-language which Jones had hypothesized about, known as Proto-Indo-European. The first attempt to analyze the relationships between the Indo-European languages was made by the German linguist Franz Bopp in 1816. Professor of Oriental literature and general philology at the University of Berlin (1821–67), Bopp published a Sanskrit grammar (1827) and a Sanskrit and Latin glossary (1830). He sought to trace the common origin of Sanskrit, Persian, Greek, Latin, and German, a task never before attempted. The task which Bopp endeavoured to carry out in his Comparative Grammar was threefold - to give a description of the original grammatical structure of the languages as deduced from their intercomparison, to trace their phonetic laws, and to investigate the origin of their grammatical forms.
3. In 1818, the Danish philologist Rasmus Rask developed the principle of regular sound changes to explain his observations of similarities between individual words in the Germanic languages and their cognates in Greek and Latin. Rask was the first to indicate that the Celtic languages, which include Breton, Welsh, and Irish, belong to the Indo-European family and also stated that Basque and Finno-Ugric do not. He established the relationship of Old Norse to Gothic and of Lithuanian to Slavic, Greek, and Latin.
It was another German, Jacob Grimm - better known for his Fairy Tales - who formulated the law of consonants correspondence in older Indo-European, and Low Saxon and High German languages. The main merit of Grimm was that he ascertained the so called sound law (later – Grimm’s law) stating an internal connection between acts of general shift in languages development (voiced → voiceless→fricatives→ voiced and so on (by circle):
4. Both Rask and Grimm were unable to explain apparent exceptions to the sound laws that they had discovered. It was in 1875 that a Danish scholar, Karl Vernerz:\wikiKarl_Verner, made a methodological breakthrough when he formulated the sound law which now bears his name, and which was the first sound law to use comparative evidence to show that a phonological change in one phoneme could depend on other factors within the same word, such as the neighbouring phonemes and the position of the accent: in other words, the modern concept of conditioning environments.