THE STORY OF THE BACTERIA. 15 



primitive simplicity which we imagine to have 

 belonged to the earth's earliest denizens. 

 These are the bacteria. 



So small are the bacteria, and so simple in 

 their structure and activities, that it has not 

 been an easy task for scientific men to decide 

 whether they belonged among animals or 

 plants. It is now definitely settled, however, 

 that they are plants, and are closely related 

 to the algae. 



The bacteria vary a good deal in shape, but 

 in general they are either spheroidal or ovoidal, 

 like a billiard-ball or an egg ; or rod-shaped, 

 like a lead-pencil ; or spiral-shaped, like a cork- 

 screw. 



They are in general so very small that we 

 can hardly form a conception of them except 

 by comparison with some well known objects. 

 One of the most common of the bacteria is a 

 little rod, so small that if you were to put fif- 

 teen hundred of them end to end, the line 

 would scarcely reach across the head of an 

 ordinary pin. If you look at them with a 

 magnifying power so great that, if it could be 

 applied to him, it would make a man look 



