458 ADAPTATIONS TO MEDIA AND SUBSTRATES 



animal would be in misery without a spectacle — at least, so long as the 

 eyes were retained in useful condition. 



In some blind burrowing lizards (e.g., Amphisbaenidae) , the tertiary 

 spectacle might seem a mere sign of ocular degeneracy — as a secondary 

 spectacle so often is, in limicolous fishes. But if we imagine the evolution 

 of a burrowing reptile, we see at once that the spectacle would have to 

 antedate the blindness, and that it must have been truly protective at 

 first. Later, of course, the spectacle came to have no meaning — when the 

 organ it had been protecting degenerated beneath it. A morphological 

 equivalent of the tertiary spectacle (but one which never was trans- 

 parent) is seen in some moles, in which the lid opening has constricted 

 to the vanishing point, leaving the furry skin unbroken over the vestigial 

 eye. Similarly, in one cave salamander — Typhlotriton spelceus — the lids 

 develop at metamorphosis, as usual, but then fuse edge-to-edge (except 

 at their very centers) over the eye. Such opaque spectacles are in quite 

 another category from those of burrowing reptiles which still had func- 

 tional eyes long after they had evolved their spectacles. 



The spectacles of nocturnal, non-burrowing lizards call for still an- 

 other explanation. Geckoes* and night lizards (Xantusiidae) may hide 

 under objects, but they are not true burrowers. Most of them live far 

 from deserts, and the desert xantusiids live under boulder flakes or in 

 yucca tops — not in the sand. Moreover, other reptiles such as Spbeno- 

 don, the beaded lizards, and the crocodiles are just as nocturnal, yet have 

 normal lids. These latter forms are large-bodied, however; and the an- 

 swer to the gekkonid or xantusiid spectacle is seen in the fact that these 

 little chaps place their eyes in constant jeopardy from gravel and stubble, 

 amidst which they crawl under such poor visual-acuity conditions that 

 they cannot possibly see them clearly enough to avoid them. 



The spectacles of some serpents might seem to explain themselves as 

 do those of bottom fishes, Emyda and Cbelodina, or the lizards: some 

 snakes feed in mud, some burrow, some are nocturnal. All of them rep- 

 tate or crawl with the eyes very close to the potentially injurious sub- 

 strate. But so do many diurnal lizards, and these have no spectacles. 

 Most snakes are diurnal and live in above-ground habitats. Many of 

 them are arboreal, and some are permanently aquatic. Why then do all 

 snakes have spectacles when only a small minority really need them? 

 Manifestly, because the original snake or its immediate lacertilian an- 

 cestor was nocturnal or subterranean. We shall see later (Chapter 16, 



*The 'eublepharids' have mobile lids fringed with stiff lash-like scales. 



