SENSATION AND THE SENSIFEROUS ORGANS 317 



epidermic layer of the general integument. Hence 

 it follows that the rods and cones of the vertebrate 

 eye are modified epidermic cells, as much as the 

 crystalline cones of the insect or crustacean eye 

 are ; and that the inversion of the position of the 

 former in relation to light arises simply from the 

 roundabout way in which the vertebrate retina is 

 developed. 



Thus all the higher sense organs start from one 

 foundation, and the receptive epithelium of the 

 eye, or of the ear, is as much modified epidermis 

 as is that of the nose. The structural unity of 

 the sense organs is the morphological parallel to 

 their identity of physiological function, which, as 

 we have seen, is to be impressed by certain modes 

 of motion; and they are fine or coarse, in 

 proportion to the delicacy or the strength 

 of the impulses by which they are to be 

 affected. 



In ultimate analysis, then, it appears that a sen- 

 sation is the equivalent in terms of consciousness for 

 a mode of motion of the matter of the sensorium. 

 But, if inquiry is pushed a stage farther, and the 

 question is asked, What then do we know about 

 matter and motion ? there is but one reply 

 possible. All that we know about motion is that 

 it is a name for certain changes in the relations of 

 our visual, tactile, and muscular sensations; and 



