PHYLOGENY, CHEMISTRY OF FILTERS 203 



ten rid of all droplets. So have many lizards and the snakes; and it is 

 in these two groups that we find evidence that once the oil-droplets have 

 been lost, they can never be regained : 



Most lizards are diurnal and have bright yellow and colorless oil-drop- 

 lets. In the chameleons, there is claimed to be an additional yellow pig- 

 mentation of the inner layers of the retina in the region of the fovea, 

 though this matter requires further investigation. Secretive and fossorial 

 lizards such as Anniella have lost most of the pigment of the oil-drop- 

 lets, and nocturnal above-ground forms like Xantusia and Heloderma 

 have only completely colorless droplets. The logical final step has been 

 taken by the geckoes, which probably passed through a Xantusia-Vkt 

 stage (consult Fig. 25, p. 62) but later eliminated the useless, color- 

 less oil-droplet entirely. Some geckoes are so small, with such tiny eyes 

 (e. g., Sphcerodactylus and Gonatodes spp.), that they are able to be 

 more or less diurnal without benefit of a slit pupil. A couple of genera of 

 good-sized geckoes {Phelsuma and Lygodactylus) are round-pupilled 

 and diurnal, and have eyes large enough to demand special provisions for 

 this habit. It is probable that in them the visual cells have been recon- 

 verted into cones, and in Lygodactylus at least the lens is known to be 

 yellow. 



The bearing of the structure of the eye upon the problem of the origin 

 of the snakes will be discussed later (Chapter 16, section D) ; suffice it to 

 say here that their lack of oil-droplets shows them to have originated as 

 light-shunning forms. The yellow lens has appeared here (as in Lygo- 

 dactylus and the squirrels) because the oil-droplets could not reappear, 

 upon the adoption of diurnality by forms whose photophobic ancestors 

 had discarded them. It is safe to say that any group which has oil-drop- 

 lets has had unbroken ancestry in forms similarly provided. Thus, the 

 presence of yellow droplets in frogs indicates that the early amphibians, 

 the Stegocephali, had droplets and were diurnal— as indeed we should 

 surmise from their bulk, their consequent need of the warmth of the sun, 

 and their complete freedom from terrestrial enemies during their evolu- 

 tion from the fishes. A similar argument would attribute diurnality to 

 the dinosaurian ancestors of the birds. 



Of the mammals, the monotremes are usually called nocturnal though 

 the duck-bill is not strictly so, and has oil-droplets whose color or lack of 

 it has not been ascertained. Droplets occur in marsupials but are always 

 colorless so far as is known, though once erroneously reported to be pig- 

 mented in kangaroos. The placental mammals are nearly all crepuscular 



