62 The Animal Mind 



ferent reactions. When Jennings, for instance, says that 

 Amoeba " reacts to all classes of stimuli to which higher 

 animals react" (211, p. 19), we cannot conclude that it pos- 

 sesses all classes of sensations that higher animals possess, 

 for its reactions to these different stimuli are but little varied 

 according to the kind of stimulus. 1 



1 6. Evidence from Structure and Behavior Combined 



As a matter of fact, the argument from structure needs 

 confirmatory evidence from behavior. For clearly the mere 

 presence of a sense organ bearing sufficient likeness to our 

 own to admit of conjecturing its function would be of no 

 value as proof unless it were shown that the sense organ actu- 

 ally functioned. In order to do this, it would be necessary to 

 show that the animal reacted to the stimulus conjectured as 

 appropriate to the sense organ, and that removal of the organ 

 profoundly modified the reaction. Thus we shall find that 

 many experiments to test sensory discrimination have been 

 made by the method of extirpating a sense organ and studying 

 the effect on behavior. Thelnethod has many disadvantages, 

 the chief of which lies in the fact that it is hard to say which 

 disturbances in behavior are due actually to the loss of the 

 organ and which to the more widespread effects of the opera- 

 tion. Yet this much may be said for the combination of 

 proof from structure and behavior involved in the Method 

 of Extirpation, if we may so call it : where an animal reacts 

 to a certain stimulus, for instance light, when a sense organ is 

 intact, and fails to react to light, though otherwise normal, 

 when the organ is removed, there arises a possibility that light 



1 One of many reasons for the unsatisfactoriness of a recent article by A. 

 Olzelt-Newin, entitled " Beobachtungen iiber das Leben der Protozoen" 

 (299), lies in the author's uncritical acceptance of the hypothesis that reac- 

 tion to a special kind of stimulus means a special kind of sensation. 



