THE SCOPE OF ZOOLOGY 



mosquitoes? Since your previous observations indicate that mosquitoes 

 do not reproduce without water, you might formulate the hypothesis 

 that water is necessary for mosquito reproduction. A survey of the 

 literature on the subject will give you a background of what has al- 

 ready been investigated. Let us assume that you find no conclusive 

 evidence on the subject and, with the information you have, you plan 

 your methods of attack. 



You would probably decide to collect mosquitoes of different species 

 and keep them in cages. In some of the cages you would place bowls 

 of water and in the others there would be none. In the course of time 

 you would notice that the mosquitoes in the cages having the water 

 laid eggs in the water which later hatched into wigglers and finally came 

 out as full grown mosquitoes. In the other cages there would be no 

 reproduction. This would tend to support your hypothesis, but you 

 would be careful not to jump to conclusions. You would want to study 

 the mosquitoes in their natural environment to see if there might be 

 something about your method of handling the mosquitoes in the lab- 

 oratory that could have resulted in misleading findings. You might 

 find mosquitoes living in an area where there were no apparent bodies 

 of water in which to lay their eggs. How did they get there? You 

 might have to conduct experiments on mosquito migration to see if 

 they migrated from other regions. This might not explain the num- 

 ber of mosquitoes which you find. After much careful work you find 

 that in some of the plants of the area rain water accumulates between 

 the leaves and the stems. This water remains long enough to serve as 

 a breeding place for the mosquitoes. 



After many such investigations you may feel that you have con- 

 clusive proof that water is necessary for the reproduction of mos- 

 quitoes of the species which you have investigated and you could 

 publish the information as a scientific fact. This would not prove that 

 all mosquitoes need water, for there would be species in other localities 

 which you have not been able to investigate. However, you might 

 wish to propose a theory that all mosquitoes need water for reproduc- 

 tion, since your work indicates that this, in all probability, is true. 

 This would be a very valuable theory to keep in mind in all work with 

 mosquitoes even though we may never find it possible to investigate 

 every species of mosquito on earth and prove the point conclusively. 



The Controlled Experiment 



The controlled experiment is one of the most important tools of 

 the scientist. In most investigations it is necessary to conduct two 



