THE VITAL IMPETUS 151 



in the sense that several simple reactions proceed 

 simultaneously, upon simultaneous stimulation of 

 different receptors. Now the extension of all this to 

 movements of a " higher " grade is obvious. 



Let us note in the first place, that the stimuli so 

 far considered in all the examples quoted are simple 

 elemental ones. There are, of course, relatively few 

 such stimuli : gravity, conducted heat (the kinetic 

 energy of material bodies), radiated heat (the energy 

 of the ether), electric energy, chemical energy, and 

 mechanical contact or pressure (including atmospheric 

 vibrations). In all these cases we have a definite, 

 measurable, physical quantity, with which we must 

 relate a definite response in the form of a definite 

 measurable, physico-chemical reaction. There should 

 be a functionality between the stimulus and response, a 

 definite, quantitative energy-transformation. To take 

 a concrete example, a certain quantity of light energy 

 falling upon the receptor organs of Loeb's caterpillar 

 ought to transform into another quantity of " nervous 

 energy," and this travelling in an analogous way to a 

 " wave of explosion " ought to transform into an 

 energy quantity of some kind, which initiates another 

 " wave of explosion " in the muscle substance. All 

 these transformations must be quantitative ones, and 

 the energy of the individual light must be traced from 

 the receptor organ to the points in the muscle where 

 it disturbs a condition of false equilibrium in the sub- 

 stance of the latter. Nothing less than this is required 

 to demonstrate the purely physical nature of a reaction, 

 on the part of the organism, to an external stimulus. 

 It may safely be said that physiological investigation 

 has not yielded anything even approximating to such 

 an experimental demonstration. 



What are the stimuli to the actions of a higher 



