根据以下内容回答题： Many things about language are a mystery，and many will always remain so.But sonethings we do know． First，we know that all human beings have a language of some sort．Thero is no race of men anvwhere on earth so backward that it has no language，no set of speech souBds by whieh the people communicate with one another．Furthermore，in historical times，there has never been a race of men without a language． Second．there is no such thing as a primitive language．There are many people whose cal-tures are undeveloped，who are，as we say，uncivilized，but the languages they speak are not Drimitive．In all known languages we can see complexities that must have been，tens of thou-sands of years in developing. This has not always been well understood；indeed，the direct contrary has often been stated．Popular ideas of the language of the American Indians will illustrate．Many people have supposed that the Indians communicated in a very primitive system of noises-Study has proved this to be nonsense．There are，or were，hundreds of American Indian languages，and all of them tum out to be very complicated and very old．They are certainly different from．the lanlguages that most of us are familiar with，but they are no more primitive than English and Greek.A third thing we know about language is that all languages are perfectly adequate．That is，each one is a perfect means of expressing the culture of the people who speak the language.Finallv，we know that language changes．It is natural and normal for language to change；the only languages which do not change are the dead ones．This is easy to understand if we look backward in time．Change goes on in all aspects of language．Grammatical features change as do speech sounds．and changes in vocabulary are sometimes very extensive．and may occur Very rapidly．Vocabulary is the least stable part of any languages. In the second paragraph the author thinks that__________.