WATER AND THE PLAN OF THE EYE 369 



few of these ever even put their heads out of water for more than a 

 moment, and are not then demonstrably trying to see through air. 



These animals which fit our definition can be expected to have no 

 compromises in their eyes, and to have these organs wholly devoted to 

 seeing through water. But we do not know what to expect from the eye 

 of any amphibious vertebrate until we learn whether he uses his eyes 

 more in one medium or in the other, or equally in both. Many of the 

 adaptations discussed in this section naturally occur in some or in full 

 degree in the few amphibious fishes, as also in the amphibious species 

 which are to be found in every order of the classes of amphibians and 

 reptiles, and in many orders of both birds and mammals. Those losses 

 and new acquisitions involved in the restriction of vision to the aerial 

 medium are dealt with in the succeeding section of this chapter; and the 

 particularly stringent ocular requirements of any animal which attempts 

 'amphibious' vision are considered in the third section. 

 Effect of Water upon the Plan of the Eye — It must never be for- 

 gotten that the vertebrate eye originated in water. Only when this is 

 firmly in mind can we grasp the full meaning of some of the most fun- 

 damental features of ocular anatomy and physiology. When the verte- 

 brates finally took the eye on land with them, they had perforce to fill 

 it and surround it with simulations of its original medium of action, 

 salt water — even as the spiders, coming to the land, remained dependent 

 upon a bit of their old environment which they bottled up in their 

 gill chambers. 



Most of the physical and chemical properties of water affect the eye 

 or its operation. Many of these properties are essentially simple exagger- 

 ations of those of air — the greater absorption of light, greater scattering, 

 greater pressure-changes with altitude or depth, greater friction and so 

 on. But the quantitative disparity between air and water, with respect 

 to a given property, is so very great that in its evolution the aquatic eye 

 responds qualitatively to factors whose air equivalents are negligible 

 to it and evoke no adaptive response at all. 



Before the evolving eye produced any precise adjustments to the 

 purely optical properties of water, it had first to attain harmony with 

 properties which affect all animal organs and tissues exposed to that 

 medium, whether they are photosensory or not. The phenomenon which 

 unquestionably had a more profound effect than any other upon the 

 fundamental design of the vertebrate eye was osmosis, which therefore 

 receives first consideration in our discussion. 



