660 



THE EYE IN EVOLUTION 



Motacilla 



Hawk, Buteo 



a " private telephone wire " to the brain, so that each cone would 

 make its individual contribution to the resolution of a pattern. In 

 an eye designed to attain a high visual acuity, therefore, the retina is 

 rich in cones, its area of special differentiation or fovea is pure-cone, 

 the inner nuclear layer is thickly packed and composed of many 

 layers of cells and the ganglion cells are necessarily numerous (Fig. 756) ; 

 in such a retina there is thus little summation and the ratio between 

 the optic nerve fibres and receptors is high. 



Thus Franz (1934) estimated that the great summation in the retina of 

 Selachians (visual cells 10,800/sq. mm., ganglion cells 1,500) must reduce their 

 visual acuity to 5% of that of man (200,000 : 200,000 in the central fovea), 

 while, also owing to its high summation, the resolving power of the eye of the 

 whale can be only 2% of that of man. 



The remarkable superiority of the retina of Birds is shown not only in the 

 regularity of the arrangement of the cells but in their numbers, so that the 

 ratio of conductive to sensory cells is exceptionally high. In the American 

 " robin ", Turdus migratorius, for example, cellular counts outside the foveal 

 region give the astonishing figures of : outer nuclei, 3 rows of cells ; inner 

 nuclei, 28 ; ganglion cells, 3 (Walls, 1942). Even in the week-old chick the 

 corresponding figures for the peripheral retina are : 2-5, 18, 2-5. Similarly in 

 the peripheral retina of the white wagtail, Motacilla alba, there are 120,000 

 visual cells per sq. mm. with a corresponding 100,000 ganglion cells ; in the 

 fovea of the English sparrow. Passer domesticus, 400,000 (Franz, 1934), and in 

 the hawk, Buteo, 1,000,000 (Rochon-Duvigneaud, 1943). In the human fovea 

 the corresponding figvire is 200,000. Even in the peripheral retina of the 

 nocturnal owl. Bubo, there are 56,000 visvial elements per sq. mm. summating 

 3,600 ganglion cells, while the overall summation ratio of the human retina is 

 125 : 1 (Walls, 1942). In the comparative disability of daylight the owl would 

 thus appear to have a potential visual acuity greater than man, while the 

 resolving power of the peripheral retina of the hawk should be twice, and that 

 of its fovea eight times that of the human fovea. 



Minnow 



Stickleback 



THE VISUAL ACUITY OF VERTEBRATES 



Among FISHES the general acuity is probably relatively poor 

 (v. Hess, 1909-14), but among some Teleosts the complexity of the 

 retina and the provision of a fovea indicate the possibility of a rela- 

 tively high grade of resolution. Training experiments depending on 

 the discrimination of form in a number of Teleosts have furnished 

 interesting results. Goldsmith (1914) and Maes (1930) found that 

 goldfish were adept at this, while Rowley (1934) established that they 

 could distinguish between circles held in front of them the diameters 

 of which differed by only 3 millimetres ; Herter (1929-53) trained 

 minnows to differentiate between circles, squares, triangles and crosses ; 

 and Meesters (1940) obtained similar results with sticklebacks with 

 curved figures. It is obvious from the experiences of deep-sea divers 

 such as Beebe (1934) and Cousteau (1953) that certain fish, at any rate, 

 exhibit a degree of curiosity regarding strange elements in their 



