216 ADAPTATIONS TO NOCTURNAL ACTIVITY 



to themselves in the matter of their nerve-ceil connections, and ten cones 

 scattered among a thousand rods cannot cost the retina as much in 

 sensitivity as ten rods, scattered among a thousand cones and hooked 

 up to a single optic nerve fiber, would cost it in resolving power. There 

 is consequently simply not the urgency for getting rid of cones in noc- 

 turnal animals, that there is for weeding out rods in diurnal forms. 

 This is quite apart from any greater usefulness of 'even a few' cones 

 than of 'only a few' rods. The turtles are conspicuously exceptional in 

 having only a very few rods scattered in an almost pure-cone retina — 

 but even these may be useful since they are more numerous in light- 

 shunning forms such as Chelydra, and in the nocturnal Pseudemys. 



Pure-Rod Animals — A pure-rod retina is automatically obtained 

 where, as in some lizards (geckoes, etc.) and snakes (Hypsiglena, Phyl- 

 lorhynchus) it has been manufactured by transmuting all of the single 

 and double cones of an ancestral pure-cone retina into single and double 

 rods. Transmutation has left so very few unchanged cones in Sphenodon 

 that in an entire section of its very large eye, never more than twenty 

 can be found. Aside from these forms, absolutely cone-free retinse which 

 once were duplex, and have lost their cones, are known for a certainty 

 to occur only in deep-sea fishes, the bats, and the armadillo. Some others 

 probably have only rods — all but one or two elasmobranchs, Lepidosiren 

 among the lungfishes, caecilians, the hedge-hog, the guinea-pig, the whales 

 and seals, most lemuroids and Aotus — but all of these need addi- 

 tional histological study (since most of these were last studied, micro- 

 technical methods have improved enormously) . Still others, like the rat 

 and other nocturnal rodents, are widely believed to have no cones but 

 do indeed have a few. One ridiculous statement often seen is that rats 

 and owls "have a few rudimentary cones." In a duplex retina, no visual 

 cell is ever rudimentary, though one population of visual cells may be so 

 scant as to deserve the term, like the cones of Sphenodon or the rods of 

 turtles. As a matter of fact, owls have enough cones so that they are 

 able to see more acutely by day than by night. Rochon-Duvigneaud once 

 picketed a Bubo bubo in an open field, and found that it could detect 

 an approaching hawk which was flying so high as to be invisible, at that 

 moment, to humans. 



Summation — In nocturnal animals the rods tend to be very slender as 

 well as very numerous, causing the outer nuclear layer to thicken greatly 

 (Fig. 72, p. 177). In lungfishes and amphibians, however, the rods are 



