To demonstrate that the child learning language constructs his theory of language on the basis of the linguistic data available to him, this study investigated 21 linguistic parameters that Down's Syndrome and normal children are exposed to in their maternal linguistic environment. It was found that mothers produced certain levels of linguistic parameters more frequently than others. Psychologists and linguists suggested that the same levels emerged earlier than others in the child's language. A possible relation between the frequency of usage of certain grammatical structures by the mother and their order of appearance in the child's language is suggested. It was also found that Down's syndrome children receive a different linguistic input than normal children in terms of frequency of occurrence of certain linguistic parameters. This difference is discussed in terms of later characteristics of Down's syndrome children's different, deviant, and delayed language. (Tables comprise more than half of this document.) (Author/DP)