520 ADAPTATIONS TO PHOTIC QUALITY 



Exclusive of the placental mammals, then, color vision has been 

 elaborated perhaps only once (by the chondrosteans, passed on by 

 them to the stegocephalian-reptilian-avian series as well as to the teleosts) , 

 perhaps twice (by the holosteans or teleosts and later, independently, by 

 the early reptiles [cotylosaurs], which gave it to the birds and maybe to 

 early diurnal mammals). Then too, color vision may have been devel- 

 oped de novo within some reptilian groups. For, just as the transmutation 

 of cones into rods in such forms as Sphenodon, the geckoes, and the 

 Xantusiidae may not necessarily have abolished color vision (gecko rods, 

 Crozier and Wolf have found, respond to flicker like turtle cones) , so 

 also color vision may have been regenerated or re-invented where cones 

 have secondarily reappeared. Dryophis et al will be most interesting in 

 this connection — if color-vision researchers sometime find a way to 

 'motivate' them — as would also the diurnal geckoes such as Phelsuma, 

 whose visual cells were once lizard cones, then gecko rods, and are now 

 probably cones once more. 



However few or many times color-vision mechanisms may previously 

 have arisen in vertebrate evolution, the color vision of the higher pri- 

 mates is assuredly a law unto itself, genetically and historically speaking 

 (see Fig. 156). The absence of color vision in the lowest primates, the 

 lorises, galagos, tarsiers and the like, might mean only that these had dis- 

 carded color vision by discarding cones in order to become nocturnal. 

 The indications are overwhelmingly against such a view. The primates 

 originated as a nocturnal group, from nocturnal, rat-sized insectivore 

 ancestors which may not even have kept any of the cones of their 

 therapsidan forebears. 



The placental-mammalian cone looks most suspiciously as though it 

 had arisen by transmutation within the subclass. It is never double, never 

 has an oil-droplet or a paraboloid, never migrates. The placental mam- 

 mals evolved through the restrictions of the nocturnality of the early 

 insectivores. Like the snakes, which had an even worse time being born 

 from the lizards, they probably produced an entirely new crop of cones, 

 which consequently are quite unlike those of the lower mammals and the 

 Sauropsida. Holding this viewpoint, it becomes easier to understand why 

 it is that although cones are numerous and widespread among arhythmic 

 and diurnal placental mammals, yet color vision is not. To acquire color 

 vision, each group of such mammals would have to start from scratch; 

 and only those have made this start, whose vision means so much to them 

 that color vision is a real desideratum. 



