THE WONDERS OF LIFE 



The important idea of a release of energy^the term 

 we give to the effect of the stimulus — is also used in 

 physics. If we put a piece of burning wood in a barrel 

 of powder, the flame causes an explosion. In the case of 

 dynamite a simple mechanical shock is enough to pro- 

 duce the most enormous expenditure of force in the ex- 

 plosive matter. When we discharge a bow the slight 

 pressure of the finger on the tense cord suffices to send out 

 the arrow or bolt on its deadly mission. So also a sound 

 or a ray of light that strikes the ear or eye suffices to 

 bring about a number of complex effects by means of the 

 nervous system. In the fertilization of the ovum by the 

 male sperm the chemical conjunction of the two forma- 

 tive principles is sufficient to cause the growth of a new 

 human being out of the microscopic plasma-globule, the 

 stem-cell (cytula). In these and thousands of other 

 reactions a very slight shock suffices to provoke the 

 largest effects in the stimulated substance. This shock, 

 which we call a release of energy, is not the direct cause 

 of the considerable result, but merely the occasion for 

 bringing it about. In these cases we have always a 

 vast accumulation of virtual energy converted into living 

 force or work. The magnitude of the two forces has no 

 relation at all to the smallness of the shock which led to 

 the conversion. In this we have the difference between 

 stimulated action and the simple mechanical action of 

 two bodies on each other, in which the quantity of the 

 energy expended is equal on both sides, and there is no 

 stimulus. 



The immediate effect of a stimulus on living matter 

 can best be followed in external physical or chemical 

 stimuli, such as light, heat, pressure, sound, electricity, 

 and chemical action. In these cases physical science is 

 often able to reduce the life-process to the laws of 

 inorganic nature. This is more difficult with the internal 

 stimuli within the organism itself, which are only partly 



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