[An-lang] Proto-Dialect chains

[William H. Baxter]

Can anyone help me find the intellectual pedigree (whether within
linguistics or from elsewhere) of the principle that subgroups should
be defined by shared innovations?  I think it's a fine principle, but I
am puzzled by the fact that some historical linguists take it as a
basic assumption (and don't necessarily remember where they got the
idea from), while others seem to be unaware of it, or at least fail to
apply it.  There seems to be a good deal of variation from subfield to
subfield.

Specifically, where did this idea originate (in biology, I suspect),
who was first to introduce it to linguistics, where, and under what
circumstances?

Bill Baxter
University of Michigan

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