24 SCIENTIST 



did and what happened next. Since their ultimate purpose 

 is to arrive at some picture or theory of how events are 

 put together, they may then go on to show that what 

 happened is consistent with one idea and inconsistent with 

 other competing theories. 



For example, at about the time of the Spanish- American 

 War there were two competing theories about the trans- 

 mission of yellow fever from one person to another. One 

 school of thought held that it was carried directly from 

 one person to another through particles in the air derived 

 from the bodily excretions of the sick person. Another 

 school of thought held that the infection was carried by 

 a mosquito that sucked up blood from one person and 

 injected it into another during the process of biting. 



The Army doctor, Walter Reed, arranged with several 

 courageous volunteers to settle the matter in the following 

 way. One group was asked to live in a room furnished 

 with beds and bedding stained by the excretions of yellow- 

 fever patients but protected from mosquitoes. The other 

 group lived in a clean room but were exposed to the bites 

 of mosquitoes that had fed on yellow-fever patients. All 

 the people in the first room stayed well, while several of 

 those in the second room fell ill. The evidence was con- 

 sistent with the mosquito theory but inconsistent with the 

 older one. Immediately it became clear that one way of 

 controlling epidemics of this very dangerous disease was 

 to keep down the mosquito population. A campaign was 

 undertaken to do just this and yellow fever was in a very 

 few years eliminated as a serious public health problem. 



Much scientific work is of this same general character. 

 It turns out that it is not really necessary or actually very 

 sensible to ask whether mosquitoes are the cause of yellow 

 fever. What we are really after is an accurate description 

 of the circumstances in which yellow fever has occurred 



