20 THE TROPISMS 



could be applied with only a slight modification to other 

 kinds of tropisms. "These tropisms," says Loeb, "are 

 identical for animals and plants. The explanation of them 

 depends first upon the specific irritability of certain elements 

 of the body-surface, and second, upon the relation of sym- 

 metry of the body. Symmetrical elements at the surface of 

 the body have the same irritability; unsymmetrical elements 

 have a different irritability. Those nearer the oral pole 

 possess an irritability greater than that of those near the 

 aboral pole. These circumstances force an animal to orient 

 itself in such a way that symmetrical points on the surface 

 of the body are stimulated equally. In this way the animals 

 are led without will of their own either toward the source of 

 stimulus or away from it. Thus there remains nothing for 

 the ganglion cell to do but to conduct the stimulus, and this 

 may be accomplished by protoplasm in any form." 



If, for instance, a worm were near a bit of food we might 

 suppose that the substances diffusing from the food would 

 strike one side of the body of the worm causing the muscles 

 there to contract more strongly than the opposite ones. 

 The worm would turn toward the food until both sides 

 of the body were equally affected, when it would proceed 

 directly toward the source of stimulation. Negative re- 

 sponses receive an essentially similar interpretation. The 

 point that is emphasized by the theory is that choice or 

 volition on the part of the animal is excluded; the actions of 

 the creature are supposed to be mechanically determined. 

 It is "forced" to go toward or away from the source 

 of stimulation as the strength of the stimulus and the 

 organization of its body determine. The moth flies to the 

 candle, not because it is drawn by curiosity, as suggested 

 by Romanes, or from any other conscious motive, but 

 because it is compelled to orient its body so that sym- 



