448 ADAPTATIONS TO MEDIA AND SUBSTRATES 



dioptric system as effectively as a pinhole could do. At the same time, it 

 will admit more light than will a pinhole of the same width as the slit. 



The primitive seal, seeking a means of obtaining sharp images in both 

 water and air, may have considered the usual method — that of the turtle, 

 cormorant, and otter — but decided that it involved too much intra-ocular 

 work to employ both ciliary muscle and iris sphincter to wring the lens. 

 Much simpler to develop just enough accommodation to give himself 

 emmetropia under water, and eliminate entirely the need of any great 

 reserve of accommodation for use in seeing through the air. To make 

 extensive accommodation in air unnecessary, he developed a high degree 

 of corneal astigmatism, with its axis and his slit pupil so oriented as to 

 give an approach to the performance of a pinhole camera. The quasi- 

 pinhole reduced the retinal illumination so greatly in air that a sensitive 

 retina, backed even by a tapetum, became necessary. Under water, the 

 corneal astigmatism conveniently vanishes and the spherical lens, oper- 

 ated by a quite ordinary ciliary body, goes into action. Its accommo- 

 dation has now to combat the hypermetropia which replaces the aerial 

 emmetropia or myopia. The widened pupil lets in enough of the dimmed 

 subaqueous light, and the seal eye is then as useful in deep water as that 

 of a shark. 



We may be sure that the system works, if not always (Phocd barbata!) 

 just in this way. Considering their food and feeding habits, seals would 

 starve without clear underwater vision. On land or ice, a seal is decidedly 

 alert — not wholly because of his excellent olfaction. He is visually alert, 

 never sleeping for more than four minutes at a time. True, the elephant 

 seal appears near-sighted out of water, like a penguin; but even the most 

 eye-minded vertebrates have a deadline, located afar by fear or nearby 

 by fearlessness, to which they will allow approach without showing alarm 

 even though they see clearly far beyond it. The elephant seal's apparent 

 aerial myopia may really have such a basis. Under water his vision is 

 surely good, for he feeds on swift cuttlefishes. In great contrast to 

 Macrorhinus, the average seal will take flight from a man 150 yards 

 away. 



Even a wild seal is reputed to catch in its mouth a stone tossed to it. 

 The reader may not want to believe this — and can hardly be blamed. 

 But if he has ever watched a trained sea-lion on a dry stage going through 

 a repertoire of catching balls, sticks, and finny rewards, he cannot doubt 

 that the seals in general are as eye-minded, as readily able to see well 

 through air, as he himself. 



