A peculiar interest attaches to this bird on account of its change of plumage from brown in summer to snow-white in winter, and of the fact that it inhabits only the summits and slopes of high mountains. These two things--the white winter plumage and the mountain habit--have a close connection. The periodical change to white is a common phenomenon in arctic animals, both birds and mammals, and all the species of grouse of the genus to which the ptarmigan belongs assume the white dress in winter, with one exception--the red grouse of the British Islands. Thus, in Britain we have two grouse of this group (Lagopus), one of which turns white like the continental grouse, while the other keeps its brown dress throughout the year. To explain this difference it must be assumed that both species inhabited Britain at a period when its climate was an intensely cold one, and that both species changed their colour to protective white in the season of snow. When the British climate changed, and became so mild that the snow no longer remained unmelted for months at a time on the lower levels, all such creatures as had the arctic habit of becoming white in winter would be in danger of extermination, since their intense whiteness on the brown or green earth would make them fatally conspicuous to their enemies. A white grouse on a brown moor would be visible for miles to high-soaring birds of prey. The red grouse escaped destruction by losing its white winter dress: the change in it from two distinct liveries to one colour for all seasons was doubtless gradual, extending over a period of very many centuries, keeping pace with the slowly improving climate. He ended by becoming a bird that was wholly brown in winter, while the willow-grouse of northern Europe and Asia--the continental form, and, it may be added, the parent, form of our bird of the moors--continued to change to white periodically. Meanwhile no such change took place in the ptarmigan’s plumage: he alone continued to assume the pure white winter dress, as if to keep alive the tradition of an ancient arctic Britain; and yet he survived. He escaped destruction because he was a hardier bird, and preferred the higher grounds, where the snows never melted in winter. At the northern limit of its range, north of the arctic circle, the ptarmigan inhabits the fells and level country; in Europe it is everywhere confined to the higher slopes of lofty mountains; in other words, wherever found--and it ranges as far south as the mountains of Spain--it still has an arctic climate. The bird exists ‘islanded’ on high mountains, separated from the rest of its kind by wide spaces of low-lying country as impassable to it as the sea.