THE MICROBE 
21 
chains, but more often separate entirely and 
continue to live and divide just as the original 
microbe did. This process goes on, once or 
twice an hour, so long as the microbe is attached 
to a substance from which it can obtain food. 
It has been estimated that if the growth of 
microbes went on unchecked, and the division 
of each cell took place as often as once an hour, 
the descendants of each cell would in two days 
number more than 281 billion. 
Fortunately, this amazing increase is pre¬ 
vented by many natural means, which keep the 
microbes from overrunning the earth and 
crowding everything else out of existence. In 
absorbing its food, the microbe forms acids and 
other injurious products, which remain around 
it and often prevent the birth of more microbes. 
There may not be enough food for the increased 
numbers, or the temperature may become too 
hot or too cold for them to live; or another 
kind of microbe may drive them out, thus pre¬ 
venting them from increasing so rapidly. 
A few kinds of microbes, in addition to form- 
Why dorit they 
overrun the 
world? 
