Schleicher claimed that he himself had been convinced of the natural descent and competition of languages before he had read Darwin'sOrigin of Species. He invented a system of language classification that resembled a botanical taxonomy, tracing groups of related languages and arranging them in a genealogical tree.
Schleicher's tree model
His model, the Stammbaumtheorie (family-tree theory), was a major development in the study of Indo-European languages. He first introduced a graphic representation of a Stammbaum in an article published in 1853 entitled Die ersten Spaltungen des indogermanischen Urvolkes. By the time of the publication of his Deutsche Sprache (German language) (1860) he had begun to use trees to illustrate language descent. Schleicher is commonly recognized as the first linguist to portray language development using the figure of a tree. Largely in reaction to this, Johannes Schmidt later proposed his 'Wave Theory' as an alternative model.
Schleicher believed that languages pass through a life cycle, similar to that of living beings. They start simpler than they will become. This state of primitive simplicity was followed by a period of growth, which eventually slowed, and then gave way to a period of decay (1874:4):
As man has developed, so also has his language (...): even the simplest language is the product of a gradual growth: all higher forms of language have come out of simpler ones.... Language declines both in sound and in form.... The transition from the first to the second period is one of slower progress.
Schleicher was an advocate of the polygenesis of languages. He reasoned as follows (1876:2):
To assume one original universal language is impossible; there are rather many original languages: this is a certain result obtained by the comparative treatment of the languages of the world which have lived till now. Since languages are continually dying out, whilst no new ones practically arise, there must have been originally many more languages than at present. The number of original languages was therefore certainly far larger than has been supposed from the still-existing languages.
Schleicher's ideas on polygenesis had long-lasting influence, both directly and via their adoption by the biologist Ernst Haeckel. Ernst Haeckel was a German evolutionist and zoologist known for proposing the gastraea hypothesis.
In 1866, August Leskien, a pioneer of research into sound laws, began studying comparative linguistics under August Schleicher at the University of Jena.
Sprachvergleichende Untersuchungen. / Zur vergleichenden Sprachgeschichte. (2 vols.) Bonn, H. B. Koenig (1848)
Linguistische Untersuchungen. Part 2: Die Sprachen Europas in systematischer Uebersicht. Bonn, H. B. Koenig (1850); new ed. by Konrad Koerner, Amsterdam, John Benjamins (1982)
Formenlehre der kirchenslawischen Sprache. (1852)
Die ersten Spaltungen des indogermanischen Urvolkes. Allgemeine Zeitung fuer Wissenschaft und Literatur (August 1853)
Handbuch der litauischen Sprache. (1st scientific compendium of Lithuanian language) (2 vols.) Weimar, H. Boehlau (1856/57)
Litauische Maerchen, Sprichworte, Raetsel und Lieder. Weimar, H. Boehlau (1857)
Kurzer Abriss der Geschichte der italienischen Sprachen. Rheinisches Museum fuer Philologie 14.329-46. (1859)
Die Deutsche Sprache. Stuttgart, J. G. Cotta (1860); new ed. by Johannes Schmidt, Stuttgart, J. G. Cotta (1888)
Compendium der vergleichenden Grammatik der indogermanischen Sprachen. (Kurzer Abriss der indogermanischen Ursprache, des Altindischen, Altiranischen, Altgriechischen, Altitalischen, Altkeltischen, Altslawischen, Litauischen und Altdeutschen.) (2 vols.) Weimar, H. Boehlau (1861/62); reprinted by Minerva GmbH, Wissenschaftlicher Verlag, ISBN3-8102-1071-4
Die Darwinsche Theorie und die Sprachwissenschaft – offenes Sendschreiben an Herrn Dr. Ernst Haeckel. Weimar, H. Boehlau (1863)
Die Bedeutung der Sprache für die Naturgeschichte des Menschen. Weimar, H. Boehlau (1865)
Darwinism Tested by the Science of Language. (Transl. by Alexander V. W. Bikkers) London, J. C. Hotten (1869)
A Compendium of the Comparative Grammar of the Indo-European, Sanskrit, Greek, and Latin Languages, translated from the third German ion by Herbert Bendall. London: Trübner and Co (1874) (Actually an abridgement of the German original.)
Laut- und Formenlehre der polabischen Sprache. reprinted by Saendig Reprint Verlag H. R. Wohlwend, ISBN3-253-01908-X
Sprachvergleichende Untersuchungen. reprinted by Minerva GmbH, Wissenschaftlicher Verlag, ISBN3-8102-1072-2
Die Formenlehre der kirchenslavischen Sprache erklaerend und vergleichend dargestellt. Reprint by H. Buske Verlag, Hamburg (1998), ISBN3-87118-540-X
^E. F. K. Koerner, Practicing Linguistic Historiography, John Benjamins Publishing Company, 1989, p. 193: "Schleicher historicism ... was in effect radicalized by the Neogrammarians."
^ abHadumod Bussmann, Routledge Dictionary of Language and Linguistics, Routledge, 1996, p. 85.