FUNDAMENTAL FACTS 7 



elements which compose these complicated instincts, 

 thetropisms (heliotropism, chemotropism, geotropism, 

 stereotropism) play an important part. These trop- 

 isms 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 relations of symmetry of the 

 body. Symmetrical elements at the surface of the 

 body have the same irritability ; unsymmetrical ele- 

 ments 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 toward a source of 

 stimulation 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 the 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. For 

 the inheritance of instincts it is only necessary that 

 the egg contain certain substances which will de- 

 termine the different tropisms--and the conditions 

 for producing bilateral symmetry of the embryo. The 

 mystery with which the ganglion-cell has been sur- 

 rounded has led not only to no definite insight into 

 these processes, but has proved rather a hindrance in 

 the attempt to find the explanation of them. 



It is evident that there is no sharp line of demarc- 

 ation between reflexes and instincts. We find that 



