This bird, the Lagopus scoticus of naturalists, is entirely confined to
the British Isles. It is, however, very closely allied to the willow
grouse (Lagopus albus), a bird which ranges all over Europe, Northern
Asia, and North America, but which, unlike our species, changes to white
in winter. No difference in form or structure can be detected between
the two birds, but as they differ so decidedly in colour--our species
being usually rather darker in winter than in summer, while there are
also slight differences in the call-note and in habits,--the two species
are generally considered to be distinct. The differences, however, are
so clearly adaptations to changed conditions that we can hardly doubt
that, during the early part of the glacial period, when our islands were
united to the continent, our grouse was identical with that of the rest
of Europe. But when the cold passed away and our islands became
permanently separated from the mainland, with a mild and equable climate
and very little snow in winter, the change to white at that season
became hurtful, rendering the birds more conspicuous instead of serving
as a means of concealment.
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