So THE ORIGIN OF THE NERVOUS SYSTEM 



excitation is transmitted as a wavelike change and that 

 it is completely reversible. This is undoubtedly true in 

 large measure for certain protoplasmic mechanisms, but 

 it does not follow that it is true for all or under all con- 

 ditions. In fact the existence and persistence of the 

 metabolic and physiological gradients in protoplasms 

 without specialized mechanisms indicates, if it does not 

 prove, that neither wavelike transmission of the excita- 

 tion process nor complete reversibility is a universal 

 characteristic of the excitation-transmission relation. 

 As I have pointed out, the excitation itself does not 

 necessarily travel in these gradients, but the electro- 

 motor changes determine different degrees of excitation 

 at different distances from the point of original excita- 

 tion. Moreover, in most protoplasms these gradients 

 are not rapidly reversible but persist after the factors 

 inducing them have ceased to act, and become factors in 

 determining the developmental order and relation of 

 parts. They are in a sense fixed or static excitation 

 gradients, and the electrical changes rather than the 

 excitation itself are transmitted. The various lines of 

 evidence which have been considered in preceding chap- 

 ters seem to me to indicate that the primitive excitation- 

 transmission relation in protoplasm approaches more or 

 less closely this condition and that oxidation is an 

 important factor in it. With the specialization of 

 protoplasm and the appearance of one mechanism or 

 another changes occur. Perhaps the most significant 

 of these changes is the change in the relation between 

 the exciting factor and the excitation produced. In the 

 more primitive mechanisms, as pointed out above, the 

 energy, the intensity, or, in lack of definite knowledge 



