THE DIURNAL RETINA 175 



are still to focus on the retina; hence the reduction of its sharpness of 

 curvature. If we imagine the acuity-requirements of an animal to be stead- 

 ily increasing through evolution, we may visualize the consequent gross 

 changes in the eye thus : 



1. A steady increase in absolute size until the eye is relatively large 

 if the animal is small. If the animal is large, the eye may then still be 

 relatively small though absolutely large. The result is an enlargement 

 of the image and an increase in resolving power since the visual cells do 

 not enlarge proportionately, but instead become more numerous per 

 angular unit of the image. 



2. A faster growth of the fundal portion, the anterior segment be- 

 coming, more and more rapidly, relatively small as compared with the 

 posterior. The result is an increase in the size of the image relative to 

 the size of the eye, with a consequent increase in resolving power. 



3. A relative or an absolute forward movement (or both) of the 

 optical center of the cornea-lens system, further expanding the image 

 owing to the increased distance from optical center to retina (Fig. 71). 



4. A relative diametral shrinkage and flattening of the lens or the 

 cornea (or both) , increasing the focal length to suit it to the increasing 

 distance from optical center to retina. 



5. A relative diminution of the size of the pupil and of its excursion 

 of movement, there being abundance of light entering the eye under 

 diurnal conditions (so that the pupil can be small) and, in the pure-cone 

 retina in which diurnality tends to culminate, a restricted range of sensi- 

 tivities (so that there is no point to having the pupil capable of opening 

 very widely or of closing extremely) . 



(B) The Diurnal Retina 



ConeiRod and Receptor:Conductor Ratios — The diurnal retina is 

 invariably rich in cone-substance. This clumsy term must be used in at 

 least this one place, for the sake of emphasizing that it is the relative 

 total masses, not the numbers, of cones and rods which count in retinal 

 adaptations to sensitivity. For, an animal may have dozens of rods to 

 every cone and still be suited best for diurnal activity — if the rods are 

 tiny and the cones massive. This is actually the case in the bright-light 

 teleost fishes (Fig. 22b, 23c, p. 54). Apart from them, the rule is that 

 relative numbers of cones-per-rod are high in diurnal forms, low in 

 nocturnal. And within the teleost group, this rule of numbers of course 



