“The importance of the concept of a language structure is that in any language, but also during development this structure follows rules. In other words, language and language development is not without a goal. There is an end state in development, which is described and explained with the aid of linguistic theory. Language development, i.e. the change from no language to that end state, is not a proliferation of change that is adrift, but a series of learning events in time that is rule guided, although these rules do not need to be innate or explicitly learned.”
From: Going the distance: A Non-Linear Approach To Change In Language Development. H.G. Ruhland. Groningen, 1998.