480 The Outline of Science 



a curious flat and feeble creature, with soft white spines pointing 

 backwards, and a pale blue-grey skin. It is not for some time able 

 to roll itself up, yet it develops quickly, and is able to follow the 

 mother in a month or two. 



12 



Hibernation 



Many creatures, such as reptiles, amphibians, snails, and in- 

 sects, pass into a lethargic state when winter sets in, and lie low 

 until the spring. But it is only in mammals that we find true 

 hibernation, a very peculiar physiological condition, which is not 

 sleep, nor necessarily connected with winter. It is exhibited by 

 Hedgehog and Hamster, Dormice and Bats, Marmot and Sous- 

 lik, the Spiny Ant-eater of Australia and the Jerboa of the 

 Kirghiz steppes. 



To understand the hibernation or so-called winter sleep of 

 these mammals, it is necessary to recall the main facts in regard 

 to animal heat. Inside the body heat is produced by various 

 chemical processes, but mainly by the muscles; it is of great im- 

 portance in facilitating the operations of the living laboratory. 

 But the heat tends to be lost by radiation into the outer world 

 through the skin, and in the hot breath and in sweating. The 

 non-conducting fur in ordinary mammals and the blubber of 

 whales lessen the loss from the skin, as do the feathers of birds. 

 But there is in birds and mammals a self -regulating system, which 

 keeps the temperature approximately constant, day and night, 

 year in and year out ; and this is what is meant by warm-blooded- 

 ness. The regulating centre is in the brain, whence orders issue 

 to the muscles, blood-vessels, and skin. If too much heat is being 

 produced or lost, an adjustment is effected. But all mammals are 

 not perfect as regards this heat-regulating arrangement, and it is 

 among these that hibernation occurs. A good example may be 

 found in the Spiny Ant-eater (Echidna) , whose temperature may 

 vary ten degrees Centigrade according to that of the outside 



