Language contacts : two models of the XlXth century and their offsprings nowadays. The genealogical tree theory for indo-european languages (Schleicher) and the waves theory (Schmidt) are shortly exposed within their own historical frame, and the difficulties of the emergence of a theory of language contacts, in spite of Schuchardt, is underlined. The effect of this historical past on the evolution of the issues of contact and of bilingualism within linguistics and in ideological discourses about language, and especially about State language, is described in the second part of the paper.La théorie de l'arbre généalogique des langues indoeuropéennes (Schleicher) et la théorie des ondes (Schmidt) sont brièvement exposées dans leur contexte h...