518 ADAPTATIONS TO PHOTIC QUALITY 



Of possible color vision in some of the most nearly diurnal marsupials, 

 the kangaroos and wallabies, we know nothing. For the monotremes and 

 marsupials together, there is only the single entirely negative report of 

 Salzle on an opossum species, Didelphis paraguayensis. But the retenti<!^n 

 of the cone oil-droplets during so much of mammalian evolution, past 

 the monotreme level and into the marsupials, suggests that these lower 

 subclasses were not always as strongly nocturnal as their surviving repre- 

 sentatives (the kangaroos excepted) are today. 



Phytogeny of Color Vision — In digesting the above survey, the 

 reader may have been struck by the fact that the groups of vertebrates 

 which possess full-blown color vision are the very ones which have 

 evolved excellent mechanisms of accommodation: the teleosts, the sau- 

 ropsidans, and the primates. This relationship is not accidental. These 

 are the groups which are more eye-minded than otherwise, and whose 

 retinal visual acuity is high enough to deserve refined optical images and 

 to make hue-differences a useful factor in the perception, identification, 

 and evaluation of visual objects. They are the only groups in which a 

 fovea is ever seen. The bright-light habit depends upon cone-richness, 

 affords high visual acuity, demands good accommodation, and supports 

 good hue-discrimination. It is only natural, then, that these phenomena 

 are found in association. 



The color-vision systems of these three vertebrate groups are probably 

 just as independent of each other, in point of origin, as are their methods 

 of accommodation. If they are physiologically identical or nearly so (and 

 they certainly appear to be), it is because, like so many other simpler, 

 discontinuously-distributed and repeatedly-evolved entities (lentiflavin, 

 rhodopsin, melanin etc.) they have developed out of a substrate of chem- 

 ical and physiological potentialities which is common to all vertebrates. 

 In other words, the systems are homoiologous. 



We know nothing about the possible color vision of non-teleost fishes. 

 The lampreys, some species of which have at least 50% cones, may con- 

 ceivably have it, though it would be hard to say what its value might be 

 to them. The elasmobranchs are all pure-rod excepting Myliobatis aquila 

 and Mustelus spp., which have few cones. But at least one of the hol- 

 osteans, Amia, is known to have a teleost-like retina and habits; and 

 when eventually investigated this may prove to be the group which really 

 invented teleostean color vision. 



The extinct crossroads group of the Stegocephali, which were almost 

 certainly diurnal, may have shared with the teleosts the inheritance of 



