640 REPTILES 



In the vipers (Viperidae) and pit- vipers (Crotalidae), derived inde- 

 pendently from colubrid ancestors, the diurnal colubrid pure-cone pat- 

 tern has again been taken over and converted into a duplex one by the 

 transmutation of the Type C cone into a rod. All of the Crotalidae 

 (moccasins, rattlesnakes, fer-de-lances) show the pattern of Figure 188, 

 with the rods outnumbering the cones about as extensively as in man. 

 The viperid retina is rather more complex. The primitive vipers (Causus, 

 A tract as pis) are crepuscular and have nearly-round pupils. The Causus 

 retina (Fig. 189a) looks at jfirst glance like a diurnal colubrid one to 

 which a few rods (C) have been added. The retinas of the highly 

 specialized vipers (Cerastes, Bitis, et al) show the same four types of 

 cells (Fig. 189b) ; but here the C' rods outnumber the combined cone 

 types by three to one (in sections). The mystery of the C' elements 

 clears up when one looks at the retina of the central genus of the family, 

 Vipera. In V. berus the Type C cones and the Type C rods intergrade 

 structurally through an unbroken series of intermediate conditions. 

 Causus and Bitis have obtained their four types of visual cells simply by 

 getting rid of the intermediates between two of them, which Vipera — 

 fortunately for the comparative retinologist — ^has never eliminated. 



Two colubrid genera, Farancia and Abas tor, resemble Vipera closely; 

 but this is of course no implication that the Viperidae stemmed from such 

 colubrids, any more than the essential similarity of the crotalid and 

 Leptodeira patterns implies a genetic relationship. 



The plasticity of the ophidian retina is thus enormous. The snakes 

 alone have rung as many changes upon their visual-cell patterns as have 

 all the other vertebrates put together (Plate I). If anything could make 

 a snake-hater learn respect and admiration for this abused group of 

 animals, it would be the study of their eyes. The writer speaks from 

 personal experience! 



