60 THE STORY OF LIFE'S MECHANISM. 



we need not hesitate in recognizing a correlation 

 between physical and nervous energy. Even 

 though nervous energy is, very subtle and only 

 affects our instruments of measurement under 

 exceptional conditions, the fact that nervous 

 forces are excited by physical forces, and are 

 themselves directly measurable, indicates that 

 they are correlated with physical forces. Up to 

 this point, then, we may confidently say that the 

 nervous system is part of the machine. 



But when we turn to the more obscure parts 

 of the nervous phenomena, those which we com- 

 monly call mental, we find ourselves obliged to 

 stop abruptly. We may trace the external force 

 to the sensory organ, we may trace this force 

 into a nervous stimulus, and may follow this 

 stimulus to the brain as a wave motion, and 

 therefore as a form of physical energy. But 

 there we must stop. We have no idea of how 

 the nervous impulse is converted into a sensa- 

 tion. The mental side of the sensation appears 

 to stand in a category by itself, and we can not 

 look upon it as a form of energy. It is true 

 that many brave attempts have been made to 

 associate the two. Sensations can be measured 

 as to intensity, and the intensity of a sensation 

 is to a certain extent dependent upon the inten- 

 sity of the stimulus exciting it. The mental 

 sensation is undoubtedly excited by the physical 

 wave of nervous impulse. In the growth of the 

 individual the development of its mental powers 

 are found to be parallel to the development of 

 its nerves and brain a fact, which, of course, 

 proves that mental power is dependent upon 



