ALTERATIONS FOR AERIAL VISION 417 



ings of the severe aquatic environment. The great whales would still 

 need their thick scleras, even if they never left the surface at all. 



Scleral cartilage has been allowed to disappear in all vertebrate groups 

 which have allowed the eyeball to become spherical — in salamanders, 

 snakes, and mammals. Where, as in the large whales, the globe has later 

 ballooned and flattened, the sclera has had to be thickened very greatly; 

 for it takes a deal of connective tissue to give the same stiffness as a 

 much thinner piece of cartilage. It was a great piece of luck for the 

 vertebrates to chance upon the plan, for their eyes, of a membranous 

 sac kept turgid by internal fluid pressure. The whales have crowded that 

 luck about as far as can safely be done. 



(B) Aerial Vision 

 The emergence of the vertebrates upon the land necessitated several 

 changes in the eye, which in turn made possible certain improvements 

 which would never have been brought about in the water. Some of these 

 changes (those of the eyeball itself) were mostly related to the major 

 optical difference between air and water — the difference in refractive 

 index. Other changes (those in the adnexa) were demanded by the loss 

 of the moistening and cleansing action of water, and by the new jeopardy 

 from sharp blows and abrasive objects. 



Changes in Dioptrics — The alterations of the eyeball for vision 

 through air were essentially optical. No longer was the eye shielded 

 from harmful ultra-violet light, and the lens and cornea had to become 

 able to absorb this light, or to change it by fluorescence into harmless 

 visible light. No longer could the cornea with impunity have an irregular 

 surface, and with the Amphibia it becomes smooth and optically perfect. 

 Nor was a flat corneal surface any longer necessary or desirable. Exposed 

 to air instead of to water, it became the most important refractive surface 

 in the eyeball, where it had formerly been a nonentity. When the cornea 

 became arched, as it first did in the amphibians, this drew the optical 

 center of the eye forward. This in turn enlarged the image, even though 

 the lens, now relieved of the lion's share of image-placement, became 

 flatter and receded into the eye (cf. Figs. 105 a and 106; pp. 261, 266). 

 The backward shift of the lens was of inestimable importance for the 

 future, for it placed the lens in such a relation to the ciliary body that 

 the latter could eventually (in the Sauropsida) take over the labor of 

 accommodation, and could accomplish this adjustment with greater 

 speed, and over a far greater range, than had hitherto been possible. 



