CHEMICAL STRUCTURE AND METABOLISM 105 



and animal kingdoms depends upon one compound or another in this 

 group. Apparently we have here another example of far-reaching chemical 

 homology uniting widely diverse living things. 



Aside from this broad homology, however, is the interesting question of 

 why fresh-water fisjies have a different carotenoid, porphyropsin, than do 

 otli£r_veirtebrates. Added significance is given the question by the fact, 

 mentioned earlier, that fresh-water fishes are believed to have been an- 

 cestral to both_marine bony fishes and lancd jvertebjates. This being the 

 case, and assuming that ancestral fresh-water fishes resembled modern 

 ones in this respect, both the line leading to marine bony fishes and the 

 line leading to land vertebrates (amphibians and their descendants) ap- 

 parently underwent an evolutionary change (chemical mutation or muta- 

 tions) from porphyropsin to rhod_fl psia. This would afford another exam- 

 ple of parallel evolution. It is interesting that the chemical difference 

 between the two types of visual pigment is small. The molecule of rhodop- 

 sin incorporates one more atom of hydrog en than does the molecule of 

 porphyropsin, or, to put it differently, the ring structure of the porphy- 

 ropsin molecuIeTias one more double bond than does that of rhodopsin 



(Waia; I958T ' 



We may well ask: why did the chemical change in visual pigment occur 

 at all? Perhaps the porphyropsin system may be, in some way still unex- 

 plained, an adaptation for life in fresh water. Evidence comes from inves- 

 tigation of various animals. For example, the sea lamprey (a cyclostome) 

 spends most of its adult life in the ocean but spawns, and undergoes early 

 development, in fresh water. Wald (1958) reported that young lampreys 

 on their way downstream to the ocean have rhodopsin in their retinas, 

 while older lampreys on their way upstream to spawn have porphyropsin. 

 It is as though the pigment had changed in anticipation of the environ- 

 ment to which the lamprey was going. Wald has regarded this change as 

 biochemical metamorphosis. 



As we noted previously (p. 92), the newt Thturus spends its larval pe- 

 riod in the water and then emerges on land for a sort of adolescent 

 period (the "red eft" stage); after two or three years it returns to the wa- 

 ter to live as an adult. During the red eft stage it not only excretes most 

 of its nitrogenous wastes in the form of urea (Nash and Fankhauser, 

 1959) as other land-dwelling amphibians do, but it also possesses a pre- 

 ponderance of rhodopsin in its retina, although porphyropsin is also pres- 

 ent (Wald, 1958). When the newt has undergone a sort of second meta- 

 morphosis and returned to the aquatic environment for its adult life, 

 porphyropsin is found to be the only visual pigment in its eyes (and the 



