CHAPTER XIII 

 THE EYES OF REPTILES 



The portrait of Gordon l. walls (1905 ) (Fig. 417) could suitably serve 



as an introduction to many chaj^ters in this book for he has done much to corre- 

 late and rationalize our ideas on the structvire and function of the eyes of 

 Vertebrates. Originally trained as an engineer, he branched into zoology at 

 Harvard University ; here, expecting to work on Rotifers, he was arbitrarily 

 assigned a problem on the retina for investigation and for many years devoted 

 all his energies to the study of the finer structure and function of this tissue 

 throughout the vertebrate phylum. His most striking contribution in this field 

 was his enthusiastic advocacy of the theory that the cones were more primitive 

 than the rods and that in the evolutionary process the cones of an ancestral 

 species transmuted into rods in a descendant species. It was in the eyes of 

 Reptiles, particularly snakes, that he found the most satisfying evidence for 

 his views, and his observations led him to formulate new ideas about the evolu- 

 tionary history of groups such as these. His work in this field was summarized 

 in his classical book. The Vertebrate Eye and its Adaptive Radiation, published 

 in 1942, which is undoubtedly the most comprehensive and readable vokime 

 on this subject in the English literature ; to it I have been greatly indebted in 

 the writing of this volume. This task completed, he forsook comparative 

 ophthalmology and, as Professor of Physiological Optics at the University of 

 California, he devoted his attention to the still more complex problems of colovu" 

 vision and colour blindness, a subject wherein his contributions will be noted 

 in a subsequent volume of this series. 



Of the five main groups of extant Reptiles, the chelonl\ns (turtles, 

 tortoises) are the most archaic and primitive ; the rhynchocephalians (the 

 sole extant representative of which is Sphenodon) have relatively simple eyes 

 largely adapted for noctumality ; the crocodilians (crocodiles, alligators) 

 again have relatively simple eyes largely adapted for vision under water ; the 

 LACERTLLiANS (lizards), active and (with many exceptions) typically diurnal 

 creatures, have the most elaborately formed eyes among the entire class and the 

 most typically reptilian in their characteristics; while ophidians (snakes) have 

 eyes peculiar to themselves and in most of their essential features widely different 

 from all other members of the group, bearing little resemblance to the eyes of 

 their immediate ancestors, the lizards. 



We shall therefore describe the eyes of lizards in some detail as the essential 

 reptilian type, enumerate shortly the main simi^lifieations seen in the first three 

 groups, and finally discuss the unique eyes of snakes. 



THE EYES OF BEPTILES are tlie first to be finally and completely 

 adapted to terrestrial life. We have already seen that those of the 

 Ichthyopsida have many features in common and that although 

 Amphibians, leaving the water after the larval stage, have acquired 

 many adaptations for vision on dry land, their eyes still exhibit a 



S.O.— VOL. I. 3.>3 -ii 



