GEORGE VVALD 737 



their peculiar capacity for geometrical isomerization. All organic 

 chroniophorcs owe their color to the presence of conjugated systems 

 of alternate single and doidile bonds. In all other types of natural 

 pigment, ho^vever, the conjugated systems are largely condensed into 

 rings, which hold the atoms rigidly in position, preventing isomeriza- 

 tion. The carotenoids and haplocarotenoids are unique in possessing 

 straight-chain conjugated systems, which can for this reason readily 

 undergo extensive isomerization. One of the most general procedures 

 for inducing isomerization is exposure to light. I think that it may 

 he the unique capacity to undergo profoimd changes in shape when 

 exposed to light that has led to the widespread selection of carotenoids 

 for photoreception. 



This thought can be carried a step further. Well-formed, image- 

 resolving eyes are almost entirely restricted to three animal phyla: the 

 molluscs, arthropods, and vertebrates. Each of these phyla has evolved 

 its o^vn characteristic types of eye, apparently in complete independ- 

 ence of the others. No connections, genetic, anatomical, or embryo- 

 logical, seem to link the eyes in any one of these phyla with those 

 in the others. 



It is highly significant, therefore, that in each case not only do the 

 visual pigments that govern photoreception possess retinene 7 or 2 as 

 chromophore, but that in every case yet analyzed this is the ll-cis 

 isomer. AVhy shoidd three phyla independently have selected for vision 

 such a fundamentally improbable, hindered cis configuration of reti- 

 nene? I would suggest the reason that among all the geometric 

 isomers of retinene, the ll-cis isomer possesses the highest photo- 

 sensitivity, in the sense that it is photoisomerized to a.ll-trans with 

 the highest quantum efficiency, and hence at the highest intrinsic rate. 

 The quantimi efficiency for isomerization to M-trans is only about 1/3 

 as great for the 9-cis isomer, the chromophore of the iso-pigments; 

 and the isomerization of the l3-cis isomer, though rapid and effi- 

 cient, leads to only a minor change of shape. It seems probable that 

 its unique photosensitivity and large change of shape on isomeriza- 

 tion are the factors that have led to the repeated selection of the 

 ll-cis isomer for the construction of visual systems. 



The Problem of Visual Excitation 



Some component in the sequence of changes initiated in a visual 

 pigment by the absorption of light accounts for visual excitation. 

 Since, however, even in a cold-blooded animal the first electrical 

 response to light follows the stimulus by a small fraction of a second, 



