30 The Story of the Bacteria 



material which was best suited to their needs, 

 or until the material which they had set free 

 had so far accumulated as to prevent their fur- 

 ther growth, and then they died, self -poisoned, 

 just as a man might who should be shut up in 

 a tight room until the accumulation of the 

 products of his respiration and excretion had 

 made further life impossible. Or they may 

 die because other species of bacteria growing 

 in the same fluid furnish material which 

 poisons their neighbors. So the proces- 

 sion of life goes on, until the bottom of 

 the jar becomes a veritable graveyard of 

 races. 



Some forms of the bacteria, however, which 

 seem dead, and fall with the rest to the bot- 

 tom of the jar, are really only in a resting 

 stage; they have formed spores within them- 

 selves in the manner described above, and may 

 lie dormant until the proper conditions come 

 again, when they may spring into renewed 

 activities. But new species may from time to 

 time fall into the jar from the air and find in 

 the water, which was rank poison for the dead 

 species at the bottom, just the food they need, 



