176 ADAPTATIONS TO DIURNAL ACTIVITY 



holds. Wunder made counts in a number of species, and found the 

 greatest number of rods (810,000 per square miUimeter of retina) in the 

 nocturnal Lota. Lota also had the fewest cones (3400/sq. mm.), the 

 diurnal Tinea the most (9000/sq. mm.). The catfishes, however, have 

 thick rods in their crude eyes. Wunder found no other teleost with so 

 few rods as Amieurus (l8,400/sq. mm.), whose rods are almost am- 

 phibian in plumpness (c/. Figs. 63 and 64, pp. 147-8) . 



The most strictly diurnal vertebrates have only cones in their retinae. 

 Among these are the great majority of lizards and snakes (all of those 

 with round pupils), some (perhaps many) birds, and the majority of 

 the members of the squirrel family — at least, the marmotines (ground- 

 squirrels, prairie-dogs) are certainly pure-cone, and all others except the 

 flying-squirrels are probably pure-cone. 



In many birds, only a few rods can be found and these may be 

 present over only a part of the whole retinal area. Cones outnumber rods 

 very greatly in all diurnal birds which have any rods at all. Turtles have 

 very few rods among their cones, and some species may have none. In 

 freshwater lampreys, the cones and rods are equal in numbers; but in 

 marine species the rods are more numerous to give the added sensitivity 

 demanded by deeper water. 



The most nearly diurnal of the amphibians — the frogs — have much 

 higher cone-to-rod ratios than do some vertebrates which are more 

 strictly diurnal than they; but in the amphibians the rods are so large 

 and the cones so small that we have here a situation which is the reverse 

 of that in the teleosts. The actual effect is of a preponderance of rods — 

 just as in teleosts, with the rods very numerous but very tiny, there is 

 an effective preponderance of cone-substance. 



Except for the vertebrates above-mentioned, none is known to exceed 

 by very much the cone-to-rod ratio of man, which is about 1 : 20 and 

 seems very low — until we take account of the great size of the eyes of 

 primates, large carnivores, and ungulates, whose retinal image sizes are 

 such that many rods may be allowed to leak in between the cones with- 

 out the visual acuity being pulled down below that of a small bird whose 

 retina is pure-cone and whose cones are contiguous. Thus, where an 

 animal has room for a large-enough eye, he can afford to have a duplex 

 retina without sacrificing too much visual acuity, and then has the 

 opportunity of seeing something in twilight or moonlight, whether he 

 takes the opportunity or not. Most do — and thus it is that ungulates, 

 large carnivores, and primates are able to stay up after the birds have 



