organs of
any {40} kind. Without possessing separate organs for the different
vital functions, these little creatures do nevertheless take in and
digest food, reproduce their kind, and move. Every animal shows at
least two different motor reactions, a positive or approaching
reaction, and a negative or avoiding reaction.

The general notion of a reaction is that of a _response_ to a
_stimulus_. The stimulus acts on the organism and the organism acts
back. If I am struck by a wave and rolled over on the beach, that is
passive motion and not my reaction; but if the wave stimulates me to
maintain my footing, then I am active, I respond or react.

Now there is no such thing as wholly passive motion. Did not Newton
teach that "action and reaction are equal"?--and he was thinking of
stones and other inanimate objects. The motion of a stone or ball
depends on its own weight and shape and elasticity as much as on the
blow it receives. Even the stone counts for something in determining
its own behavior.

A loaded gun counts for more than a stone, because of the stored
energy of the powder that is set free by the blow of the hammer. The
"reaction" of the gun is greater than the force acting on it, because
of this stored energy that is discharged.

An animal reaction resembles the discharge of the gun, since there is
stored energy in the animal, consisting in the chemical attraction
between food absorbed and oxygen inspired, and some of this energy is
utilized and converted into motion when the animal reacts. The
stimulus, like the trigger of the gun, simply releases this stored
energy.

The organism, animal or human, fully obeys the law of conservation of
energy, all the energy it puts out being accounted for by stored
energy it has taken in in food and oxygen. But at any one time, when
the organism receives {41} a stimulus, the energy that it puts forth
in reaction comes from inside itself.

There is another way in which the organism counts in determining its
reaction. Not only does it supply the energy of the response, but its
own internal arrangements determine how that energy shall be directed.
That is to say, the organism does not blow up indiscriminately, like a
charge of dynamite, but makes some definite movement. This is true
even of the simplest animals, and the more elaborate the internal
mechanism of the animal, the more the animal itself has to do with the
kind of response it shall make to a stimulus. The ner