250 DEATH AND THE DURATION OF LIFE 



higher animals construct nests, and, even before the 

 coming of the winter, withdraw into these retreats, and 

 enter upon a period of sleep lasting from two to four 

 months. No food is taken during this time, the pulse is 

 very feeble, breathing movements are slight and occur at 

 long intervals, and the heat generated by the oxidation 

 of the fatty tissues, especially when it is retained by the 

 walls of the nest, is sufficient to keep the animal from 

 freezing. During hibernation, therefore, all of the chief 

 functions of the body are being carried on, it is believed, 

 but they are slowed down almost to the point of com- 

 plete inactivity. 



Summer Sleep. — In tropical and desert countries,^ 

 where drought prevails for months at a time, many species 

 of animals and plants undergo the so-called summer sleep 

 that resembles hibernation in certain of its features. 

 Many snakes, beetles, crocodiles, and alligators, upon the 

 approach of the dry season, bury themselves in the mud, 

 and, with activities at low ebb, exist in a dormant con- 

 dition for months. Among the lower animals there are 

 certain snails and slugs that surround the exposed portions 

 of their bodies with a coat of slime, and in this state may 

 exist without feeding for at least two years. Practically 

 all of the freshwater unicellular animals may be dried 

 into inert shrivelled masses, exposed to the burning sun, 

 and blown about by the wind, and continue to exist in 

 an apparently lifeless condition until a more favorable 

 time. Numerous plants likewise dry up and shrink to 

 small size, while the seeds, like the eggs of many animals, 

 are protected by heavy coats' to enable them to withstand 

 excessive drying for several years. 



Effects of Freezing. — It has long been known that 

 numerous species of animals and plants may be frozen 

 solid yet regain their customary activity when again 

 brought into favorable conditions. As a matter of fact 

 this normally occurs every winter in cold climates. For 

 example, in the Arctic region of North America the 



