162 ADAPTATIONS TO ARHYTHMIC ACTIVITY 



rich or even pure-rod. Many species in both categories are fond of 

 basking, and the geckoes are often active in the brightest of Hght. This 

 twenty-four-hour activity is made possible by great pupil excursions, 

 which are perhaps exaggerated by the absence of movable lids. Thus, 

 some geckoes if not most or all, and even one or two snakes {Leptodeira 

 annulata, for example) can close their pupils completely. The gecko, 

 with a large eye and retinal image, and a retina which, though pure- 

 rod, often has excellent resolving power (since the rods are but little 

 summated and owe their sensitivity to their size and to their rich content 

 of rhodopsin) has probably the best allround, night-and-day eye of 

 any vertebrate below the mammals. 



The crocodile group is nocturnal. Its members, notoriously fond of 

 basking, depend for the protection of their sensitive duplex retinae upon 

 the lids and pupil rather than the retinal photomechanical changes, which 

 are here at a low ebb. Highly active pupils, among the reptiles, thus 

 go with pure-rod and duplex retinae and are the more mobile, the more 

 the species or group scorns concealment in the daytime. The circular 

 pupils of the diurnal majority are relatively or quite inactive, as would 

 be predicted from their pure-cone retinae. 



Bird pupils are very active, but the photomechanical changes have 

 made a phylogenetic 'come-back' in this group. The paradox is resolved 

 when one notes the lack of precise adjustment of the avian pupil to 

 illumination. It plays so much that, although experimental proof is as 

 yet lacking, many workers have suspected it of being under the bird's 

 voluntary control. At any rate, it is easy to understand why in the birds 

 the retina has had to re-assume the responsibility of regulating its own 

 stimulation — the pupil cannot be trusted to do so. 



Mamimalian pupils — except those blocked by enormous lenses in some 

 strongly nocturnal forms (Fig. 71, p. 173) — are comparable in mobility 

 with those of birds, but are better-behaved with respect to intensities and 

 thus make retinal migrations quite superfluous. Excursion is greatest, of 

 course, in nocturnal forms which love to bask, like the cats and foxes. 

 It is reduced in many ungulates, and is least in crepuscular forms such as 

 the bats, in secretive night-prowlers, and in such sun-worshippers as the 

 ground-squirrels. In short, the more constant are the illumination-condi- 

 tions in which a group prefers to be active, the less mobility the pupil 

 exhibits. The pupil closes almost completely in Tarsius, Pedetes, dormice, 

 and cats, very far in the otters and (by means of an operculum) in some 

 whales. It has an exceptional range of movement in the seals — but not 



