THE TROPISM THEORY 269 



From the relations above set forth, it follows that for determination 

 of the direction of movement in accordance with this tropism theory, 

 a stimulus must act upon one portion of the body differently from or 

 more intensely than on other parts. Without such differential action 

 on different parts of the body there is nothing to cause the animal to 

 turn in one direction or another. 



This tropism schema is made by its upholders the basis for the 

 larger part of the directed activities of the lower animals. "Thus the 

 phenomena of positive and negative chemotaxis, thermotaxis, photo- 

 taxis, and galvanotaxis, which are so highly interesting and important 

 in all organic life, follow with mechanical necessity as the simple results 

 of differences in biotonus, which are produced by the action of stimuli 

 at two different poles of the free-living cell" (Verworn, 1899, p. 503). 

 Verworn (1899) and Loeb (1900) have developed the theory as a general 

 explanation for all sorts of directed activities, and many authors have 

 accepted it for reactions to particular stimuli. In recent times, Holt 

 and Lee (1901) have applied it in detail to the responses to light, Loeb 

 (1900, p. 186) and Garrey (1900) to chemicals, Loeb (1897) and Verworn 

 (1899) to gravitation, Mendelssohn (1902 a) to heat and cold. 



In the foregoing chapters we have examined the behavior of a con- 

 siderable number of lower organisms, of many different kinds. How 

 far does this examination support the above theory? How far is the 

 observed behavior due to orientation produced by the local action of 

 stimuli on the different parts of the body? To what extent does this 

 tropism theory aid us in understanding the behavior of these organisms ? 



In Amoeba there are no permanent body axes ; anterior and posterior 

 ends continually interchange places in the rolling movement, and any 

 part may become at any time the advancing portion. Under these 

 conditions the term "orientation" can have little meaning, and we can 

 hardly say that stimulation causes the body to become oriented in a 

 certain way. But stimulation does determine the direction of motion, 

 and anything like orientation that can be distinguished is a result of the 

 direction of motion, not its cause. Under stimulation the direction of 

 movement is changed first, then in consequence the animal takes an 

 elongated form which furnishes the only possible basis for the use of the 

 term "orientation." 



In the fact that to produce directed movement, local action of the 

 stimulus on a certain part of the body is necessary, causing local contrac- 

 tion or extension, the conditions in Amceba agree with the fundamental 

 postulates of the tropism theory. The agreement is most precise in the 

 positive reactions, where the part stimulated is the part that extends 

 and determines the direction of movement. In the negative reactions 



