272 SENSATION AND THE SENSIFEROUS ORGANS. [LECT. 



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 sensa- 

 tion 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 all that we know about 

 matter is that it is the hypothetical substance of 

 physical phenomena the assumption of the existence 

 of which is as pure a piece of metaphysical speculation 

 as is that of the existence of the substance of mind. 



Our sensations, our pleasures, our pains, and the 

 relations of these, make up the sum total of the 



