CYCLES OF CHANGE 303 



Thus even decay may be looked upon as a process friendly 

 to man. 



Decay is a very complex process. It is produced by forms 

 of life so small that they can be seen only with a microscope. 

 There is good reason to believe that there are forms so 

 small that even the most powerful microscopes will not re- 

 veal them. The most important of these minute forms of 

 life are called bacteria. They exist in uncountable millions 

 almost everywhere. Scientists are acquainted with over 1500 

 different kinds of bacteria, and each kind has its own peculiar 

 characteristics. Molds and yeasts are other low forms of life 

 that help in the processes of breaking down, or disintegration. 



All these minute forms of life must have considerable 

 moisture and some of them, at least, must have free oxygen 

 in order to thrive and to accomplish their work. Almost 

 every one who has walked through the woods has noticed 

 how much more rapidly damp wood decays than dry wood. 

 It is to keep moisture and air from wood that we paint it, 

 so that bacteria may not have in it living quarters favorable 

 to their work of destruction. 



Cycles of Change. Sometimes areas where soils have 

 accumulated for centuries and centuries have been grad- 

 ually submerged below the waters of the sea. There these 

 soils, and even undecayed plant growths, have been consoli- 

 dated into sedimentary rocks. Ages afterward these areas 

 have again emerged and the whole process of tearing down 

 has begun anew. And so the cycles of building up and tear- 

 ing down continue. Sun, water, ice, bacteria, the move- 

 ments of the atmosphere, and the slow movements of 

 the earth's crust are constantly working in league with one 

 another to tear down what many of the same agencies 

 have worked steadily to build up. 



