80 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 
