260 THE ORIGIN OF THE NERVOUS SYSTEM 



excitation in protoplasm. Certainly the intensity of the 

 excitation process increases in general in development 

 and evolution, and such change undoubtedly determines 

 more effective transmission or conduction, just as a rapid 

 change in electric potential is more effective than a slow 

 change in producing excitation. It is not improbable 

 also that in the more highly specialized excitatory 

 processes the electrical changes are greater than in the 

 primitive processes, and this again must contribute to 

 more effective transmission. Other changes may also 

 be concerned, but the point which I wish to make is 

 that the changes are such as to make it possible for 

 conduction of. a local excitation to occur to a greater 

 distance up a physiological gradient in the higher 

 animals than in the lower. If the conception of exci- 

 tation advanced in earlier chapters is correct, the 

 change is probably primarily electrical. 



It may be also that the gradients themselves are less 

 steep in the higher than in the lower forms, but as yet 

 no adequate basis exists for comparison of steepness in 

 different forms, though the electrical data will probably 

 give some evidence along this line. Finally, the fact 

 that the higher animals consist of segments and in con- 

 nection with segmentation develop a secondary posterior 

 growing region and a secondary longitudinal gradient 

 in the opposite direction to the primary gradient is 

 unquestionably a factor of great importance in deter- 

 mining that the more posterior levels of the body are 

 able to send impulses all the way to the head. This 

 gradient determines the primary reflex mechanism in 

 Amphibia, as already pointed out, and the definitive 

 relations arise later after the gradients have undergone 



