DUPLICITY AND TRANSMUTATION 



167 



There are other cases in which nearly all (S phenodon) or absolutely 

 all the cones of a pure-cone forebear have been transmuted into rods; 

 and the result is not necessarily a strictly nocturnal animal, for the pupil 



O.N. 





J 



:^ L^rsen 



(^ ^ e-G. 



G LarsGn 

 b 



Fig. 69^ — Duplex snake retirice. x 500. 



a, a nocturnal colubrid, Leptode'tra annulata, whose rods (derived by transmutation from 

 the Type C cones of a diurnal ancestor — compare Fig. 26) are very numerous, and whose 

 cones have taken a position which, in terms of the photomechanical changes of lower verte- 

 brates, might be called one of permanent dark adaptation. 



b, for comparison, a boid, Tropidophis melanurus, whose simple, duplex retina is more 

 ancient than the cone-simplex one from which the Leptodeira pattern was evolved. 



C- cone; D.C.- double cone (= Type B); S.- ganglion-cell layer; I.N.- inner nuclear layer; 

 L.- limitans; O.N.- outer nuclear layer; P.E,- pigment epithelium; R.- rod; S.C.- single 

 cone (=Type A). 



is so well developed in some reptiles that it can often make possible 

 twenty-four-hour activity even when, behind it, lies a pure-rod retina. 

 Round-pupilled lizards have only single and double cones, which in 



