152 HARRY BEAL TORREY 



tion to the problems of behavior. That we may see the fruits 

 of physiological analysis against a background which will throw 

 them into significant relief, it will be desirable first of all to 

 consider the influence of selection theory on the physiological 

 field. 



Allusion has already been made to Weismann's application 

 of selection theory to the problem of variation, which as the 

 theory of germinal selection marks its extreme development in 

 this direction. In the field of behavior it has appeared in its 

 most extreme form in connection with the interpretation of the 

 activities of the lower organisms. Here it is found in signifi- 

 cant association with the method of trial. 



The method of trial was first used in this field as a measure 

 of the learning capacity of the higher animals. As such it has 

 been eminently successful. In the obscure and baffling field of 

 psychology it has encouraged the establishment of objective 

 standards and the prosecution of vigorously scientific investiga- 

 tion. With its extension to the behavior of organisms in gen- 

 eral, certain difficulties have arisen that have more frequently 

 been subjects of dispute than sources of common understanding. 

 Let me illustrate. 



An interesting parallel to the learning process by which a 

 cat is taught to escape from a puzzle box is the behavior of 

 Paramecium in the presence of an obstruction in its path. The 

 unsuccessful trials that accompany the learning process of the 

 cat are represented in Paramecium by the motor reflexes that 

 follow contact with the obstacle. These reflexes are the means 

 by which the organism is adjusted to the situation. Passing 

 the obstruction is evidence of successful adjustment. 



To what extent is the behavior of Paramecium a key to the 

 behavior of organisms in general, especially to that peculiar 

 type in many of the lower organisms that is characterized by 

 the direction of the reaction with reference to the source of 

 stimulation ? Is the motor reflex an essential factor in tropic 

 responses ? 



IV 



Of the answers which have been given to these questions, 

 the affirmative alone is of present concern. According to this 

 view, the final orientation of a phototropic organism to light 



