CH. XII.] ADJUSTMENT, DIRECT AND INDIRECT. 63 



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 substantially the same argument will apply to it. 

 The survival of a primeval savage in the struggle for life 

 would certainly depend to a considerable extent on his ability 

 to discriminate certain colours as well as outlines by the 

 eye, as also upon his ability to recognize the twibre or quality 

 of certain sounds. But the power of distinguishing the 

 delicate shades in a painting of Correggio could be no more 

 useful, from a zoological point of view, than the power of 

 appreciating the most subtle harmonic effects in a symphony 

 of Schumann. For this extreme differentiation there would 

 seem to be no assignable cause save the direct action of 

 luminous waves upon the wonderfully sensitive and responsive 

 nerve-tissue of civilized man. 



Were it needful for the further illustration of our position, 

 I might show how Mr, Spencer has proved that the structure 

 of vertebral columns is also primarily due to directly adaptive 

 changes. Many peculiarities in the shapes of plants and 

 animals are probably thus to be explained. And in regard 

 to the hues of organisms — those phenomena which are so 

 beautifully explained by the Darwinian theory — there are 

 some exceptions to be cited. The magnificent tints of many 

 corals, of certain caterpillars, and of the shells of sundry 

 luollusks, must undoubtedly be due to the direct working of 

 such chemical affinities as produce our wonderful aniline 

 dyes, or the rich tints of our American autumn woods. 



But passing over all these interesting points, enough has 

 been said to show that there are many phenomena o^ organic 

 evolution which natural selection, when considered alone, 

 will not suffice to account for. But, with the amendments 



^ This is the opinion of Helmholtz, the gi-eatest living authority ; and it in 

 Btren<,'tl!ened by Dr. Brown Seqiiard's discovery of the number of fibres lu th« 

 ■piual cord which are specialized for the reception of particular seL'sationa. 



