THE OUTER AND INNER WORLDS 217 



of modern physics, matter is in a constant 

 state of movement, and we also assume the 

 movements of the ether as the cause of the 

 phenomena of heat, light, and electricity. 

 Such movements impinge on the body of an 

 animal of the simplest type, and by a slow 

 process of evolution through countless ages 

 sense organs and a nervous system have been 

 produced. Thus a pigmented spot has slowly 

 become an organ of vision, and a few special- 

 ized hairs have been developed into a recipient 

 organ for variations of pressure, a rudimentary 

 organ of touch or of hearing. As we ascend 

 the scale of animal life, the sense organs 

 become more and more complicated until we 

 find them as in man and in the higher animals. 

 Each sense organ, as already pointed out, 

 is adapted to its specific kind of stimulus. 

 Thus the retina is attuned to receive the 

 vibrations of light, and in the skin and in the 

 internal ear we have structures adapted for 

 receiving variations of pressure. These end 

 organs are composed, putting the matter in a 

 general way, of (a) modified epithelial cells 

 to protect and support ; (b) a highly special- 



