48 MOUNTAIN AND MOORLAND 



have spoken of these in another part of this book. The 

 Arctic Fox (Canis lagopus) and the Collared Lem- 

 ming (Cuniculus torquatus) may be noted as two other 

 examples of turning white in winter. Permanent 

 whiteness is characteristic of many northern animals, 

 such as Polar Bear, American Polar Hare, Greenland 

 Falcon, and Snowy Owl. It is plain that what suits 

 well for the Far North is likely to suit also for high 

 altitudes. 



We have called the whiteness of the winter Ptar- 

 migan a garment of invisibility, but it is not to be sup- 

 posed that this protection is perfect. Very few of 

 these fitnesses are. We remember, for instance, the 

 fact that the Snowy Owl (Nyctea scandica), a native of 

 the barren grounds of the Far North both in Europe 

 and America, and an occasional visitor to Britain in 

 very cold winters, will pick up snow birds like the Ptar- 

 migan and snow mammals like the Mountain Hare. 

 Its own plumage is white with variable dark spots and 

 bars, and it has a strong silent flight. It hunts by day, 

 wary but fearless, master of its fate and defiant of 

 difficulties. 



The other adaptation of the Ptarmigan to high alti- 

 tudes is to be found in its heart. It is well known 

 that going up a mountain is very trying to people with 

 a weak heart, and that is seen even when they ascend 

 in a rack-and-pinion railway. The reason is that at a 

 great height the air is rarer, there is less oxygen to be 

 got out of a breath. One way of meeting the diffi- 

 culty is to take big breaths at rapid intervals, but this 

 panting is very exhausting. Another way is to get 

 the heart to beat more quickly so that more blood 

 than usual is sent to the lungs every minute. But this 



