124 Professor Huxley on Sensation. [March 7, 



epidermic cells, as mucli as the crystalline cones of the insect or crus- 

 tacean 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 sensation is the 

 equivalent in terms of consciousness for a mode of motion of the 

 matter of the sensoriura. But, if inquiry is pushed a stage further, 

 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 phe- 

 nomena — the assumption of the existence of which is as pure a piece 

 of metaphysical speculation as that of the substance of mind. 



Our sensations, our pleasures, our pains, and the relations of these 

 make up the sum total of the elements of positive, unquestionable 

 knowledge. We call a large section of these sensations and their 

 relations matter and motion ; the rest we term mind and thinking ; 

 and experience shows that there is a certain constant order of suc- 

 cession between some of the former and some of the latter. 



