Where Gaelic is spoken: languages of the UK.
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This article, describing the evolution of the Welsh language in particular and that of the other Celtic tongues of the British Isles and Spain by way of background, illustrates this principle. Almost by historical parallel with the Latin spoken by the Romans who occupied Britain at the beginning of this narrative, native classical British was once widely spoken throughout what are now England, Wales and much of Scotland and Ireland. Then came fragmentation.
Even as Latin fragmented with the Roman empire, splitting into local dialects that eventually became Spanish, Portuguese, French, Catalan, Italian and Romanian (in addition to some others now extinct or moribund), British was fractured by waves of invaders, raiders and conquerors that divided the Celts into geographically isolated salients. As with biological evolution, isolation led to divergence: In different locations, classical British became Pictish, Breton, Cornish, Manx, and the principal tongues that have survived into modernity: Welsh, Gaelic and Celtiberian. Also as with its biological counterpart, linguistic evolution sometimes favors one participant over others, so that most of the scattering of such tongues is now a subject for the historian rather than the student of living languages.
But this is precisely as we should expect, for language is the product not of method and logic, but of the vicissitudes of history. Thus languages wax, wane, change and disappear in response to specific events: As its speakers fare, so fares the tongue. And if indeed one might sufficiently trace and correlate the perpetual evolution of languages over time, one might find in them a map of all history.