Real-life linguistic systems undergo changes, often on a day-to-day basis. Such changes are most obvious in children, whose linguistic systems undergo rapid development during the first few years, from essentially nothing at all at birth to huge capacity and fluent operation by age five. But adults also acquire new lexical items from time to time, in some cases quite often, as when they undertake learning some new body of knowledge. They also sometimes acquire new syntactic constructions. And so a model of linguistic structure, to be considered realistic, must incorporate the ability to develop and to acquire new capabilities of production and comprehension. This criterion may be called the requirement of developmental plausibility. It provides another easy way to distinguish a theory of linguistic structure from a theory of the outputs of linguistic structure. Valuable as they are for their own purposes, theories of the outputs, are in such a form that there is no plausible avenue that could lead to their development. This statement applies also to some network theories, such as the well-known connectionist theory of Rummelhart and McClellan (1986).
Lamb, Sydney M. "Linguistic structure: A plausible theory" in Language Under Discussion, published online June 2, 2016.