A DEFINITION OF EVOLUTION 



protochordates, while failing to sho\\' relationship of the annelids to either 

 of these groups. 



Aniinine and creatine are verv closely related chemicallv, and the for- 

 mer is actually used by yertebrates in the synthesis of the latter. In the 

 embryos of sharks, arcrinine is abundant, but its occurrence in adults is 

 more restricted. Thus it is possible that the creatine metabolism of verte- 

 brates is a biochemical recapitulation, comparable to the embryological 

 recapitulations discussed in the preceding chapter. 



Visual Pigments. \'ision among \ertebrates depends upon one or the 

 other of t\yo chemical s\'stems in the rods of the retina. Fresh-water fishes 

 have visual purple, a porphyropsin-vitamin Ao system, while marine fishes 

 and land vertebrates have visual red, a rhodopsin-vitamin Ai system. That 

 marine and land yertebrates should both contrast with fresh-water fishes 

 is surprising until one remembers that \'ertebrates probably arose in fresh 

 water, then migrated from it in the tN\'o directions. 



However, some anadromous fishes, like the salmon, \\\q principally in 

 the sea, but rctinn to fresh \\'ater to breed; while others, like the eel, are 

 catadromous; thev live much of their life cycle in fresh water, but return 

 to the sea to breed. And amphibians, of course, may live much of their 

 adult lives on land, but tliev return to fresh water to breed. Wald and his 

 collaborators have studied the visual pigments in tliese animals, and the 

 results are most illuminating. Anadromous fishes and amphibians are both 

 hatched in fresh water, and they imdergo larval development there. 

 After a metamorphosis involving profoimd anatomical and physiological 

 changes, they migrate to salt water and to land, respectiveh', where they 

 live much of their adult lives. Finalh , after changes which are parth- a 

 reversal of those occurring at metamorphosis, and which Wald has called 

 a second metamorphosis, they return to fresh water to spawn. 



Actually tadpoles and young anadromous fishes ha\'e porphyropsin in 

 their rods. At metamorphosis, that is, when they are preparing to migrate 

 to land or to the sea, the morphological changes are accompanied b\- a 

 change to a predominantly rhodopsin \isual s\stem. Later, when the 

 mature animals are ready to retiu'n to fresh water for breeding purposes, 

 they once again revert to a porplnropsin system. In catadromous fishes, 

 the facts are similar, but the sequence of changes is reversed. 



On other grounds, it had alreach' Ixhmi believed that the vertebrates 

 originated in fresh water, and there tlu^ ancestral fishes differentiated, 

 giving rise to fresh-water and marine fishes, and to amphibians. If this is 

 correct, then the sequence of changes in visual pigments is best inter- 

 preted as a recapitulation— a condensed repetition ol ancestral liistory. 

 Wald summarizes this storN as iollows: 



Marine fishes— Ai Land vertebrates— Aj 



Catadromous fishes— Ai — Aj / 



\ Amphibia— Ao and A 



Anadromous fishes— A2 >Ai / 



Fresh-water fishes— A^ 



Lampreys— A2 and Ai 



Ancestral vertebrates— ,'\j? 



( See Sfient-e, V. 12.S, p. 1 t8:3, DcvciuImi- 12. 1958.) 



