Chapter 12 



ADAPTATIONS TO PHOTIC QUALITY 



(A) Color Vision in Animals 



The Limits of the Spectrum — The first and foremost adjustment of 

 vertebrates to the quality (that is, frequency) of light, as opposed to 

 its quantity or intensity, was the positioning of the limits of the visible 

 spectrum. When a student first learns that the visible spectrum occupies 

 barely a single octave on a great keyboard of radiant-energy frequencies, 

 he may well wonder why the eye evolved with such narrow limitations 

 of its capacity. Since all sorts of organic substances exist which are 

 absorbent of {i.e., opaque to) frequencies far beyond the visible band 

 in both directions, could not a much more broadly sensitive retina have 

 been as easily devised, so that we could see much more in the world with- 

 out benefit of fluoroscopic screens and infra-red-sensitive camera films? 

 Such a retina might have evolved, but not where the vertebrate eye 

 took its origin — in water. Of what point to an aquatic eye, to evolve 

 sensitivity to lights which can never strike it? By Visible spectrum' we 

 usually mean the assortment of contiguous wavelengths which stimulates 

 the cones. This spectrum has complexities — of limits, peaks of maximal 

 stimulating value, etc. — due to the behavior of the color-vision mechan- 

 ism, which is far from primitive and, in land animals like ourselves, has 

 been partly released from slavery to the properties of water. What the 

 really primitive, pre-color-vision cone spectrum may have been like, we 

 cannot know; but the next-most-ancient absorption spectrum is that of 

 fish rhodopsin, which in its shortening at the red end and in the position 

 of its maximum is clearly adjusted to the kind of light in which it has 

 to operate. It is thus no mere coincidence that the risible spectrum is 

 roughly the transmission spectrum of water. The rod spectrum is closely 

 fitted to water, the cone spectrum a little better fitted to air. 



Value and Origin of Color Vision — But the fitting of the sensitivity 

 of the eye to the kind of light available is not the most conspicuous 

 adjustment to photic quality. It is overshadowed by 'color vision', by 

 which we mean the capacity to respond differently to lights which differ 

 only in frequency. Where vertebrate color vision is a conscious process, 



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