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The widespread European vole species Microtus arvalis (Pallas) occurs in the British Isles only on Guernsey and various of the Orkney islands. The traditional explanation for this peculiar distribution has been that the island races are the sole relicts of a previously cosmopolitan form. This idea is not supported by the present study, in which population samples are compared with voles from a number of localities from Continental Europe. Using the frequencies of non-metrical skull variants as genetical markers, it emerged that although the Guernsey population (Microtus arvalis sarnius) was virtually indistinguishable from typical German specimens, and therefore likely to be a relict from the time when Guernsey was connected to France, the Orkney populations (Microtus arvalis orcadensis) formed a distinct group with affinities more to south Europe than elsewhere. It seems probable that arvalis was brought to Orkney by some of the earliest human colonizers about 4000 years ago, and then radiated to the different islands. It is argued that this manner of peripheral differentiation may be an important method of evolutionary change.