THE PHYSIOLOGICAL ANALYSIS OF BEHAVIOR 155 



unterschieds empfindlichkeit or differential sensibility. They are 

 of the nature of trigger effects, depending on the release of 

 internal energy by a sudden change of potential in the stimu- 

 lating medium. It is this change in potential or intensity rather 

 than any addition of energy from without that represents the 

 stimulus in all such cases. Can this conception of stimulus be 

 applied to tropic responses as well ? 



It is noteworthy that probably the most typical reactions of 

 this type are executed by various organisms when subjected to 

 sudden changes of potential in the galvanic current. Parame- 

 cium, for instance, responds to the making and breaking of the 

 current by characteristic motor reflexes. And this is quite re- 

 gardless of the fact that the galvanic current is unknown to 

 Paramecium in a state of nature and could have played no 

 part in the evolution of its behavior. 



The responses of Paramecium to the galvanic current are, 

 accordingly, of two types. And these it is quite impossible to 

 reduce to the common denominator of the motor reflex. How 

 is it then with the responses of photosensitive organisms to 

 light, which fall similarly into two groups ? For light, unlike 

 the galvanic current, belongs to the world of nature. It is 

 only exceptionally a laboratory product. Hence its influence 

 on the behavior of organisms cannot be disregarded. Can these 

 two types be reduced to one whose key is the motor reflex in 

 one form or another ? Are the remarkably direct and precise 

 turning movements that characterize tropic reactions resultants 

 of the selection of random movements of this type ? Or are 

 they compelled by the constant application of energy, as is 

 Paramecium in the constant galvanic current ? 



There can be no doubt that a successful attempt to resolve 

 the difference here presented, in terms of a single fundamental 

 factor, would greatly simplify the problem. But, assuming the 

 existence of such a factor, the view that its application to tropic 

 responses would involve selection of random movements does 

 not square with the fact that various organisms, among them 

 Euglena, the earthworm (Allolobophora) , the sow bug (Porcellio) , 

 and house fly larvae have exhibited unmistakable evidence of 

 responding to light by reactions whose direction can be definitely 

 predicted. 



