METABOLISM 73 



the phenomenon of its slowly rotting away. With 

 animal remains the process is much more rapid. 

 In the end, however, the myriads of animal and 

 plant individuals, after their brief existence on the 

 earth, dissolve back again into the elements from 

 which they were built up. " Dust to dust " is a 

 very real and constantly recurring cycle. The com- 

 plex substances composing living tissue by successive 

 cleavages become resolved into H 2 O, CO 2 , NH 3 , and 

 similar compounds. Were this not so, the material 

 to build the organic world would soon be exhausted, 

 and the earth would be so covered with the remains 

 of animals and plants, accumulating during long 

 periods of time, that life would be impossible. In 

 very hot, dry climates, as in deserts, animal remains 

 actually do dry up and " mummify," without de- 

 caying. In the presence of moisture, however, 

 dissolution begins as soon as life is extinct. 



Putrefactive Organisms. - - This process of dis- 

 solution, which is merely the cleavage of the proto- 

 plasmic compounds into their simpler components, 

 doubtless would go on automatically, at least to a 

 certain point, but the process is hastened and carried 

 to a final conclusion by the assistance of various 

 kinds of bacteria and molds, known as the putre- 

 factive organisms. In the light of what has just 

 been said these obscure and unpleasant atoms of 

 life are one of the most important agents in the econ- 

 omy of nature. They are the wreckers, tearing 

 down that Nature may build herself anew. Through 



