72 VERTEBRATE PHOTORECEPTORS 



relative distribution in different forms, become matters of 

 primary importance in a comparative study of retinal adap- 

 tation to light and darkness, and also give us a structural 

 foundation which is prerequisite to an adequate study of the 

 light-perceiving value of the retina as well as to the study of 

 color vision. 



From his studies upon the retina of reptiles. Walls (1934a) 

 has concluded that many groups have become nocturnal sec- 

 ondarily — the condition having been brought about by a 

 " . . . transmutation of some or all of the cones of an ancestral, 

 diurnal, pure-cone retina into functional rods ..." This 

 'transmutation' theory of Walls, derived from an assemblage 

 of facts, has led him to the following conclusions : 



" 1) Cones have been secondarily, phylogenetically, trans- 

 formed into rods during the evolution of a nocturnal habit 

 by forms whose ancestors were strictly diurnal and pure-cone. 



''2) Such transmutations have occurred many times, inde- 

 pendently of each other, so that unrelated forms have 

 convergently attained similar final conditions. 



''3) Intermediate stages can be seen in extant genera, some 

 of which may be in a dynamic condition with regard to a 

 change in their habits with respect to light. 



*'4) The reverse transmutation — of rods into cones — 

 has probably taken place at least once, and in this case is a 

 tertiary change. 



^'5) Within limits, habits rather than taxonomic position 

 determine the visual-cell pattern which a given form will 

 display. 



^^6) It was practically inevitable that visual-cell trans- 

 mutation should occur in reptiles, and it is improbable that 

 man:, cases will be found outside the reptilian group." 



Walls stresses the point that if the rods of some animals 

 have been derived from the cones of others, the differentia- 

 tion of the two types of visual cells is more physiological 

 than morphological, and suggests that perhaps they should 

 be given names that have solely physiological connotation 

 such as photocyte for the cone and scotocyte for the rod. 



