THE SCOPE OF ZOOLOGY 



experiments simultaneously. These two should always be as nearly 

 alike as possible in all details except for the one thing under investi- 

 gation. For instance, in the mosquito experiment, if we are to feel 

 reasonably sure that water is necessary for reproduction, all factors, 

 except the presence and absence of water, must be exactly the same. 

 Then, any differences which we observe can be assigned to the presence 

 or absence of water. 



A very good illustration of the value of such a controlled experi- 

 ment was brought out in studies on poliomyelitis (infantile paralysis). 

 Some years ago a serum was developed which gave promise of acting 

 as a preventive for this dread disease. Experiments were devised to 



Courtesy of E. R. Squibb and Sons 



Fig. 1.1. The controlled experiment in vitamin research. These two rats were raised 

 exactly alike except for the fact that the diet of the one on the left was deficient in 

 vitamin A. Had it not been for a group of control rats, like the one on the right, 

 who developed normal eyes, it would not have been possible to be certain that vitamin 

 A deficiency caused the eye abnormalities of the experimental group, illustrated by the 



one on the left. 



test the hypothesis that it would prevent the disease. During an epi- 

 demic year in a large New York clinic, parents who brought their chil- 

 dren to be examined were asked if they would like to have this serum 

 given to their children. The fact was made very clear to them that 

 the treatment was still in the experimental stage and might not be of 

 any benefit. About half of the parents decided to have their children 

 inoculated. The other half felt that they did not want their children 

 to serve as human "guinea pigs" and these were not inoculated. There 



