Can Parent and Daughter co-exist?

[petegray]

Does the whole debate reduce to a matter of definition?

Imagine a situation where a group of spakers of a language go and settle
elsewhere, where substrate and other factors make their language change
swiftly, while those who stayed at home enjoy a very much slower rate of
change.    After some years, and political upheaval, we can see a situation
where the settlers are deemed to be speaking a different language from that
which they brought with them years before.

I suspect that Rich Alderson is saying that the stay-at-homes are also
speaking a different language, by definition;  while some others are saying
that if the changes are few enough, it should be defined as the parent
language.   And of course, both sides are right - which is why there is so
little understanding on both sides.   The debate is ultimately based simply
on our definition of what a single language is.

Peter