428 ADAPTATIONS TO MEDIA AND SUBSTRATES 



of eyes exposed to air and to injurious terrestrial objects has been pro- 

 duced, in the form of a tertiary spectacle (section D), in some verte- 

 brates — but unfortunately only in those which were absolutely driven to 

 make this logical modification of the mobile palpebral system. 



But the lids have not been without their purely optical influences upon 

 the eyes of land animals. The very choice of an upper-lower combination 

 instead of a nasal-temporal or diagonal one (the turtles excepted) was 

 dictated by the predominantly vertical direction of the incident sunlight. 

 Again, as long as the animal's eyes were carried close to the ground and 

 exposed to bright upward reflections from the substrate, it was desirable 

 to have the lower lid in control of eye closure. Only in those forms in 

 which, by and large, the eyes are carried higher (crocodiles, some birds, 

 nearly all mammals) does the upper lid become the more active of the 

 two. In very small mammals (e.g., the house mouse) the lower lid may 

 move more than the upper, as in the creepers and crawlers of the lower 

 classes. 



The horizontal orientation of the palpebral fissure has had at least 

 two effects upon the structure of the eyeball itself. It has allowed the 

 development of 'ellipticity', of horizontally extended corneas and pupils, 

 in those mammals which have great need of a wide visual field. It 

 accounts also for the well-nigh universally vertical orientation of slit 

 pupils in terrestrial forms. In bright light the lids, partially closed as we 

 so often see them in a basking cat, are not unimportant in aiding the 

 pupil to control intra-ocular illumination — as witness the fact that where 

 the slit pupil can be entirely closed, it is most often in forms which lack 

 mobile lids (see Chapter 9, section C). Where the slit pupil is vertical, 

 the squinting of the lid opening at right angles to the slit makes of it a 

 better stenopaic aperture, combatting the optical imperfections of the 

 peripheries of lens and cornea, yet still admitting enough light because 

 of the great retinal sensitivity of slit-pupilled eyes. It seems significant 

 that the vertical orientation of the slit pupil was not finally adopted until 

 the vertebrates came on land and developed lids (Table VI, pp. 220-1). 



Peculiar Status of the Elasmobranchs — Our whole philosophy of 

 the basis of the contrast between the fish eye (with its lack of a ciliary 

 corona, lids, and glands, and its spherical lens in contact with a flat cor- 

 nea in a shallow globe) and the typical 'air' eye (in which ciliary folds 

 are present, the lens flattened and drawn back from an arched cornea 

 kept moist by glands and the lids which spread their products) is rather 

 rudely disturbed by the elasmobranchs. In some of these fishes all of the 



