ADJUSTMENT, DIRECT AND INDIRECT 



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 composed of little 

 rods of nerve tissue, packed closely together like 

 organ pipes ; and it is probable that each of these 

 rods vibrates in unison with a particular ray of 

 light.' Here is a case of extreme differentiation 

 just like that witnessed in the ear ; and substan- 

 tially the same argument will apply to it. The 

 survival of a primeval savage in the struggle for 

 life would certainly depend to a considerable ex- 

 tent on his ability to discriminate certain col- 

 ours as well as outlines by the eye, as also upon 

 his ability to recognize the timbre or quality of 

 certain sounds. But the power of distinguishing 

 the delicate shades in a painting of Correggio 



^ This is the opinion of Helmholtz, the greatest living au- 

 thority ; and it is strengthened by Dr. Brown-Sequard's dis- 

 covery of the number of fibres in the spinal cord v^^hich are 

 specialized for the reception of particular sensations. [The 

 passage in the text above, and in this note, regarding the func- 

 tions of the retina, would have been greatly modified had 

 Fiske rewritten it in the light of recent knowledge. The the- 

 ory of the colour sensations has been much changed by the 

 researches since Helmholtz.] 



91 



