Languages change in ways that make old sounds into new sounds and words into grammar, and they shift in different directions, so that eventually there are languages different as German and Japanese. At all times, any language is gradually on its way to changing into a new one; the language that is not gradually turning upside-down is one on the verge of extinction. This kind of change is so relentless that it even creates "languages within languages." In separate populations who speak the same language, changes differ. The result is variations upon the language-that is, dialects. Often one dialect is chosen as the standard one, and when it is used in writing, it changes more slowly than the ones that are mostly just spoken. because the permanency of writing has an official look that makes change seem suspicious. But the dialects that are mostly just spoken keep on changing at a more normal pace. Then, the languages of the world tend to mix together on various levels. All languages borrow words from one another; there is no "pure" vocabulary. But some borrow so much vocabulary that there is little original material left, such as in English. And meanwhile, languages spoken alongside one another also trade grammar, coming to look alike the way married couples sometimes do. Some languages are even direct crosses between one language and another, two Languages having "reproduced" along the lines of mitosis. There are several factors which make languages behave like human beings. The circumstances and situation that lead languages to change and adapt are the changing times, new requirements, societal construction and negotiations of identity, power, prestige, gender, age, occupation and the like.
INTRODUCTION TO LINGUISTICS & PHONETICS – LANGUAGE CHANGE