Passage Summary: Even if everyone starts speaking the same language for business, people in different places will still have their own local ways of talking because they have different things to talk about.

Conclusion: A future universal trade language will inevitably split into various regional dialects.

Reasoning: Dialects arise because local populations have unique communication needs, and these needs will persist even with a global language.

Analysis: The linguist's prediction relies on the assumption that the 'local communicative needs' that caused dialects in the past will continue to exist in a unified global economy. If the global economy made everyone's lives and needs identical, the pressure to create dialects might vanish. To find the necessary assumption, ask what must be true for the conclusion to hold: the argument requires that local needs remain distinct enough to force changes in the universal language.