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Meaning of family-tree theory

fam'i•ly-tree' the"ory

Pronunciation: (fam'u-lē trē', fam'lē-), [key]
— Historical Ling. Historical Ling.
  1. a theory that describes language change in terms of genetically related languages developing in successive splits from a common parent language, such as Indo-European, as depicted by a family tree diagram. Cf. wave theory.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.