242 EVOLUTION AND ANIMAL LIFE 



death, may be killed by accident or by enemies, before the life 

 cycle is completed. And this is the fate of the vast majority 

 of animals which are born or hatched. Or death may come 

 before the time for birth or hatching. Of the millions of eggs 

 laid by a fish, each egg a new fish in simplest stage of develop- 

 ment, how many or rather how few come to maturity, how 

 few complete the cycle of life! 



Of death we know the essential meaning. Life ceases 

 and can never be renewed in the body of the dead animal. It 

 is important that we include the words " can never be renewed, ^^ 

 for to say simply that ''life ceases, ^^ that is, that the perform- 

 ance of the life processes or functions ceases, is not really death. 

 It is easy to distinguish in most cases between life and death, 

 between a live animal and a dead one, yet there are cases of 

 apparent death or a semblance of death which are very puzzling. 

 The test of life is usually taken to be the performance of life 

 functions, the assimilation of food and excretion of waste, the 

 breathing in of oxvgen, and breathing out of carbonic-acid 

 gas, movement, feeling, etc. But some animals can actually 

 suspend all of these functions, or at least reduce them to such 

 a minimum that they cannot be perceived by the strictest exam- 

 ination, and yet not be dead ; that is, they can renew again the 

 performance of the life processes. Bears and some other 

 animals, among them many insects, spend the winter in a state 

 of deathlike sleep. Perhaps it is but sleep; and yet hibernat- 

 ing insects can be frozen solid and remain frozen for w^eeks and 

 months, and still retain the power of actively living again in 

 the following spring. Even more remarkable is the case of 

 certain minute animals called Rotatoria and of others called 

 Tardigrada, or bear animalcules. These bear animalcules live 

 in water. If the water dries up, the animalcules dry up too ; 

 they shrivel into formless little masses and become desiccated. 

 They are thus simply dried-up bits of organic matter; they are 

 organic dust. Now, if after a long time — years even — one of 

 these organic dust particles, one of these dried-up bear ani- 

 malcules, is put into water, a strange thing happens. The body 

 swells and stretches out, the skin becomes smooth instead of 

 all wrinkled and folded, and the legs appear in normal shape. 

 The body is again as it was years before, and after a quarter 

 of an hour to several hours (depending on the length of time 

 the animal has lain dormant and dried) slow movements of 



