6 GENERAL BIOLOGY 



for the fact that if we add a little calcium chloride 

 to the salt solution, the heart begins to beat again. 

 The calcium has neutralized the ill effect of the 

 sodium. If, then, contracting is any criterion of 

 whether the heart is alive or not, its life would seem 



FIG. 1. A tardigrade (Macrobiotus) : a, in the creeping active con- 

 dition ; 6, dried, in the state of apparent death. 



to depend upon the presence or absence of something 

 wholly outside the living matter itself. 



Under perfectly natural conditions such a state 

 of "ceasing to live " and then resuming life again is 

 not uncommon. Some animals that live in shallow 

 puddles exposed to the chance of drying up are capa- 



