634 REPTILES 



either nocturnal, or else lived underground, and those of their descend- 

 ants which are neither nocturnal nor fossorial have been unable to trade 

 the spectacle in for a pair of mobile lids. Again, the absence of retinal 

 oil-droplets in all snakes, and the presence instead of a yellow lens in 

 diurnal species, has been emphasized as indicating that the early snakes 

 shunned bright light. Their invention of the spectacle and their discard 

 of the oil-droplets had a common basis. 



Mere above-ground noctumality would not, however, have called for 

 any greater changes in the ancestral lizard eye than have occurred in the 

 night-lizards, snake-lizards, and geckoes. The pattern of the whole snake 

 eye is consistent only with the hypothesis that the first snakes lived 

 underground or originated there from lizards which had become fos- 

 sorial. Two whole families of snakes and several families of lizards have 

 this habit even today. 



Quite aside from the structure of their paired eyes, there are a number 

 of other ophidian peculiarities which seem puzzling when one considers 

 how much alike the habits of snakes and lizards are, but are at once 

 explained by the fossorial-origin hypothesis: it accounts nicely for the 

 loss of not only the limbs but the ears as well, and the parietal eye, 

 dermal color-changes, retinal photomechanical changes,* and some of 

 the same cranial elements which are lacking in the subterranean amphis- 

 baenid lizards. All of these things are present in the Varanidae, and all 

 would certainly have been retained by the snakes if they had originated 

 on the earth's surface. 



As the lizard ancestor took more and more strongly to an under- 

 ground life, its eye probably at first increased in sensitivity. The pupil 

 may even have become a slit, as it is in burrowing boas; and the retina 

 would in any case have lost the oil-droplet pigment, then the droplets 

 themselves, even if the cones were not converted temporarily into rods. 

 The long persistence of the light-shunning habit would permit the de- 

 generation of the whole apparatus of accommodation — and this com- 

 prises a good part of the eye : the atrophy of the ciliary muscle made it 

 no longer necessary to maintain a ringwulst, or scleral ossicles, or even 

 scleral cartilage; and of course the ciliary processes were already gone 

 in the diurnal lizard ancestor. As the eye shrank, then, it also became 

 spherical. The spectacle had to be provided early — though as the eye 

 degenerated beneath it, it eventually lost its usefulness for a time. The 



*Though this meant only a hastening of a degradation which is seen in all other reptiles 

 as well (see Table II, p. 150). 



