THEORIES OF INDIVIDUALITY 33 



If the external factor acts only momentarily at a, the 

 increase in rate of reaction at a is usually only momentary 

 or of short duration, and a sooner or later returns to or 

 approaches its original condition, perhaps in some cases 

 with a gradually disappearing rhythm of increase and 

 decrease in rate. The transmitted change consists in 

 this case of a wave or a series of successively decreas- 

 ing waves of change. 



It is probable that even the occurrence and passage 

 of such momentary changes as these in a substratum so 

 sensitive and so intimately associated with the reactions 

 as protoplasm produce changes which persist for a longer 

 or shorter time after the metabolic change has dis- 

 appeared, but such changes are usually slight or inap- 

 preciable. If, however, the external factor continues 

 to act on a for a sufficiently long time, or if it acts 

 intermittently with sufficient and not too great fre- 

 quency or intensity, it produces sooner or later more or 

 less permanent changes in the protoplasm, which are 

 most marked in the region a and decrease with the 

 transmission-decrement. The exact nature of these 

 changes is not certainly known, but their effect is to 

 increase the reactive capacity, to alter the protoplasm 

 so that in the absence of external stimuli, or with a 

 given intensity of external stimulus, a rate or intensity 

 of chemical reaction exists higher than the rate under 

 similar conditions before the change. In the terms 

 generally employed, the irritabiHty of the protoplasm is 

 increased. 



Since this change is greatest in the region a, Fig. i, 

 where the excitation is greatest, and decreases with 

 increasing distance from this region, the result of 



