DOUBLE RODS 



the same structure except for the size and shape of the outer segments. 

 They contain no rhodopsin, and owe their sensitivity to the large vol- 

 ume of their outer segments and to their multiple connections to single 

 nerve cells. The gecko double rod does contain a rhodopsin, indicating 

 that this substance, like other pigments such as hemoglobin and melanin, 

 can be evolved repeatedly and was not invented once and for all. 



This whole matter of the conversion of one type of visual cell into 

 another will be discussed at some length later (Chapter 7, section D). 

 It has a considerable bearing upon the ability of animal species to change 

 their characteristic behavior with respect to light, and upon the question 

 of the capacity of animals for discriminating colors (see Chapter 12). 



Fig. 26 — Double rods in snakes, and their ancestry. 



a, the three cell-types of the pure-cone retina of a diurnal colubrid, the European gcass 

 snake, Matrix natrix; parts as in Figs. 22b and 24d. Type A is the ordinary single cone; 

 type B is the double cone, equal in numbers to A; type C is an uncommon single cone with 

 dark-staining ellipsoid. 



b, the homologous rod types of the spotted night snake, Hypsiglena o. ochrorhynchus. In 

 this genus and in some other colubrids, the ancestral cones have all been converted into rods, 

 through intermediate conditions shown by such forms as Cemophora, Arizona, Rhinocheilus, 

 and Trimorphodon. See Figure 68a, p. 166. 



Since cones can and do change into rods in evolution — and rods into 

 cones, as well, though less often — it is not surprising that numerous 

 halfway stages in such derivations occur in living forms. These are, of 

 course, grist to the mill of those few who insist that any distinction be- 

 tween rods and cones is wholly artificial. Naturally, such cells do defy 

 classification, and will not be considered here as discrete types. 



