278 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
all the physicochemical laws of action and reaction which have been 
observed to occur within living things. In all physicochemical 
processes within and without the organism by which energy is cap- 
tured, stored, transformed, or released the actions and reactions are 
equal, as expressed in Newton’s third law. 
Actions and reactions refer chiefly to what is going on between 
the parts of the organism in chemical or physical contact, and are 
subject to the two dynamical principles referred to above. Inter- 
actions, on the other hand, refer to what is going on between material 
parts which are connected with each other by other parts, and cannot 
be analyzed at all by the two great dynamical principles alone without 
a knowledge of the structure which connects the interacting parts. 
For example, in interaction between distant bodies the cause may be 
very feeble, yet the potential or stored energy which may be liberated 
at a distant point may be tremendous. Action and reaction are 
chiefly simultaneous, whereas interaction connects actions and reac- 
tions which are not simultaneous; to use a simple illustration: when 
one pulls at the reins the horse feels it a little later than the moment 
at which the reins are pulled—there is interaction between the hand 
and the horse’s mouth, the reins being the interacting part. An 
interacting nerve-impulse starting from a microscopic cell in the brain 
may give rise to a powerful muscular action and reaction at some 
distant point. An interacting enzyme, hormone, or other chemical 
messenger circulating in the blood may profoundly modify the growth 
of a great organism. 
Out of these physicochemical principles has arisen the conception 
of a living organism as composed of an incessant series of actions and 
reactions operating under the dynamical laws which govern the 
transfer and transformation of energy. 
_ The central theory which is developed in our speculation on the 
Origin of Life is that every physicochemical action and reaction 
concerned in the transformation, conservation, and dissipation of 
energy, produces also, either as a direct result or as a by-product a 
physicochemical agent of interaction which permeates and affects the 
organism as a whole or affects only some special part. Through such 
interaction the organism is made a unit and acts as one, because the 
activities of all its parts are correlated. This idea may be expressed 
in the following simplified scheme of the functions or physiology of the 
organism: 
