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 wouid 
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- 
