Long-tailed field mice (Apodemus sylvatkus) live on the Castle Rock in the centre of Edinburgh city, which has been cut off from open country for nearly one and a half centuries. However they do not differ significantly from mice caught 4 km away on the edge of Edinburgh. The Edinburgh samples have different frequencies in 5 out of 20 nonmedical skull variants when compared with a Durham sample, and in eight when compared with a Perthshire one. The most likely explanation for the lack of differentiation of the Castle Rock sample is that it is a relic of a population once continuous with the main breeding population of Britain, that has survived in its restricted area despite its small numbers.
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