144 ADAPTATIONS TO ARHYTHMIC ACTIVITY 



In either type of rhythmic animal we may have fancy adaptations, yet 

 an ocular situation which is simple in that it is static. But, for an animal 

 to become capable of arhythmic, twenty-four-hour activity, it is incum- 

 bent upon him to evolve a more flexible set of ocular features, capable 

 of physiological change to embrace a wide range of stimuli — in other 

 words, a dynamic eye in which, when the animal passes from one ex- 

 treme of illumination into the other, something or several things can be 

 seen to happen, and can be seen to be adjustive. The photomechanical 

 changes of the iris and the retina are the most conspicuous 'somethings' 

 referred to. Adaptation to twenty-four-hour vision has its static end- 

 products as well, in the evolutionary alteration of the cone: rod ratios 

 of a rhythmic ancestor, or even in the production of a duplex retina 

 from an ancestrally simplex one by the transmutation of cones into rods 

 or vice versa. 



Before we take up these physiological and phylogenetic methods of 

 adaptation toward all-round visual capacity, it will be well to have 

 certain ecological definitions well in hand. We find that animals may 

 be classified as: 



A. Diurnal; by which we shall take to mean that they are active 

 chiefly in the daytime, occasionally also in bright moonlight. Such 

 animals have eyes which are incapable of dim-light vision. 



B. Crepuscular ; that is, active only in either or both of the evening 

 and morning twilight periods. Requires more sensitive eyes, which are 

 truly neutral, with few or no adaptations for extremes of illumination. 



C. Twenty-jour-hour — more properly, 'arhythmic', the former term 

 applying better to both eye and animal, and both terms signifying that 

 the animal is about equally active by night and day. Such animals, if 

 they sleep at all, do so by irregular cat-naps. 



D. Nocturnal; being active chiefly at night and confining daytime 

 activities largely to passive basking. Eyes usually more sensitive than 

 those of twenty-four-hour animals, and with much better devices for 

 greatly reducing sensitivity in daylight, 



E. Strictly Nocturnal; with such sensitive eyes, so lacking in sensi- 

 tivity-reducing devices, that the animal is secretive or quiescent by day. 



Each of these categories blends and intergrades with the next. Par- 

 ticularly is this true between 'C and 'D\ in which groups fall nearly all 

 of the mammals with the larger ones leaning toward 'C and the smaller 

 species inclining strongly toward 'D' or 'E'. The chief assemblages of 



