August Schleicher in the context of "Comparative method"

In historical linguistics, the tree model (also Stammbaum, genetic, or cladistic model) is a model of the evolution of languages analogous to the concept of a family tree, particularly a phylogenetic tree in the biological evolution of species. As with species, each language is assumed to have evolved from a single parent or "mother" language, with languages that share a common ancestor belonging to the same language family.

Popularized by the German linguist August Schleicher in 1853, the tree model has been a common method of describing genetic relationships between languages since the first attempts to do so. It is central to the field of comparative linguistics, which involves using evidence from known languages and observed rules of language feature evolution to identify and describe the hypothetical proto-languages ancestral to each language family, such as Proto-Indo-European and the Indo-European languages. However, this is largely a theoretical, qualitative pursuit, and linguists have always emphasized the inherent limitations of the tree model due to the large role played by horizontal transmission in language evolution, ranging from loanwords to creole languages that have multiple mother languages. The wave model was developed in 1872 by Schleicher's student Johannes Schmidt as an alternative to the tree model that incorporates horizontal transmission.

Indo-European studies (German: Indogermanistik) is an interdisciplinary field that examines the Indo-European languages and related cultural history through historical linguistics, comparative philology, archaeology, and genetics.

The discipline coalesced in late eighteenth- and nineteenth-century Europe as comparative linguists including Marcus Zuerius van Boxhorn, Franz Bopp, and August Schleicher refined methods for reconstructing a shared ancestry among the languages. Research centers on rebuilding the Proto-Indo-European language, homeland, and society by combining comparative analysis with archaeological, mythological, and genetic evidence to test models such as the Kurgan hypothesis. Dedicated programs, research centers, journals, and book series sustain Indo-European studies across Europe and North America.

Structural linguistics, or structuralism, in linguistics, denotes schools or theories in which language is conceived as a self-contained, self-regulating semiotic system whose elements are defined by their relationship to other elements within the system. It is derived from the work of Swiss linguist Ferdinand de Saussure and is part of the overall approach of structuralism. Saussure's Course in General Linguistics, published posthumously in 1916, stressed examining language as a dynamic system of interconnected units. Saussure is also known for introducing several basic dimensions of semiotic analysis that are still important today. Two of these are his key methods of syntagmatic and paradigmatic analysis, which define units syntactically and lexically, respectively, according to their contrast with the other units in the system. Other key features of structuralism are the focus on systematic phenomena, the primacy of an idealized form over actual speech data, the priority of linguistic form over meaning, the marginalization of written language, and the connection of linguistic structure to broader social, behavioral, or cognitive phenomena.

Structuralism as a term, however, was not used by Saussure, who called the approach semiology. The term structuralism is derived from sociologist Émile Durkheim's anti-Darwinian modification of Herbert Spencer's organic analogy which draws a parallel between social structures and the organs of an organism which have different functions or purposes. Similar analogies and metaphors were used in the historical-comparative linguistics that Saussure was part of. Saussure himself made a modification of August Schleicher's language–species analogy, based on William Dwight Whitney's critical writings, to turn focus to the internal elements of the language organism, or system. Nonetheless, structural linguistics became mainly associated with Saussure's notion of language as a dual interactive system of symbols and concepts. The term structuralism was adopted to linguistics after Saussure's death by the Prague school linguists Roman Jakobson and Nikolai Trubetzkoy; while the term structural linguistics was coined by Louis Hjelmslev.

Schleicher’s fable is a text composed as a reconstructed version of the Proto-Indo-European (PIE) language, published by August Schleicher in 1868. Schleicher was the first scholar to compose a text in PIE. The fable is entitled *Avis akvāsas ka (‘The Sheep [Ewe] and the Horses’). At later dates, various scholars have published revised versions of Schleicher’s fable, as the idea of how PIE should be presented and pronounced has changed over time. The resulting parallel texts serve as an illustration of the significant changes that the reconstruction of the language has experienced during the last 150 years of scholarly efforts.

The first revision of Schleicher’s fable was made by Hermann Hirt (published by Arntz in 1939). A second revision was published by Winfred Lehmann and Ladislav Zgusta in 1979. Another version by Douglas Q. Adams appeared in the Encyclopedia of Indo-European Culture (1997:501). In 2007 Frederik Kortlandt published yet another version on his internet homepage.