70 life's beginning on the earth 



The question therefore is to be answered: what are the 

 smallest living things? To find the answer we embark on 

 a journey of exploration into the domain of the most dimin- 

 utive living creatures. Passing in review small worms and 

 primitive one-celled animals whose body diameter averages 

 I mm. or ^ of an inch, we arrive at the much smaller bac- 

 teria some of which measure only 2 ooo °f a millimeter. 

 But they are real giants as compared with the much smaller 

 filtrable viruses some of which measure only 20 ] Qm of a 

 millimeter. Objects of such diminutive size cannot be 

 seen in any microscope, since 500 o millimeter is about the 

 lowest limit of any directly visible object, no matter how 

 efficient a microscope we use. On account of their diminu- 

 tive size they will pass through the finest openings in filters. 

 Their approximate size can be estimated from the size of 

 the holes through which they are able to pass. 



The reader will ask, how we know of their existence. The 

 answer is that these filterable viruses arc disease-producing 

 agents; a few of them, when inoculated into an animal, will 

 multiply at an appalling rate. They can be readily trans- 

 ferred from animal to animal, or from man to man. The 

 most commonly known human diseases caused by them are 

 influenza, small pox, rabies, yellow fever, and trachoma. 

 Each disease is produced, of course, by a different virus. 

 Their entire behavior resembles closely that of the visible 

 germs, the bacteria, which cause other infectious diseases 

 like cholera, typhoid fever, diptheria, and pneumonia. If 

 the bacteria are believed to be alive, the viruses should also 

 be so considered, in spite of their invisibility. 



The production of disease is the only sign we have of the 

 presence of a virus. 11 can be demonstrated by placing 

 material containing viruses on a filter and by showing that 

 the liquid passing the filter remains as an agent to propagate 

 the disease. This finding is in contrast to the observation 

 made with nitrates from liquids containing bacteria. Such 



