THE TWENTY-FOUR-HOUR HABIT 145 



class 'C, twenty-four-hour vertebrates, and their principal bases for all- 

 round visual capacity, are: 



The teleost fishes, relatively few of which are strictly diurnal, noc- 

 turnal, or crepuscular. Their ability to regulate ocular sensitivity resides 

 aknost wholly in their rod-rich retinae, in the form of efficient photo- 

 mechanical changes. Very few have mobile pupils. 



The frogs, which again rely chiefly upon retinal adjustments and 

 possess at least one diurnal adaptation (yellow oil-droplets) which the 

 toads and the salamanders have had to eliminate, in order to become 

 respectively nocturnal and secretive. 



Many slit-pupilled reptiles, which, being poikilothermous, tend to 

 bask in the sunshine rather more than would a warm-blooded animal 

 with the same general type of eye. The crocodiles and particularly the 

 geckoes have such excellent pupillary control of sensitivity that they 

 are practically arhythmic though tending to feed more at night. 



The larger terrestrial mammals — ungulates, elephants, and large car- 

 nivores such as the wolves, bears, lion, etc. Here alone do we find 

 twenty-four-hour eyes which physiologically are relatively static, with 

 neither special retinal nor, as a rule, extensive pupillary regulation of 

 sensitivity. These forms straddle the fence by having enough rods-per- 

 cone to secure fair intrinsic retinal sensitivity, with large eyes and large 

 retinal images to obtain good resolution of details despite the paucity 

 of cones. They compensate for the lowered illumination of the larger 

 image by placing behind the retina a sensitizing device, the tapetum, 

 which is elsewhere found chiefly among the best-adapted of nocturnal 

 vertebrates. The vision of these mammals both by night and by day is 

 good enough so that they depend on it. Hearing and scent are important 

 enough at long range, but the serious business of stalking involves vision, 

 whatever the illumination. Day or night, a sightless carnivore would be 

 helpless — and so would a blinded ungulate. 



(B) Retinal Photomechanical Changes 



The phenomena which are grouped under this heading were discovered 

 one by one in the 1877-1887 decade. They consist of changes of position, 

 in bright and dim light or darkness, of the retinal pigment and the visual 

 cells, and of minor changes in shape and position of some of the retinal 

 nuclei. The nuclear changes are largely passive and are of no known 

 significance for vision; but the migrations of the rods, cones, and retinal 

 pigment are of great importance in the lower vertebrates. 



