442 Dynamic Theory. 



color blind. ' This is a defect that extends to red and green only, or 

 chiefly. Color blind people, as a rule, have no trouble with yellow, 

 blue and violet. This fact indicates that these latter colors are of 

 earlier evolution in the race history, and are therefore faster colors. 

 As we have observed, of all the parts subject to evolution by habit, the 

 older the habit, as long as it is active, the less is the liability to rever- 

 sion and loss of function. Again, if we have but three kinds of cones, 

 one for violet arid the other two for seeing red and green, color blind 

 people could get no idea at all of yellow ; whereas, the fact is, the 

 sense of yellow survives when red and green have become reversions. 

 I have spoken of the rods as having the sensibilit} r to light as contrasted 

 with darkness. 



As the lower octaves of the vibrations of the ether produce upon 

 animal and vegetable protoplasm and tissue a peculiar molecular move- 

 ment, and the sensation of heat in degrees of varying intensit} T , so the 

 44th octave as a whole produces other peculiar movements, and the 

 peculiar subjective sensation of light. Some kinds of organic tissues 

 are more influenced by one part of this octave and some by another. 



It is the general influence of the various vibrations of the octave that 

 together give the sensation of light, and differentiate the rods from 

 the nervous tissues of the retina to respond to these general mixed 

 vibrations. By the law of differentiation, as explained elsewhere, organs 

 or parts subjected to various movements under varying stimuli do not 

 respond perfectly to any one. As long as the stimulus is compound 

 the response is imperfect and uncertain, and it is only when the com- 

 pound stimulus is split up into simple stimuli by the specializing of 

 organs for and b} r each one, that the action becomes a regular and 

 constant specialized function. The rods then as first differentiated are 

 movable in an indifferent and imperfect way by all the various vibra- 

 tions of the whole octave, or at any rate by the greater part of them. 

 And the sensation as far as it goes would be that of dim and ineffectual 

 colorless light like twilight. The first specialization from this state of 

 things would be caused by the necessar} r fact that the retinal tissues 

 are somewhat more mobile under the stimulus of some special ra}~ of 

 vibration, and this fact in turn may result from the circumstance that 

 such special ray is the one most common in the surroundings in which 

 the organ is environed. That most potent ray is the yellow, because it 

 is experimentally demonstrated to be the most luminous ; which is only 

 another way of saying that it actually has the most powerful effect on 

 organic tissues and the visual organ ; and it is proved likewise to be the 

 most powerful promoter of plant action. 



Animals whose visual organs have reached this first differentiation 

 would see sunlight as yellow instead of white. If now we suppose a 



