n8 FUNCTIONAL INERTIA 



that the property of inertia in protoplasm is a mo- 

 lecular affair neither more nor less dependent on 

 molecular structure than is affectability. Just as 

 the property of gunpowder to explode when a spark 

 falls on it is due to its molecular structure expressed 

 here as an affectability explosive susceptibility 

 towards sparks as stimuli, so the failure of cordite 

 to explode when ignited is as much due to its molec- 

 ular structure expressed here as a non-affectability 

 towards sparks or flames ; but this latter might be 

 pefectly well called the explosive inertia of cordite. 

 It has affectability towards an appropriate stimulus, 

 e.g., concussion. Just as no organism is all inertness 

 so neither probably is any non-living mass. 



There are, of course, certain differences between the 

 affectability of the non-living and of the living. One 

 of these is noted by Donaldson in his work, " The 

 Growth of the Brain." * He points out that the 

 same force of explosion of gunpowder will be the 

 result of the application of either a small spark or of 

 a huge flame, whereas, within limits, the discharge 

 from a nerve-cell varies in intensity according to the 

 intensity of the stimulus impinging on it. 



So in sensory stimulation : the effect on the retina 

 is, within limits, the greater according as the stimulus 

 is greater ; the light from a glow-worm produces 

 much less retinal disturbance than the electric flash. 



There is a superior limit set by functional inertia : 

 the supra-maximal stimulus produces no more effect 



* Donaldson, " The Growth of the Brain," Contemp. Science 

 Series, p. 277. (London : Scott.) 



