August Schleicher (February 19, 1821 - December 6, 1868) was a German linguist. His great work was A Compendium of the Comparative Grammar of the Indo-European Languages, in which he attempted to reconstruct the Proto-Indo-European language.

August Schleicher was born in Meiningen. He began his career studying theology and Indo-European, especially Slavic languages. Influenced by Hegel, he formed the theory that a language is an organism, with periods of development, maturity, and decline. In 1850 Schleicher completed a monograph systematically describing the languages of Europe, Die Sprachen Europas in systematischer Übersicht (The languages of Europe in systematic perspective). He explicitly represented languages as perfectly natural organisms that could most conveniently be described using terms drawn from biology e.g., genus, species, and variety.

Schleicher claimed that he himself had been convinced of the natural descent and competition of languages before he had read Darwin’s Origin 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. 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 articles published in 1853. 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.. August Schleicher died from tuberculosis in age of 47 in Jena.

Language evolution and the comparative method
Languages change over time. Historical linguists construct family trees, an idea pioneered by the 19th century historical linguist August Schleicher. The basis for the trees is the comparative method: languages presumed to be related are compared with one another, and linguists look for regular sound correspondences based on what is generally known about how languages can change, and use them to reconstruct the best hypothesis about the nature of the common ancestor language from which the attested languages are descended.

Use of the comparative method is validated by its application to languages whose common ancestor is known. Thus, when the method is applied to the Romance languages (which include French, Spanish, Portuguese, Italian, and Romanian), the reconstructed common ancestor language comes out rather similar to Latin - not the classical Latin of Horace and Cicero, but Vulgar Latin, the colloquial Latin spoken in various dialects in the late Roman Empire.
The comparative method can be used to reconstruct languages for which no written records exist, either because none were preserved or because the speakers were illiterate. Thus, the Germanic languages (which include German, Dutch, English, Norwegian, Swedish, Danish, Faroese, Icelandic, Yiddish, and the extinct Gothic) can be compared to reconstruct Proto-Germanic, a language that was probably contemporaneous with Latin and for which no records are preserved.
Germanic and Latin (more precisely, Proto-Italic, the ancestor of Latin and a few of its neighbors) are themselves related, being co-descended from Proto-Indo-European, spoken perhaps 5000 years ago. Scholars have reconstructed Proto-Indo-European on the basis of data from its nine surviving daughter branches, which are: Germanic, Italic, Celtic, Greek, Baltic, Slavic, Albanian, Armenian, Indo-Iranian, and from the two dead branches Tocharian and Anatolian.
The comparative method is used to distinguish true linguistic descent - that is, the passing of a language from parents to children, down through the generations - from accidental resemblance due to cultural contact.

The comparative method has been successfully used to reconstruct some very large language families, notably Austronesian (which includes Hawaiian, Tagalog, Javanese, and Malagasy) and Niger-Congo (the majority of the languages of modern Africa). Once the various changes in the daughter branches have been worked out, and a fair amount of the core vocabulary and grammar of the protolanguage are understood, then scholars will quite generally agree that a relationship of genetic relatedness has been proven.]

Non-comparative method theories
Vastly more controversial are hypotheses about relatedness which are not supported by application of the comparative method. Scholars who attempt to probe deeper than the comparative method supports (for example, by tabulating similarities found by mass lexical comparison without setting up sound correspondences) are often accused of scholarly wishful thinking. The problem is that any two languages have a huge number of opportunities to resemble one another just by accident, so merely pointing out isolated resemblances has little evidentiary value. However, by ignoring known historical changes in the languages, mass lexical comparison incorporates known randomness, and therefore appears to be willfully inaccurate.

The ultimate in remote reconstruction is the recovery of a Proto-World language. Not all scholars believe that such a language even necessarily existed. Moreover, it is difficult to reconcile Proto-World with what we know about prehistory. Joseph Greenberg has suggested that people coming out of northeast Africa around 50,000 BC spoke Proto-World. But that would violate the claim that no relationships would be recognizable after 10,000 years; if that figure is accurate, then if all languages are observably related, such a relationship must have somehow formed more recently.
Schleicher argued that human beings, in their acquisition of language, went through three periods of developement: a pre-linguistic period, a pre-historical period of language emergence and developement, and then a historical period of language decline.

Schleicher is generally regarded as totally superseded. Since he flourished immediately before the neogrammarians, it is scarcely remarkable that their reputation has eclipsed his. Probably the most commonly maintained segment of his writings is his model for displaying languages, the family tree, though it too is held to be superseded by other interpretations of language spread and interrelationships, such as the wave theory. In part Schleicher seems supplanted because so many of his ideas were taken over by his successors.
The reconstructed form of a proto-language, rather than the earliest known form of a selected language which has developed from it, is now the accepted way of indicating linguistic relationships. In Indo-European linguistics Schleicher broke the practice of citing Sanskrit for this purpose and introduced the starred form.
But possibly the most important influence he has had is that on the neogrammarians-his aim (credited to them) to account for relationships to the extent possible and then to admit residues. In his brief sketch of the history of Indo-European linguistics.

Schleicher did not sharply formulate a need for complete accounting for phenomena and explanation of residues.
Schleicher looked on language as a whole. His introduction states specifically that linguists should deal with the construction of sentences-a statement found again in the excerpt from Sievers below; but Schleicher also admits that he cannot handle sentences adequately, and hence he confines himself to sounds and forms. This limitation was largely maintained for the next eighty years, with the emphasis on phonology that Schleicher introduced.
He also attempted to get away from the detail of language to its form, using for this purpose formulae. In this attempt he foreshadows the repeated efforts to increase rigor in linguistics. His formulae for descriptive linguistics, were not maintained though his reconstructed-or fundamental- forms have been. It should be noted that Schleicher looked on these as abstractions, not as real language material; he says specifically that he does not assert they ever existed.

The grammar of the Indo-European languages is therefore a special grammar: because it treats of these languages as products of growth, and exhibits their earlier and earliest gradations, and would therefore be more accurately called a special historical grammar of Indo-European language.
Schleichert believed that all the languages are derived from one original-language – Indo-European Proto language.
Within this Indo-European class of speech however certain languages geographically allied point themselves out as more closely related to one another: thus the Indo-European speech-stem falls into three groups or divisions.

These are:
1. The Asiatic or Aryan division, comprising the Indian, Iranian (or more correctly Eranian), families of speech, very closely allied to one another. The oldest representative and fundamental-language of the Indo-European family, and generally the oldest known Indo-European language, is the Old Indian, the language of the oldest portion of the Vedas; later on, after it had become fixed in a more simplified form, and subject to certain rules, as a correct written language, in opposition to the peoples' dialects, called Sanskrit.
2. The south-west European division, composed of the Greek, next to which we must perhaps place the Albanian, preserved to us only in a later form; Italian (the oldest known forms of this language are the Latin-especially important for us is the Old-Latin, as it was before the introduction of the correct literary language formed under Greek influence-the Umbrian and the Oscan), Keltic, of which family the best known, though already highly decomposed, language is the Old Irish. Italian and Keltic have more in common with one another than with the Greek.
3. The North-European division, composed of the Sclavonic family with its closely-allied Lithuanian,-the most important language for us of this group,-and the German, widely separated from both. The oldest forms of this division are the Old-Bulgarian; the Lithuanian (and of course the High-Lithuanian, South-Lithuanian, Prussian Lithuanian), first known to us 300 years ago, but clearly of far greater antiquity; and the Gothic from the fourth century. Beside the Gothic, however, are the oldest representatives of German and Norse, Old High German, and Old Norse, which we may bring forward when they present earlier forms than Gothic.