168 ADAPTATIONS TO ARHYTHMIC ACTIVITY 



Xantusia have enlarged their outer segments and lost their oil-droplet 

 pigment. The geckoes have still further enlarged the outer segments, 

 discarded the colorless oil-droplets, and re-invented rhodopsin. In 

 Coleonyx, for example, the transmuted rods are enormously long cyl- 

 inders (Fig. 25, p. 62) and, though sensitive in the extreme, are ade- 

 quately protected by the slit pupil from dazzlement in the daylight. At 

 the same time, the rods of Coleonyx are slender enough, and little-enough 

 summated, to aflFord respectable visual acuity. Thus Coleonyx has been 

 able to become arhythmic by installing a hinge in the middle of the sensi- 

 tivity-acuity seesaw. The geckoes can be comfortable in bright light with 

 a pure-rod retina, while their diurnal lizard relatives, with pure-cone ret- 

 inae, are completely blind in dim light. It may be a little clearer now, why 

 diurnality is a more restrictive habit than noctumality; for while a pure- 

 cone animal cannot see anything, even hazily, at night, a duplex or even 

 pure-rod species can always see in the daytime, though perhaps not acute- 

 ly — the real danger being that he will see too much light (bats, owls) if 

 his share of photomechanical changes, pupil mobility, or lid apparatus is 

 unable to reduce the stimulation of his rods to a comfortable value. 



Pure-rod snakes, as well as lizards, exist by virtue of transmutation. A 

 few pure-cone ones (e. g. Lampropeltis , Rhinocheilus) have increased 

 sensitivity somewhat by enlarging the outer segments, eliminating color- 

 filters (yellow lenses) , and by hooking up more cones to each optic nerve 

 fiber. Arizona and Trimorphodon have carried these processes so far that 

 their pupils have had to become elliptical, and in Hypsiglena and Phyl- 

 lorhynchus (Fig. 26b, p. 63) the visual cells are all morphologically rods 

 though devoid of rhodopsin. When we can observe so clearly the second- 

 ary, apparently easy derivation of unquestionable rods from indubitable 

 cones, it becomes easier to understand why both of these so diverse cell- 

 types are usually required for a well-rounded visual capacity. And, it is a 

 little easier to see that in order to become duplex, and thus more widely 

 useful, the cone-like receptors of the provertebrate retina could spawn 

 rods without necessity of their having to be formed de novo from a sepa- 

 rate cellular ancestor. The first rods in the world were produced by the 

 transmutation of cones, and the process has been occasionally repeated, 

 wherever needed, ever since the vertebrates came on land. 



