SENSATION. 103 



In my opinion tliis theory, in its main outlines, seems a 

 probable, as it certainly is a plausible one. I do not, indeed, 

 quite understand why, in accordance with the theory, the 

 " neutral point " of the colour-blind should not merely be 

 found to be shifted to another part of the spectrum, nor am I 

 quite clear about the explanation of the fact that the warm 

 colours are those havincj the lowest and not the hi^diest order 

 of vibrations, as analogy would lead us to expect. But the 

 theory has the merit of being antecedently probable, when we 

 remember that in all likelihood the visual sense arose by the ' 

 progressive elaboration of nerve-endings in particular parts of 

 the skin, which before their special elaboration presumably 

 ministered to the senses of touch and temperature. 



And this remark leads me to the last topic that I have to 

 dwell upon in the present chapter. I refer to the body of 

 morphological evidence which we now possess, showing that 

 all the organs of special sense have had their origin in special ' 

 elaborations of these nerves of the integument. For the 

 uniform result of histological and embryological investigation 

 is to show that all organs of special sense, wherever they 

 occur and whatever degree of elaboration they present in the 

 adult animal, are fundamentally alike in that their receptive 

 surfaces are composed of more or less modified epithelium 

 cells which originally constituted part of the external layer 

 of the animal. Thus, the origin of the olfactory membrane 

 in the embryo of the Vertebrata is found to consist in a pitting 

 of the skin of the fore part of the head — the pits, therefore, 

 being lined by the general layer of epidermic cells. The 

 subsequent grow^th of the surrounding parts of the face 

 eventually brings this lining to occupy the position which it 

 does in the hollow parts of the nose. Similarly, the organs 

 of hearing first begin as a pair of pits on the side parts of the 

 head, situated somewhat far back, and likewise lined by the 

 cells of the general integument. These pits rapidly deepen, 

 so that their lining is pinched off or separated from the 

 general integument of which it originally formed a part. The 

 deep pit then becomes a closed sac, and the adjacent tissues 

 becoming first cartilaginous and then osseous, this sac is 

 enclosed well within the skull by bony w^alls. While its 

 structure is undergoing further anatomical and histological 

 changes, the drum, the chain of ear-bones, and the external 

 ear are being formed, and thus eventually the auditory organ 



