PTABMIGAN 271 



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 conti- 

 nental 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 des- 

 truction 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 periodi- 

 cally. 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 

 moimtains of Spain— it still has an arctic cHmate. 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. 



