200 COLIN CLOUT'S CALENDAR. 



as divergent lateral branches from a single original 

 common stock? Probably the last, and for these 

 reasons. 



The bird which came northward at the close of the 

 glacial period, to inhabit the now thawed plains of 

 northern Europe, much as the American partridge might 

 take possession of Greenland if all its glaciers were to 

 clear away in a more genial era, was doubtless a more 

 or less southern and temperate type of grouse-kind. 

 Coming into Britain, it would soon be entirely isolated 

 from all its allies elsewhere ; for it is of course a poor 

 flyer for distance, and it inhabits only the northerly or 

 westerly parts of our island which lie furthest from the 

 Continent, separated from Holland and Scandinavia by 

 a wide sea. Here it could not fail to be subjected to 

 special conditions, differing greatly from those of the 

 European mainland, partly in the equable insular 

 climate, partly in the nature of the vegetation, and 

 partly in the absence of many mammalian foes or com- 

 petitors. These conditions would be likely first to affect 

 the colouring and marking of the feathers, the spots on 

 the bill, the naked scarlet patch about the eye, and so 

 forth : for we know that even freer-flying birds in the 

 south, which cross often with Continental varieties, tend 

 slightly to vary in such ornamental points ; and a very 

 isolated group like the red grouse would be far more 

 likely to vary in similar directions. Meanwhile, the 

 main branch of the family, separated on the great 

 continents from this slightly divergent group, would 

 probably acquire the habit of changing its plumage in 

 winter among the snows of the north, by stress of 

 natural selection, just as the Arctic fox and so many 



