Current, rule-based theories of grammar do not provide much
insight into how languages can develop new behaviors over
time. Yet, textual data indicate that languages usually evolve
new grammatical patterns by gradually extending existing ones.
I show how a grammar model that is sensitive to prototype structure can model innovation as a process of extrapolation along
salient dimensions of the category clusters. A Connectionist network provides a usefully interpretable implementation.
Confirming evidence comes from a study of the development
of English be going to as a marker of future tense.
