238 THE EYE IN EVOLUTION 



ear, the heart and most other organs. In its essentials the eye of a 

 fish is as complex and fully developed as that of a bird or man ; the 

 differences between the members of the series are relatively minor 

 in character, adaptations to the habits of the animals rather than 

 expressions of phylogenetic evolution. All Vertebrates have a three- 

 layered retina and a pigmentary epith&lium, all have the same dioptric 

 apparatus of a cornea and an epithelial lens, all have the same nutrient 

 mechanism. It is true that the essential visual components except the 

 three-layered retina are found in many invertebrate eyes ; but at the same 

 time it is to be remembered that the optic ganglion of the latter group 

 corresponds essentially to the nervous layers of the retina of Verte- 

 brates. Despite these similarities, however, a revolution has taken 

 place. 



Throughout the whole phylum paired lateral eyes are present, 

 although occasionally, as in specialized predators such as the hagfish, 

 Myxine, or in cave-dwelling or abyssal fishes, subterranean amphibians 

 and reptiles and the mole, they may degenerate.^ In the most primi- 

 tive vertebrates known to man — the long extinct agnathous fishes 

 {Pteraspis, Cephalaspis, etc.) the fossil remnants of which are found in 

 the rocks of the Silurian era ^ — a median and two lateral eyes were 

 present. In the extant representatives of this primitive stock, the 

 lampreys (Petromyzon), the lateral eyes are rudimentary and hidden in 

 the arnmocoete (larval) stage ; but in the adult they become well- 

 developed and reach the surface (Figs. 276-7), while the animal is also 

 provided with median pineal and parietal " eyes ".^ Although 

 primitive, however, and lacking the diagnostic characteristics of true 

 fishes, the lateral eyes of this most primitive type emerge as fully 

 differentiated organs and shed little light on the origin of the eyes of 

 the higher species. It would seem, therefore, that the vertebrate eye 

 evolved not as a late off-shoot from the simple eye of Invertebrates 

 after the latter had reached an advanced stage ; it probably emerged 

 at a very early stage, further back than geological evidence can take us, 

 and developed along parallel but diverging lines. The apposite remark 

 of the great German anatomist, Froriep (1906), that the vertebrate eye 

 sprang into existence fully-formed, like Athene from the forehead of 

 Zeus, expressed the frustration of the scientists of half a century ago 

 to account for its appearance ; today we are little wiser. 



The apparently revolutionary changes in morphology which 

 characterize Vertebrates are not, of course, confined to the eyes. The 

 abruptness of the separation between the backboned and backboneless 

 animals -Aab evident to Aristotle and was firmly drawn by Lamarck 

 in 1801), but the pedigree of the former — presumably from the latter — 

 still ren! V '^s unknown and all the theories which have been advanced 

 ^ p. 72i. 2 320 to 350 million years ago, p. 754. * p. 713. 



