430 ADAPTATIONS TO MEDIA AND SUBSTRATES 



elaborate adnexal pattern which terrestrial animals have found essential. 

 If they happen to belong to the great sauropsidan-mammalian majority 

 of air-and-water lookers they have no worries on this latter score; for 

 though their lids and glands are not needed under water, neither are 

 they any great handicap. But these secondarily aquatic forms which still 

 cling to the land for feeding and breeding purposes have to compensate 

 somehow for the optical loss of the cornea, when this important refrac- 

 tive structure is 'gone with the water'. 



In attempting to combine two very diverse optical arrangements within 

 one visual organ, amphibious vertebrates are in a position analogous to 

 that of the twenty-four-hour animal with respect to the extremes of 

 illumination. The arhythmic animal, be it remembered, must effect a 

 mixture of compatible adaptations to both bright and dim light. If in- 

 stead he merely 'strikes an average', he ends up not by being arhythmic 

 and maximally independent of the rotation of the earth, but crepuscular 

 and restricted more than ever in his hours of activity. Striking an average 

 in the eye for both air and water is well enough as far as the adnexa are 

 concerned. We see just such a situation in the Amphibia, whose half-way- 

 evolved lids and glands allow them freedom in the air, provided that the 

 air be humid. But half-way adaptation of the eyeball itself is impossible — 

 there is no visual medium intermediate between water and air. However 

 moist the atmosphere may be, seeing through it demands strictly aerial 

 optics — and seeing through water demands, just as sternly, aquatic optics. 



The problem boils down essentially to the production of an exceptional 

 range of accommodation — sufficient, in an amphibious fish, to overcome 

 the increased myopia which appears in the eye in air; or sufficient, in a 

 higher vertebrate, to neutralize the hypermetropia which instantly super- 

 venes when the cornea is immersed in water. These added demands upon 

 the accommodation of a given amphibious animal could never be met by 

 the mechanism characteristic of his immediate one-medium relatives, and 

 are usually countered by supplementary devices which increase the defor- 

 mation of the lens at a considerable cost in muscular effort. A very few 

 vertebrates, however, have found easier ways of producing interchange- 

 able aerial and aquatic systems of optics without becoming intra-ocularly 

 muscle-bound. They meet the problem with a bare minimum of muscular 

 exertion within the eyeball, or even with none whatever — just as a few 

 vertebrates restricted to either aquatic or aerial vision have been clever 

 enough to obtain good images over a range of distances without the use 

 of dynamic accommodation at all (see pp. 254-7) . 



