254 ADAPTATIONS TO SPACE AND MOTION 



which the reader will remember as the chief reason why large eyes afford 

 better-resolved cerebral images (see p. 171). Consequently a small object 

 can come very close to a small eye before its image recedes off the tips 

 of the visual cells — as close, perhaps, as the near-point of a large eye 

 possessing a good mechanism of accommodation. On top of all this, the 

 small lenses of small eyes have much greater 'depth of focus' than do 

 the lenses of large eyes; for, the shorter the focal length, the greater the 

 depth of focus of a lens. The result may well be that the small eye needs 

 no accommodation at all — so, when we find that the mechanism of 

 accommodation has undergone phylogenetic atrophy in many small- 

 eyed nocturnal mammals we are hardly justified in mournfully shaking 

 our heads at their 'degeneracy'. 



In eyes which are large enough to seem to require a capacity for accom- 

 modation, there are still four ways to dodge the demand and obtain clear 

 images of objects at various distances, successively or simultaneously, 

 with a perfectly static intra-ocular situation. All four of these substitutes 

 for accommodation have been devised and successfully employed by 

 different vertebrate animals. They are: 



1. An increase in the length, or the effective length, of the visual cells. 

 Vertebrates are strictly limited in the actual lengthening of the receptor 

 elements, for any such elongation increases the distance between the 

 inner layers of the retina and an important source of their supplies, the 

 choriocapillaris. If carried to extremes, this would be detrimental to 

 retinal nutrition. The cephalopod molluscs with their erect retinae are 

 under no such handicap, and the visual elements of a squid are so enor- 

 mously long that the image can recede and advance through their length, 

 corresponding to great excursions of the object to and from the eye, 

 without making any demands upon the inefficient apparatus of accom- 

 modation. Where vertebrates have very long visual cells, as in deep-sea 

 fishes and some geckoes (see Fig. 25, p. 62), it is of course primarily for 

 the sake of increasing their sensitivity, though as an incidental effect it 

 partially obviates accommodation. But in the great fruit-bats, the so- 

 called flying foxes (Macrochiroptera) , a unique alteration has resulted 

 in a great increase in the effective length of the rods without these cells 

 being actually elongated at all: the chorioid is beset with innumerable 

 conical eminences which deform the visual-cell layer, the elements of the 

 latter being set endwise against the chorioidal papillae like the trees in a 

 range of mountains (Fig. 102a). Thus no matter how near or far the 

 object may be, its image falls sharply upon a set of rods standing at a 



