52 MOUNTAIN AND MOORLAND 



and many Bats waken when there is a spell of fine 

 weather in midwinter. 



If it be asked why a Hedgehog hibernates, while a 

 Mole does not, part of the answer is that the Mole is 

 a burrower who finds earthworms and grubs beneath 

 the reach of the frost's fingers even in midwinter. If 

 it be asked why Bats hibernate, while Birds do not, 

 part of the answer is that the great majority of Birds 

 in North Temperate regions evade the winter by 

 migrating. If it be asked why Brown Bears hiber- 

 nate, when Stoats do not, part of the answer is in the 

 fact that the Brown Stoat becomes in winter the 

 White Ermine, and that the white robe makes the 

 problem of facing the cold much easier. If it be 

 asked why the Jerboas of the Kirghiz steppes hiber- 

 nate, while the Squirrels in the forests farther north 

 remain wakeful, part of the answer is that the Squir- 

 rels accumulate stores of food for hard times. Gen- 

 eralising all this, we may say that the Mammals which 

 do not hibernate have in many cases some special 

 adaptation or fitness that enables them to cope with 

 the winter, or, if they have none, that they have hardy 

 resistant constitutions, as in Wolves and Foxes. But 

 there is another half of the answer. 



Mammals are descended from coldblooded Reptiles, 

 and some are less perfectly warmblooded than others. 

 Those that have some imperfection in their heat-regu- 

 lating arrangements, remaining in this respect young 

 or old-fashioned, have made a virtue of necessity by 

 becoming hibernators. When great cold sets in out- 

 side, they cannot sustain their body temperature at the 

 level required for a continuance of everyday activities, 

 so they relapse into a life-saving coldbloodedness and 



