354 DEVELOPMENT AND PURPOSE CHAP. 



said to contain the developed organism. If the undiffer- 

 entiated cells are really undifferentiated, if, that is to say, 

 they are in their inmost structure just like any other cells, 

 why do they give rise to this particular organ or organism 

 and not to another ? If, on the other hand, they are really 

 different from the first, the process of differentiation is only 

 apparent. The adult is preformed in the embryo, and 

 what really happens would seem to be rather a sort of 

 unveiling of what is already there, than a true growth of 

 something new. How far does our slight account of 

 development enable us to turn this difficulty? Let us 

 take up the question again at a different point. Mechanics 

 tells us that in any system, as long as it is uninfluenced 

 from without, the sum of energy is constant. But any 

 portion of this energy may be unapparent. For example, 

 a weight lying on a scale is also suspended from a point 

 by a string so that it barely presses the scale. In this con- 

 dition it is in equilibrium. This means that several forces l 

 are being exerted, and in particular the downward pull of 

 the weight and the upward pull of the string ; but these 

 forces are balanced. So far as any change, any movement 

 is concerned, they are opposing forces and they cancel out. 

 If the arrangement is disturbed by the snapping of the 

 string, the balance is destroyed, the downward thrust of 

 the weight has its effect. There is a transference of poten- 

 tial into kinetic energy, and there is something that may 

 be called an evolution of motion and a performance of 

 work. In fact, we speak often of an evolution or develop- 

 ment of specific forms of energy as of sound, heat, light, 

 an electric current and so forth. We may, indeed, con- 

 ceive kinetic as a development of potential energy, and we 

 observe that the mode of development is this that where 

 there is potential energy certain forces oppose and balance 

 one another, and that given a readjustment which removes 

 this opposition, each force has its own way and works itself 

 out in some appropriate movement. 



The kinetic energy liberated may theoretically per- 

 petuate itself indefinitely, e.g. a projectile launched on a 



1 Meaning here and in the rest of the discussion by a * force ' whatever 

 tends to produce or inhibit motion, or, generally, any change. 



