EXCITATION AND TRANSMISSION 83 
in the rate of living, possess all the characteristics of 
excitation gradients, and are determined by differential 
exposure to the action of external factors. I believe 
we are amply justified in the light of all the facts in 
regarding them as the most general and primitive expres- 
sion of the excitation-transmission relation in protoplasm. 
The fact that these gradients are usually more or less 
persistent and not readily or rapidly reversible requires 
some consideration. From our knowledge of momen- 
tary excitations and the highly specialized processes in 
nerve and muscle we are accustomed to think of excita- 
tion as completely reversible. Excitation may be 
completely reversible in the fully developed nerve fiber, 
but certainly in many parts of the nervous system it is 
not completely reversible, as memory and the possi- 
bility of learning show clearly enough. Moreover, it is 
evidently not completely reversible in muscle, since 
frequent excitation, if not exhausting, leads to growth, 
i.e., functional hypertrophy, and after a certain stage of 
development excitation is necessary for the continued 
existence of the muscle. Whether we invoke a mys- 
terious "over-compensation" or some other hypothetical 
process to account for these facts, they show that while 
muscular contraction is completely reversible the pro- 
cess of excitation as a whole is not. 
The fully developed organism represents an approach 
to dynamic equilibrium. The protoplasmic record of 
the events of development is about as complete as is 
possible for the protoplasm concerned. The various 
mechanisms are all developed and the further changes 
in them are largely temporary and reversible except so 
far as the gradual changes which constitute senescence 
