38 VERTEBRATE PHOTORECEPTORS 



are apparently not entirely useless in daytime because some 

 nocturnal animals with rods only can see in daytime. Typi- 

 cally, however, animals with pure rod retinas become 

 more active with approaching dusk. 



The duplicity theory, since its original conception by 

 M. Schultze and its later elaboration, has been generally 

 regarded as well substantiated. There are, however, a few 

 workers who have taken exception to the validity of this 

 theory. In this connection can be cited the work of Made- 

 moiselle Verrier (1935) who regards the so-called rods and 

 cones as representing extreme structural variations of a 

 single photoreceptor cell — thus doing away with the con- 

 cept of morphological and physiological duality in the 

 retina. Regardless of this, the duplicity theory has received, 

 in recent years, indubitable support from physiologists (see 

 Hecht, 1937, 1938a; also Crozier et al. 1937-1941) in their 

 quantitative physiological measurements of the major func- 

 tions of vision, viz., dark adaptation, intensity discrimina- 

 tion, visual acuity, and the phenomenon of flicker.^ All one 

 has to do is to consult the data and the curves dealing with 



I Crozier and Wolf {J. Gen. Physiol., 1937 et seq.) have provided the most com- 

 prehensive body of data on the responses of lower vertebrates and invertebrates to 

 flickered lights. A number of fishes, the newt Triturus, and the frog, all of which 

 possess both rods and cones, yielded dual functions in these experiments such as are 

 obtained characteristically from duplex retinas. As might be expected certain 

 arthropods (Asellus, dragon fly larvae, bees, crayfish) yielded single functions. The 

 same appeared to be true of the gecko, which possesses an all-rod retina; the turtle 

 (Pseudemys scripta), the horned toad (Phrynosoma cornutum), and the zebra finch 

 (Taeniopygia castanotis) which the authors report as possessing cones only. To the 

 degree that the results from retinas containing one type of receptor were found to be 

 simple, those from duplex retinas dual, these data appear to support the essential 

 features of the duplicity (duplexity) theory. It was found, however, that the gecko 

 and the turtle (Pseudemys) responded over an almost identical range of intensities 

 intermediate between those characteristic of human rods and cones. These authors 

 (op. cit.) point out, on this basis, that the general thesis that cones are less sensitive 

 than rods falls through in this instance. It is probably true that this distinction, 

 like any other single distinction between rods and cones, fails to divide them clearly 

 in specific instances. Each of these receptors is characterized by a complex of prop- 

 erties, no one of which takes precedence over the other. It would seem, therefore, 

 that this observation could hardly be interpreted as a basic difficulty for the du- 

 plexity theory. It might be of interest to note that Pseudemys (according to Walls, 

 19426) is a turtle of relatively nocturnal habit, and that its retina contains con- 

 siderable numbers of rods. 



It may be pertinent to point out, in addition, that the intensity range over which 

 any eye functions may be affected greatly by factors extrinsic to the photoreceptors 

 themselves, e.g., size of pupil, or the presence of intraocular filters. 



