  Current syntactic theory limits the range of grammatical variation so
severely that the logical problem of grammar learning is trivial. Yet, children
exhibit characteristic stages in syntactic development at least through their
sixth year. Rather than positing maturational delays, I suggest that
acquisition difficulties are the result of limitations in manipulating
grammatical representations. I argue that the genesis of complex sentences
reflects increasing generative capacity in the systems generating structural
descriptions: conjoined clauses demand only a regular tree rewriting system;
sentential embedding uses a context-free tree substitution grammar;
modification requires TAG, a mildly context-sensitive system.
