This paper proposes a broad outline of a variable model of language development and explores several particulars of such a model in the language behavior of four two-year-old children. The process by which information about language is progressively transformed and integrated rather than merely being added together can be seen in the shifting relations between form and function in the vertical levels of language development. At the same time, knowledge of language proceeds horizontally as the child takes in more than one aspect of the linguistic system being learned. Language development is not compartmentalized with respect to comprehension and production or to phonology, lexicon, pragmatics, semantics and syntax. A study of the co-occurring lexicon, discourse and grammatical complexity variables revealed that these factors influence what children are able to say with different effects: either to facilitate or to constrain sentences. More important, however, is the fact that the variables operate at the same time to influence the child's language behavior. These observations lead to the conclusion that varying lengths of a child's sentences appear to be due to the covariance in different aspects of the child's knowledge of language. (AMH)