>I can't think of any case in which a daugther language feeds back into or
influences the >development of its parent.
Thisis possible if the parent language is fossilised, and continues into the
time when the daughter languages are established. Examples could occur with
Latin, Attic/Koine Greek, Classical Arabic or Sanskrit. This last case is
almost an example of what you suggest. The Prakrits develop from a dialect
very close to that which produces classical Sanskrit (apparently), and since
Sanskrit reaches its literary height a thousand years later, the Prakrits
are alive and kicking, and their influence on it is clear, especially in
vocabulary.
Peter