iv REFLEX ACTION 55 



well seen in the naked protoplasm of many unicellular 

 organisms, for an' excitement to spread like a wave in 

 proportion to its initial strength. A slight shake will 

 make Amoeba princeps hesitate in putting forth its c . c legs." 

 A prolonged concussion will gradually affect the entire 

 organism and make it roll up almost into a ball. In 

 strychnine poisoning, where the excitability of the nerves 

 is abnormally heightened, a slight touch produces general 

 convulsions. 



The primitive fact to which these considerations bring 

 us back is the excitability of protoplasm. Protoplasm, in 

 whatever form, is a substance in very unstable chemical 

 equilibrium. Since it is never at rest internally, the 

 equilibrium is often upset by changes within the mass of 

 the protoplasm itself, and we get internally initiated move- 

 ments. But various external events the impingement of 

 a ray of light or heat, a chemical affinity, or a purely 

 mechanical contact may also, in ways which at present 

 can only be surmised, gravely disturb the equilibrium of 

 the substance at every point. The disturbance has a 

 tendency to propagate itself, a tendency which goes on 

 until its energy is dissipated, or until a countervailing 

 force of some kind restores the balance. The result of 

 such a disturbance is or may be a movement of the mass, 

 or an alteration of its form. Unless there is something in 

 the previous history of the individual organism or its 

 species to direct the movement, there is no reason why it 

 should be in any way suited to the needs of the organism 

 in relation to that stimulus. It will be a random move- 

 ment which may be useful, harmful or neutral to the 

 organism. It is clear that variations in the direction of 

 useful action will be helpful to a race in the struggle for 

 existence, and that a race in which they occur would be 

 likely to survive. But action at this stage depends on the 



reflexes, if not held in constant quiet restraint, would run riot. The due 

 response of a lower centre is an application in an appropriate manner of 

 a portion only of the nervous discharge which the stimulus sets going. 

 This general discharge may be regarded as the matrix out of which 

 suitable or adaptive action is hewn. Or, to vary the metaphor, it is the 

 flow of water for which experience and natural selection find a useful 

 channel, while they also devise means of absorbing or damming up the 

 overflow. The primitive thing is the flow. The channel is formed later. 



