Linguistics is the scientific study of language. Synchronic linguistics studies a language's form at a fixed time in history, past or present. Diachronic, or historical, linguistics, on the other hand, investigates the way a language changes over time. The origin, growth and development of language is an important event in the life of the human race. Linguistics deals with this event.
The diachronic or historical linguistics is chiefly concerned with the growth of various languages or language as a whole through different periods of history. It divides this growth into different periods of progress, and describes them in its own manner. The various linguistic changes— phonological, grammatical and semantic—that have occurred in the past, are recorded in it. Just as history studies past events and happenings to throw light on the present social and political conditions, so linguistics studies changes and occurrences in language in the past so as to throw a light on, or account for, its present state. History is concerned with human beings; and so is language and linguistics. Thus, Linguistics and History are similar and thus become Historical Linguistics. It also forms a Typology – the classification of languages into different types; to find out how languages have developed; three areas of Comparative Historical Linguistics are of interest: Language Changes, Language Borrowings and Establishment of Language Families. The purpose of Historical linguistics has been summed by Saussure:
“Describe and trace the history of all observable languages and finding their families. To determine the forces at work in languages and deduce and the general laws to which all specific historical phenomenon can be reduced (language universals)”
Change is the law of Nature. Everything that exists on this earth, including human life and society, changes. Language changes because the society in which it is used, changes. Language is never static or stagnant. It is always in a state of flux which involves change. Change is inevitable in language but language changes are frequent, gradual, and often abrupt. What are the causes of change? The individual as well as society play a part in language change. The speech habits of one generation are based on those of the earlier one, and a change is likely to occur during the course of the acquisition of these habits by others. The rise of new concepts and discover) of new objects cause changes in the vocabulary, structure and sounds of a language. Geographical conditions also affect changes in the sound of a language. Language changes because new concepts and discoveries are born, a huge migration takes place, a prestige is required so language is molded to suit new trends and also language changes because man is accustomed to least effort in speaking. There are usually five types of changes in language: phonological, morphological, syntactical, semantic and lexical.
Phonological Change: One of the major sound changes in the history of the English language is the so-called Great Vowel Shift. In Middle English, spoken from 1100 to 1500, the word house was pronounced with the vowel sound of the modern English word boot, while boot was pronounced with the vowel sound of the modern English boat. The change that affected the pronunciation of house also affected the vowels of mouse, louse, and mouth. This illustrates an important principle of sound change. Morphological Change: An ongoing morphological change in English is the loss of the distinction between the nominative, or subject, form who and the accusative, or object, form whom. English speakers use both the who and whom forms for the object of a sentence, saying both “Who did you see?” and “Whom did you see?” The modern English word ‘stone’ has only three additional forms: the genitive singular ‘stone's’, the plural ‘stones’, and the genitive plural stones'. All three have the same pronunciation. In Old English they were different: stan, stanes, stanas, and stana, respectively. In addition, after certain prepositions, as in under stanum (under stones). Syntactic Change: In modern English, the basic word order is subject-verb-object, as in the sentence “I know John.” The only other possible word order is object-subject-verb, as in “John I know (but Mary I don't).” Old English, by contrast, allowed all possible word order permutations, including subject-object-verb, as in Gif hie ænigne feld secan wolden, literally meaning “If they any field to seek wished.” Semantic and Lexical Change: In Middle English, the word nice usually had the meaning “foolish,” and sometimes “shy,” but never the modern meaning “pleasant.” Change in the meanings of words is semantic and can be viewed as part of the more general phenomenon of lexical change, or change in a language's vocabulary.
Historical Linguistics is the study of how languages change over time and the relationship among different languages. Historical Linguistics studies the process of language change, the ‘genetic’ relationship between languages and how best to classify languages into groups. Using biological analogy, the linguist studies that languages are genetically related and are called a Language Family.