CASES OF ADAPTATIO]S[ 



159 



Every imrt of the body is protected either with a dense mass 

 of phimage, or by a homy integument on the bill and feet, 

 so that they are probably quite undisturbed while enjoying 

 the super-abundant feast nature has spread for them in those 

 remote and usually repellent lands. We may conclude, there- 

 fore, that it is to the two special features of these Arctic 



Fig. 21.— The Higher Tundra. 

 Stanavialachta at mouth of the Petchora River (N. Lat. 69°), 



tundras — their abundant berries preserved during the winter 

 in a natural ice-house, and the myriad clouds of mosquitoes 

 and their larvge — that we owe the very existence of a consid- 

 erable proportion of the bird-life in the northern hemisphere. 



The Origin of Bird-migration 



These vast Arctic plains even in Tertiary times when 

 climates were milder, would, owing to the long winter nights, 

 have always been snow-covered during several months in winter 

 although its melting might have been earlier and the sunnner 

 somewhat longer ; there can be little doubt that the short sum- 



