GENERAL PRINCIPLES 3 



taneous, i.e. it performs actions which find their starting-point 

 within the body itself, as in the case of many voluntary human 

 actions. 



Any change in the surroundings which brings the sensitive- 

 ness of an organism into play is technically known as a stimulus 

 (L. stimulus, an ox-goad), and stimuli may broadly be classified 

 as mechanical, chemical, thermal, photic, and electrical. The 

 corresponding stimulating agents are pressure, change in chemi- 

 cal nature of the surroundings, heat, light, and electricity, which 

 are scientifically defined as different forms of energy, or, to use 

 the old expression, " force". Protoplasm, like every other kind 

 of matter, may be regarded as made up of excessively minute 

 particles or molecules, much too small to be seen with even the 

 most powerful microscope, which are in a state of constant 

 vibration, throbbing, or to-and-fro movement. The pendulum 

 affords a simple example of vibratory movement. It may further 

 be said that every sort of stimulus is of the nature of a vibra- 

 tion, e.g. in a sound-wave transmitted through air the particles 

 of air move in a particular way and at a rate depending upon 

 the pitch of the sound. All the changes that take place in 

 living matter result from modifications in the movement of its 

 molecules, but we are profoundly ignorant of what exactly takes 

 place when, say, a muscle-fibre contracts or an impulse passes 

 along a nerve. The adjustment to surroundings that is necessary 

 for the maintenance of life results from these molecular changes 

 in the body, which take place in response to the action of 

 pressure, heat, light, &c., these themselves being of a vibratory 

 nature, as has already been stated. So far as an animal is 

 " sensitive " to its surroundings it is comparable to a complex 

 musical instrument capable of playing all sorts of tunes with all 

 kinds of variations, in response to external influences of different 

 kind. The reaction of an animal to its environment at any 

 given moment depends upon how external agents are acting 

 upon it at that moment: it is they which " call the tune". If 

 the supposed musical instrument could also play tunes of its 

 own accord, independently of the direct action of the surround- 

 ings, such tunes might be taken to represent the "spontaneous" 

 actions of an animal. 



That Sensitiveness and Spontaneity, as above defined, are 

 essential properties of living matter, may best be realized by 



