THE GARDEN OF THE WORLD 
than there are harmful ones. In fact, there 
would be no life upon the earth at all if it were 
not for the bacteria of decay. When a great 
tree dies and falls, these bacteria attack it, and 
gradually it crumbles and disappears. The 
task takes years, but is finally finished. When 
animals die, the bacteria of decay dispose of 
them also. If the plants and animals which 
had once lived on the earth did not decay, but 
were still lying about, it would be impossible for 
us to live at all. 
We want the milk that we drink to be sweet 
and fresh, but we need sour milk also. It is 
used to make butter and cheese. There is a 
certain kind of bacteria that turns milk and 
cream sour. Another group of bacteria turns 
cider into vinegar. There are thousands of 
helpful kinds. 
Some bacteria, for all their small size, have 
the dangerous power of making poisons. They 
seem to be able to change the food they have 
used into substances that are poisons for people 
and animals. That is what these disease bac- 
