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. s¢émulus, 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 
