Can Parent and Daughter co-exist?

[JoatSimeon at aol.com]

We're dealing with two related, but somewhat distinct, processes of change.

First, there's the inevitable change that a language undergoes even if it
doesn't spread over a wide area, but instead remains confined to one small
enough that dialects don't diverge past the point of mutual comprehensibility.

Eg., Greek, or Basque, or pre-1600 English.

Over time, such a language will change to the point that it's no longer
mutually comprehensible with its predecessor, even if it continues to bear
the same name.

On the other hand, there's the case of a language -- PIE would be the
archetypical example here -- which spreads so far geographically that its
dialects no longer share enough of the _same_ innovations to continue to be
mutually comprehensible.

All the dialects -- including the one occupying the original 'home turf' of
the linguistic family -- change and become different languages eventually;
they just don't become the _same_ successor-language.   As with PIE, or, more
recently, Romance or Germanic.