THE THEORY OF TROPISMS. IO5 



ing is due to a certain method of movement of certain motor organs. 

 In the rotifers it is the coronal cilia which accomplish the turning, 

 while in the infusoria we know that the adoral cilia are concerned in 

 the movement. We may take the coronal or adoral cilia, then, as rep- 

 resentative of the organs active in the turning. For convenience we 

 may designate these active organs simply as x. 



Now, when the animal is stimulated on the right side, we find that 

 the motor organs x move in a definite way. On the tropism theory we 

 would conclude, therefore, that the portion of the right side stimulated 

 has nervous connection with the organs x. But we find also that when 

 stimulated on the left side, the oral side, or the aboral side, the organs 

 x move in exactly the same manner. In other words, we find that it 

 does not depend on the side stimulated what organs respond, as re- 

 quired by the tropism theory. This theory, then, in its modified form, 

 is of no more service for these cases than in its original form. The 

 responses in the animals which we have considered must, therefore, 

 be conceived as reactions of the organism as a whole, and due to some 

 physiological change produced by the stimulus, not as the result of 

 direct changes in certain motor organs when they or the parts with 

 which they are most closely connected are locally affected by a stimu- 

 lating agent. The facts show that the parts act in the service of the 

 whole, not that the action of the whole is due to the more or less inde- 

 pendent irritability and activity of the parts. 



Thus the facts brought out show that the theory of tropisms is not 

 of great service in helping us to understand the behavior of these lower 

 organisms. On the contrary, the reactions of these organisms seem 

 as a rule thoroughly inconsistent in principle with the fundamental 

 assumptions of the theory. 



The facts brought out above are based on a study of what is, of course, 

 a comparatively small number of organisms. They rest chiefly on an 

 extensive study of the ciliate infusoria, with less thorough examination 

 of bacteria, flagellata, rotifers, and a few higher organisms. Doubtless 

 in organisms which are made up of many parts which are less firmly 

 bound together into a unified body than in those considered, we may 

 find greater independence of action in the parts. This seems to be the 

 case, for example, in the sea urchin, with its numerous independently 

 acting spines, pedicellariae, tube feet, etc. In this animal Von Uexkiill 

 (1900, 1900, a) concludes from his extensive study of the reactions that 

 many features in the behavior which seem at first view to be activities 

 of the animal as an individual are really due to the independent reac- 

 tions of the parts, so that he can say that while in walking, in the case 

 of the dog, " the animal moves its legs ; in the sea urchin the legs move 

 the animal." This method of behavior has a general agreement with 



