CHAPTER FIFTEEN 



THE SMALLEST OF LIVING THINGS 



The least of living things, I repeat, holds a more profound mystery 

 than all our astronomy and our geology hold. 



JOHN BURROUGHS 



Our unseen IN the chapter on insects we noted that these little 

 creatures outnumber by far all other members of the 

 animal kingdom. But there are still other living things 

 as much smaller and more numerous than insects as 

 insects are smaller and more numerous than farm animals. 

 They are in the air, in the water, and in the soil all 

 about us, and as they carry out their life processes they 

 are constantly doing us either good or harm. Among 

 these tiny organisms are included all of the bacteria 

 and many of the fungi. 



Bacteria. The most minute of organisms are called 

 " bacteria " (singular, " bacterium "). Every one should 

 know something about the bacteria, for the invisible 

 hosts of them add fertility to the soil, help to bring about 

 decay of all organic matter, cause vinegar to ferment 

 and milk to sour ; and, on the other hand, they produce 

 mosj: of the diseases of mankind, such as tuberculosis, 

 typhoid, diphtheria, and influenza. (The disease-pro- 

 ducing bacteria are commonly called " germs.") 

 The Bacteria are so small that if four hundred of some 



ojfuving kinds of them were placed side by side, they would make 

 things a li ne only as long as a single page of this book is thick. 



None of them can be seen except with the aid of a 

 powerful compound microscope, and many of them 

 cannot be seen even with the best instruments. They 



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