62 COSMIC PHILOSOPEY. [pr. U. 



when A is sounded on a musical instrument, the A-key 

 within the ear vibrates, and transmits its vibrations to a 

 special filament of the auditory nerve. If this view be 

 correct, we have here a truly marvellous instance of dif- 

 ferentiation. But now in what way can this structure have 

 ever been useful to human beings in the struggle for life ? 

 Doubtless a considerable power of discriminating sounds is 

 useful to any animal, but of what use can it be to distinguish 

 between A and A- sharp ? We may safely conclude, I think, 

 that survival of the fittest has played quite a secondary part 

 in this case. The explanation must be sought in the direct 

 effects wrought by auditory vibrations upon the molecular 

 structure of the cochlear fibres. And it is a system of effects 

 which has not even yet been wrought in its present complete- 

 ness save among highly civilized people, A savage cannot 

 distinguish the slight variations in pitch by which our ears 

 are delighted. And even among ourselves there are ears 

 which can neither in melody discriminate between the 

 ascending and the descending gamut, nor in harmony 

 distinguish between the mellifluous tonic chord and the 

 harsh inversions of the minor ninth. The defect may be 

 compared to that of colour-blindness, although it is probably 

 more common because the ear has been far less thoroughly 

 trained than the eye. Now when we consider how much can 

 be effected by individual training in enabling a moderately 

 good ear to discriminate between quarters, eighths, and 

 smaller fractions of a tone, and bear in mind that this 

 training must consist in the further differentiation of the 

 sensitive cochlear fibres, we have a strong argument in favour 

 of the production of this wonderful structure by direct 

 adaptation alone. 



Concerning the human eye I need only say that in the 

 retina it presents a structure closely analogous to the ear- 

 piano just described. The chief layer of the retina is com- 

 posed of little rods of nerve-tissue, packed closely togethej 



