TEACHING AND LEARNING. 17 



A course in physiology hinges largely upon experiment, 

 and a clear idea of its real nature and true function is therefore 

 essential. A really good experimenter is born, not made, for 

 there is a sort of experimental instinct, which is complex and 

 includes inquisitiveness, faith in one's own powers, pleasure 

 and natural skill in mechanical manipulation, and ready per- 

 ception of the value of evidence. All of these may, however, 

 to some extent be cultivated. An experiment in its essence 

 is a question asked of nature, and should always be the direct 

 definite question of a thoughtful seeker after knowledge. It 

 should call for as simple an answer as possible. It properly 

 follows upon careful observation, and usually is a testing of 

 hypotheses suggested by reasoning thereon. Most commonly 

 an experiment is undertaken to find out the relation existing 

 between the processes of the plant and some particular external 

 condition ; and practically the first and most natural step is to 

 observe the effect upon the plant when that condition is 

 removed or neutralized. The ideal experiments are those in 

 which only the single condition is altered ; but, partly on 

 account of the closeness with which different conditions are 

 yoked together, and partly because of the relative crudeness of 

 even our finest methods of experimenting, this is very rarely 

 possible. Hence in order to make sure that the result obtained 

 is reallv connected with the condition changed, and not with 



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some secondary influence introduced by the manipulation in 

 the experiment, it is usually necessary to try at the same time 

 a parallel experiment in which a similar plant is placed under 

 precisely the same conditions as the first plant except that the 

 given single condition is not changed. Here, in both experi- 

 ments, all the secondary conditions are the same, the difference 

 is only in the given primary condition ; and hence it is a fair 

 inference that an observed effect is connected with the change 

 in the primary condition. Such an experiment is called a 

 control, and an impulse to control experimenting is an essential 

 part of the experimental habit. It is in order to minimize the 

 danger of disturbance through the introduction of secondary 



