THE METHOD OF TRIAL AND ERROR. 251 



the lowest organisms with very simple factors, it is nevertheless capable 

 of development ; it leads upward. The tropism leads nowhere; it is 

 a fixed, final thing, like a crystal. The method of trial and error on 

 the other hand has been called the "method of intelligence" (Lloyd 

 Morgan, 1900, p. 139) ; it involves in almost every movement an 

 activity such as we call choice in higher organisms. With the acquire- 

 ment of a finer perception of differences the organism acting on the 

 method of trial and error rises at once to a higher grade in behavior. 

 Combining this with the development of sense organs and the differen- 

 tiation of motor apparatus, the path of advancement is wide open 

 before it. 



The most important step in advance is that shown when the results 

 of one reaction by trial and error become the basis for a succeeding 

 reaction. The method of reacting which leads to success is determined 

 by trial ; after it is once or several times thus determined, the trials are 

 omitted, and the organism at once performs the successful action. 

 This is intelligence, according to Lloyd Morgan (1900, p. 138), and 

 it is as leading to this result that the method of trial and error can be 

 characterized also as the method of intelligence. In intelligent action, 

 while the organism must react the first time by the method of trial and 

 error, it need not begin all over again each time the same circumstances 

 are presented. Do we find any indication of such action among uni- 

 cellular organisms? 



In Stentor we find action of this character to a certain extent. It 

 does not continue reacting strongly to a stimulus that is not injurious, 

 but after a time, when such a stimulus is repeated, it ceases to react, 

 or reacts in some less pronounced way than at first. To an injurious 

 stimulus, on the other hand, it does continue to react, but not through- 

 out in the same manner. When such a stimulus is repeated, Stentor 

 tries various different ways of reacting to it. If the result of reacting 

 by bending to one side is not success, it tries reversing the ciliary cur- 

 rent, then contracting into its tube, then leaving its tube, etc. (details 

 in Jennings, 1902, a). This is clearly the method of trial and error 

 passing into the method of intelligence, but the intelligence lasts for 

 only very short periods. To really modify the life of the organism in 

 any permanent way, as happens in higher animals, the method of 

 reacting discovered to be successful by the method of trial and error 

 should persist for a long time. Apparently this is not the case for 

 unicellular organisms, but further work is needed on this point. 



An application of the method of trial and error similar to that of 

 Stentor is found under certain circumstances in the flatworm. Pearl 

 (1903) found that after the animal had reacted to a repeated mechanical 

 stimulus for a long time by turning away from it, it suddenly reversed 



