162 THE WONDER OF LIFE 



the ermine, the Hudson's Bay lemming, and the ptarmigan, 

 face the dread enchantment of Winter, but turn paler 

 and paler under the spell, until they are as white as the 

 snow itself a safety-giving pallor. They have a consti- 

 tutional tendency to change their colour, and the external 

 cold pulls the trigger that sets the process at work. The 

 white suit is of service for concealment or in the chase, 

 and it is also physiologically the most economical and 

 comfortable dress for a warm-blooded animal when the 

 external temperature is very low. 



An interesting and unusual adaptation to the severity 

 of winter is exhibited by the Canadian Ruffed Grouse 

 (Bonasa umbellatus), which often takes refuge among the 

 dry soft snow of drifts, having discovered its value as a 

 non-conductor. It sometimes tunnels in, but it usually 

 gets a start by diving from a branch or off the wing. It 

 makes a passage about two feet long with an enlargement 

 at the end, and may lie there for several days. Mr. Charles 

 MacNamara observes that ' except for the one mark where 

 the tunnel begins, the surface of the snow is quite undis- 

 turbed, and no one would ever suspect that a live warm 

 bird was concealed in the drift '. 



MlGBATION AS AN INSTANCE * 



From ancient days the migration of birds has excited 

 the wonder of all thoughtful observers. The author of 

 the Book of Job took note of the hawk that stretcheth 

 her wings towards the south ; the Hebrew prophet in his 

 message to Israel recalled the fact that ' the stork in the 



1 In taking this instance we have almost inevitably repeated part of 

 the discussion of Migration to be found in several chapters of The 

 Biology of the Seasons. 



