References;
Aitchison, J. (Fifth Edition). The Articulate Mammal, An Introduction to Psycholinguistics. London and New York : Routledge Taylor & Francis Group.
Hockett, C.F. (1960). The Origin of Speech, Scientific American 203,88-111.
Victoria Fromkin, R. R. ( Seventh Edition). An Introduction to Language. Boston Massachussetts: Thomson, Wadsworth