SENSATION 



The advanced vision of the higher animals is made up 

 of a great number of different functions, with a corre- 

 sponding complexity of detail in the anatomic structure 

 of the eye. No other organ, after the brain, is so neces- 

 sary as the eye for the multifarious vital activities of 

 the higher animals, and especially for the mental life of 

 civilized man and the progress of art and science. What 

 would the human mind be if we could not read, write, 

 and draw, and have a direct knowledge through the eye 

 of the forms and colors of the outer world ? Yet this in- 

 valuable structure is only the highest and most perfect 

 stage in the long chain of evolutionary processes which 

 has its starting-point in the general sensitiveness to light, 

 or the photic irritability of plasm. However, we find a 

 number of varieties and grades of this even among the 

 unicellular protists, and, indeed, the very lowest and 

 oldest of the protists, the monera, Various species of 

 both the chromacea and the bacteria are heliotropic to 

 different degrees, and have 'a fine sensitiveness to the 

 strength of the light stimulus. 



The stimulating effect which light has on the homo- 

 geneous plasm of the monera is also found in a number 

 of inorganic bodies. In these cases the photic stimulus 

 produces partly chemical and partly mechanical changes. 

 Every chemist speaks of substances that are more or less 

 "sensitive" to light; the photographer speaks of bis 

 "sensitive plates," the painter of his "sensitive colors." 

 Many chemical compounds are so sensitive to light that 

 they are destroyed at once in sunlight, and so have to 

 be kept in the dark. There is no other word but "sen- 

 sation" to express the attitude of the atoms towards 

 each other which becomes so conspicuous in these cases 

 under the influence of sunlight. It seems to me that this 

 phettomenon is a clear justification of our hylozoic monism 

 when it affirms that all matter is psychic. In metaphys- 

 ics sensation is held to be an essential property of the soul. 



299 



