THE TROPISM THEORY 273 



left. Hence a theory like that of tropisms, based primarily on the posi- 

 tion or orientation of the body axis with reference to the direction of 

 the stimulating agent, can find little precise application. Yet it is again 

 in this group that we find behavior that is in certain respects at least in 

 accordance with the tropism theory. For locomotion in a certain direc- 

 tion the stimulus must be localized, acting in a different way on the two 

 sides; this is one of the postulates of the tropism theory. Further, a 

 local stimulation may have at least a partially local effect, and this may 

 result in movement in a certain direction. But as v. Uexkiill has well 

 pointed out, the elementary factors here are the typical reaction methods 

 (" reflexes") of the individual organs of the body surface. The tropism, 

 if we attempt to apply the concept at all, is a mere collection of these 

 elementary reactions ; it is not in any sense itself an elementary factor. 

 In other words, the tropism theory would never have been based on the 

 known behavior of the echinoderms, for the facts, even so far as they 

 agree with the fundamental postulates of the theory, can be formulated 

 more directly and simply in another way. The tropism theory is fur- 

 nished with an apparatus of relations that finds no application to the 

 starfish and sea urchin. 



Furthermore, as we have shown in detail, much of the behavior of 

 these animals is based on the method of trial. In such bilaterally sym- 

 metrical animals as the flatworm Planaria we have the most favorable 

 possible conditions for action on this tropism theory, and such animals 

 often do turn directly toward or away from sources of stimulation. But 

 when this occurs, is it due merely to the local contraction or extension 

 of the musculature on the side on which the stimulus impinges, or is it 

 a reaction of the animal as a whole ? 



This question can be answered only by a thorough study of all the 

 factors in the reaction; such a study is given us for the flatworm by 

 Pearl (1903). The positive reaction of the flatworm the direct turn- 

 ing toward the source of stimulation seem to present ideal condi- 

 tions for explanation on the simple tropism theory. But Pearl, after 

 exhaustive study, concludes that the processes in the reaction are as 

 follows : 



"A light stimulus, when the organism is in a certain definite tonic 

 condition, sets off a reaction involving (i) an equal bilateral contraction 

 of the circular musculature, producing the extension of the body; (2) a 

 contraction of the longitudinal musculature of the side stimulated, pro- 

 ducing the turning toward the stimulus (this is the definitive part of the 

 reaction); and (3) contraction of the dorsal longitudinal musculature, 

 producing the raising of the anterior end. In this reaction the sides do 

 not act independently, but there is a delicately balanced and finely co- 



