Passage Summary: New research proves pronunciation shifts happen suddenly, which ruins the old 'classical' theory. Because the classical theory is wrong, the author claims all theories about sound change must be wrong.

Conclusion: The entire field of sound-change theory must be abandoned.

Reasoning: Recent evidence shows sound changes happen quickly rather than gradually, which invalidates the classical account of sound change.

Analysis: The argument suffers from a significant 'part-to-whole' gap. The author discovers a flaw in one specific version of a theory (the classical account) and concludes that the entire category of sound-change theory is therefore doomed. To bridge this gap and guarantee the conclusion, we need an assumption that links the survival of the classical account to the survival of the field as a whole. Look for an answer that suggests the classical account is the only possible foundation for sound-change theory.