DUPLICITY AND TRANSMUTATION 165 



have come to be grateful for the shield of night over at least a part 

 of their activities. 



To regain a duplex retina and twenty-four-hour capacity — let alone 

 to go still further on into nocturnality, loose or strict — the pure-cone 

 reptiles have had actually to convert or transmute some or all of the 

 cones into low-threshold, massive, cylindrical elements. In most cases 

 these have been able to re-invent rhodopsin and thus fully deserve to 

 be called rods. Intermediate stages in these transmutations can be seen 

 in living species, which show us therefore some of the steps by which 

 the original duplex retina may have come into being in the earliest verte- 

 brates. The conversion of a diurnal reptile into an arhythmic or noctur- 

 nal one may be illustrated by considering a series of snake species which, 

 though quite unrelated to each other, each exhibit a stage of adaptation 

 through which the subsequent members of the series must once have 

 passed. 



All round-pupilled snakes have only cones, of three types as shown 

 in Figure 26a (p. 63). Two of these are single, one large and abundant 

 (Type A) , the other small and scanty (Type C) . The third is the unique 

 double cone (Type B) invented by the higher snakes to replace the lost 

 double cones of their lizard antecedents (see Fig. 24, p. 59) . 



In N. ndtrix, for example, these cone types are normal and typical. 

 Cemophora is a secretive snake in which the Type A and Type B outer 

 segments have enlarged, thus lowering their thresholds; but the biggest 

 change is in the Type C elements. These are no more numerous than 

 usual, but they have become rod-like in form (Fig. 68a). 



The mud-loving and secretive rainbow snakes, Farancia and Abas tor, 

 maintain the large cone outer segments, and in them the numbers of the 

 stubby Type C rods have increased until they equal or exceed the total 

 number of "A" and "B" cones. The Type C elements probably still lack 

 a rhodopsin at this stage of transmutation. 



Any pit-viper, such as Agkistrodon, shows the next logical steps. The 

 rods have multiplied until they have a human-like abundance relative 

 to the cones (Fig. 68b) , and they are longer than in the rainbow snakes 

 and now contain rhodopsin. With this retina, and a small eye with a 

 small, bright image, the pit-viper has enough sensitivity to require con- 

 siderable pupil mobility, and the animal can prowl at night and bask in 

 comfort and safety in the daytime, sometimes even feeding actively then. 



From the perfected duplex retina attained in the pit-vipers, among 

 many others in which this same secondary adaptation to day-and-night 



