174 ADAPTATIONS TO DIURNAL ACTIVITY 



eye. In predaceous fishes, lizards, birds, ungulates, squirrels, and other 

 swift and agile forms, large eyes go with swiftness of movement (Leuck- 

 art's ratio). 



Wherever the eye of a diurnal animal is actually small it may be that 

 visual acuity is low because, considering the animal's habits and needs, 

 it need not be any higher. This is the situation in the snakes. Far more 

 often, the eye is small because there is simply no room for a larger one. 

 Internal adaptations then appear, which compensate for inadequate size; 

 and the same adaptations occur in even very large eyes, supplementing 

 the effect of size per se, wherever maximal visual acuity is desired. 



These internal rearrangements usually consist at least of a flattening 

 of the lens and a shallowing of the anterior segment of the eyeball. (It 

 should be remembered that the anterior segment is not defined as the an- 

 terior halj of the eyeball, in front of the equator, but as the portion anter- 

 ior to a plane tangent to the back surface of the lens. This may com- 

 prise very much less than half of the volume of the eye). These changes 

 have taken place in the birds and are perhaps most marked in the 

 chameleon and in the higher primates (Fig. 71). The squirrels are 

 conspicuous for having more nearly spherical lenses than other strictly 

 diurnal vertebrates. The human eye, among mammalian eyes in gen- 

 eral, is atypical in the other direction, in its possession of so very flat 

 a lens. One gets the impression from the human, as also from most bird 

 eyes, of an ordinary-sized anterior segment grafted onto an oversized 

 posterior segment which 'doesn't belong' to it. This impression is actually 

 quite true to the facts, for it is not that the parts of the anterior segment 

 have been made smaller in order to gain visual acuity, but rather that the 

 fundus of the eyeball has been made larger, the lens then flattening in 

 order to move the focal level back onto the now more distant retina. 

 The fishes are peculiarly fortunate in that they are able, because of the 

 extraordinarily high refractive index of the lens, to obtain a broad image 

 without the eye having to be as deep as it is broad. The fish eye is con- 

 sequently flattened (Fig. 77b, p. 185) and encroaches less upon the in- 

 ternal structures of the head. 



The effect of this alteration of the relative size of the anterior and pos- 

 terior segments is to move forward the nodal points of the dioptric sys- 

 tem. The distance from the optical center to the retina being thereby in- 

 creased, the image enlarges just as it does when we draw a stereopticon 

 lantern farther away from its screen. For the greater distance of 'throw' 

 of the image, the lens must now bend the light-rays less sharply if they 



