Historical Linguistics And Descriptive Linguistics are two main schools of linguistics.You must know the difference between them.First You need to understand about the term philology in linguistics in order to understand the concept of comparative And Descriptive Linguistics.
Philology, which has its origin in the Greek word philologia, meaning study of words, is the name given by the nineteenth century linguists to that branch of knowledge which makes a scientific study of speech sounds, etymology, language change, grammar and the relationship between the collateral languages. The classical scholars were chiefly concerned with their own languages. The eighteenth century linguists made a comparative study of languages, traced their origin and developed the comparative method and the method of internal reconstruction to ascertain the relationship between sister languages and discover the etymon (the primary form) of a word. In the words of R.H. Robins,
In British usage philology is generally equivalent to comparative philology, and older and still quite common term for what linguists technically refer to as comparative and historical linguistics.
Difference Between Historical and Comparative Linguistics.
Historical and Comparative Linguistics is the modem name of philology. Some American linguists use the name Historical Linguistics. Ferdinand de Saussure, father of modern linguistics, made distinction between the study of language in its historical development and in its current form or language at a given time. He termed them diachrony and synchrony. Historical linguistics is primarily concerned with the development of language from its known beginning to the present time. Its method of investigation is comparative method and internal reconstruction.
Since the main aim of historical linguistics is to compare different languages at the sound, word and syntactic levels and trace their source language and the method is comparative, most of the linguists use the name Historical and Comparative Linguistics. In the beginning it established relations between different languages (say English, German, Danish and Norwegian) stemming from the same branch. It proved scientifically that all the languages of Europe, Middle East and North India have their root in Proto-Indo-European language. The main areas of study in historical linguistics are genealogical and typological classifications of languages, language change, writing systems, dialect geography and borrowing of words.
Descriptive linguistics is the American term for Saussure’s synchrony. According to Saussure language can be studied synchronically and diachronically. Synchrony is concerned with the description of language at a point of time. It is more important because it is on the basis of its findings that a systematic study of language development can be made. W.P. Lehmann says:
The success of historical linguistic study depends on the state of descriptive linguistics.*
The great drawback of the nineteenth century philologists was that they ignored (but we cannot blame them for it) the synchronic study of language and insisted more, as Bloomfield has pointed out, on psychological interpretation. They missed the fact that the old form of a language can be studied in relation to the present form. Synchrony precedes diachrony; historical linguistics is of great value but descriptive linguistics is its foundation.